Astronomy

What is the angle between the planes of Oumuamua's hyperbola and the Milky Way Galaxy?

What is the angle between the planes of Oumuamua's hyperbola and the Milky Way Galaxy?


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Viewing the animations of Oumuamua passing through the solar system, it is apparent that the plane of its hyperbola departs from the plane of the ecliptic by a large amount. The plane of the ecliptic departs from that of the Milky Way Galaxy by a large amount. Now that there is so much speculation about a potential artificial origin, one might be forgiven for speculating that there might be a coincidence between the Milky Way Galaxy's plane and that of Oumuama.

What is the angle between the planes of Oumuamua's hyperbola and the Milky Way Galaxy?


The inclination of the orbit is 122.7 degrees (with respect to the ecliptic), whilst the Galactic plane is inclined at 60 degrees to the ecliptic. So there is 63 degrees between the two.

Such a high inclination by no means rules out that the object is orbiting within the Galactic disk. A study by Mamajek (2017) concluded that it's approach trajectory was entirely consistent with an object travelling at the median velocity of stars around us. It had insignificant vertical or radial motion with respect to the local standard of rest, but is travelling about 10 km/s too slowly for a circular orbit at the Sun's Galactocentric radius (whereas the Sun moves about 10 km/s too fast).

This appears to be discrepant with the inclination angles discussed above until you recall that the sun is moving up and out of the Galactic plane at around 7 km/s.

Have a look at the animation at https://www.cosmos.esa.int/web/gaia/iow_20180925 , it says a thousand words. It shows time running backwards from the time of encounter and shows how Oumuamua comes in from inside the Sun's Galactic orbit and from "above", with respect to the Galactic plane.


Interstellar Visitor 'Oumuamua Was Shaped By Cosmic Particles

Artist's impression of ʻOumuamua, the first known interstellar object to pass through the Solar . [+] System.

Last year, the interstellar interloper ʻOumuamua passed through the inner Solar System. Originally thought to be a comet, then later an asteroid, this visitor turned out to have properties unlike any object ever seen before. It moved far too quickly and from too inclined an angle to originate from within our Solar System neither Jupiter nor Neptune nor an Oort cloud object could have flung it inwards with those properties. When we examined it in detail, it appeared to have a carbon-based coating over an icy interior, yet sprouted no tail, despite reaching temperatures of 550 °F (290 °C). Oddest of all, it was cigar-shaped, approximately eight times as long as it was wide. While many origin theories have been proposed, an incredibly simple possibility may provide all the answers: simply traveling through the Milky Way for billions of years may have transformed it into the object we see today.

The planets of the Solar System, along with the asteroids in the asteroid belt, orbit all in almost . [+] the same plane, making elliptical, nearly circular orbits. Beyond Neptune, things get progressively less reliable.

Space Telescope Science Institute, Graphics Dept.

When you look at our Solar System today, you can find the inner, rocky worlds, the outer, gas giant worlds, and then a slew of smaller objects clustered together in four different populations. There are:

  1. the asteroids, mineral-rich objects formed around the "frost line" between Mars and Jupiter: the border between where the Sun's radiation will allow the presence of water-ice in full sunlight,
  2. the Kuiper belt objects, ice-rich objects formed beyond Neptune, which become comets if they travel into the inner Solar System,
  3. the centaurs, which are hybrid objects found in between Jupiter and Neptune's orbits,
  4. and the Oort cloud objects, which lie beyond the Kuiper belt and are remnants from the Solar System's formation.

While Kuiper belt and Oort cloud objects are similar in composition and countlessly large in number, there were even more of them in the early days of the Solar System's formation.

Although we now believe we understand how the Sun and our solar system formed, this early view is an . [+] illustration only. When it comes to what we see today, all we have left are the survivors. What was around in the early stages was far more plentiful than what survives today.

Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)

Over billions of years, mutual gravitational interactions between objects, as well as between planets and these objects, hurls enormous numbers of them into interstellar space. For every star we have in our galaxy, we likely have anywhere from thousands to millions of these objects flying through the Universe, unbound to any star. And just as the stars typically move, relative to the Sun, at speeds of around 20 km/s as they orbit the galactic center, so should, on average, the overwhelming majority of these interstellar interlopers.

The nominal trajectory of interstellar asteroid A/2017 U1, as computed based on the observations of . [+] October 19, 2017 and thereafter. Note the differing orbits of the planets (fast and circular), the Kuiper belt objects (elliptical and roughly coplanar), and this interstellar asteroid.

Tony873004 of Wikimedia Commons

From a certain point of view, it's astounding that it took so long for us to find the first one! It's likely that these encounters happen multiple times per year, but it's rarer that relatively large objects appear so close to our own Sun, something we were able to capture only through the deep, fast, repeated surveying power of Pan-STARRS. As we discovered what it was, repeated and improved observations enabled us to determine its bizarre properties: its tumbling motion, its brightening-and-darkening light curve, its surface and interior composition, and its oddly elongated shape. The tumbling was no surprise, since without a massive object to anchor itself to, there's no reason for its orbit to regularize about a particular axis, but the other properties were a mystery.

The light curve of 'Oumuamua, at right, and the inferred, tumbling shape and orientation from the . [+] curve itself.

nagualdesign / Wikimedia Commons

We've never seen an interstellar visitor before, so astronomers and astrophysicists are scrambling to explain ʻOumuamua. Some attempt to trace its motion backwards in time, as though the extraordinarily unlikely possibility that this object was recently ejected from a star system was somehow probable. Others seek an explanation as to how such an elongated, carbon-crusted object would likely form in situ, as opposed to either the rubble-pile or solid objects we see overwhelmingly in our own backyard. Yet the most straightforward explanation may be the one that hits all the major points: that this is a common, icy object has been wandering through the galaxy for billions of years, and its interactions with the interstellar medium have worn it down to what we see today.

Just as pebbles in the ocean get worn down to smaller, smoother, and more asymmetric shapes as time . [+] goes on, so could the interstellar medium wear down a traveling comet-like body to what 'Oumuamua looks like today.

Quim Gil / Wikimedia Commons

We think of space as being an empty place, but the truth is that there are dust grains, particles, neutral atoms, ions, and cosmic rays zipping through the entirety of the galaxy, even when there are no stars. As an object moves through space, circling the galaxy at hundreds of kilometers per second (and moving relative to most other objects at tens of kilometers per second), it's constantly bombarded by large numbers of small, fast-moving bits of matter. Just as water and sand will smooth out and erode pebbles and cobbles in the ocean here on our world, the cosmic equivalent — the interstellar medium — will have the same effect over extremely long timescales on ejected icy bodies.

Comet 67P/C-G as imaged by Rosetta. 'Oumuamua is very different, in shape, size, and surface . [+] composition from this comet, but traveling through the galaxy for billions of years could have caused exactly that.

Because objects are rarely spherical, they tend to erode more in one dimension and less in the others, producing elongated, flattened shapes. The lightest molecules are eroded away the fastest, while the heavier ones, or the ones that can react with one another to form a stronger, lattice-like shape, can become bound together. The presence of carbon compounds, bombarded by particles, means they can heat heat up, bind together into more stable molecular configurations, and then freeze over again. This straightforward idea would, over billions of years, produce generically smooth, elongated, carbon-crust-rich bodies from initially icy ones.

Unless they traveled so close to a star that the interior burst out through the crust, we would expect no tails, no comas, and no comet-like behavior. Additionally, after billions of years, most of the external volatiles would have boiled away, just as they do for long-term objects in our Solar System that have been crossing Earth's orbit for millennia. It may not have an origin any more unusual than your run-of-the-mill Kuiper belt or Oort cloud object ʻOumuamua may simply have the foreign properties we observe because of a long-term journey through the galaxy. Simulations, improved observations, and greater statistics on this new class of objects will eventually provide the answer, but until that day comes, follow the golden rule of science: never attribute an exotic explanation where a mundane one would suffice.


Oumuamua Pursued

Shown again are the planets’ courses in October, November, and December, and Oumuamua’s path in magenta before its discovery, yellow from then to the end of 2017. Perihelion is marked with a tick closest moment to Earth, with a green line the vernal equinox direction, with a dashed blue line.

I’ve used slightly revised orbital elements, and a viewpoint rotated from longitude 350° to 160° and added the “line of nodes.” This is the dashed line from the descending node through the Sun to the ascending node. It is the intersection of the body’s orbital plane with Earth’s (the ecliptic), and should help you to visualize that orbital plane.

Perihelion was closer to the descending node if you look at the stalks connecting the orbit to the ecliptic plane, you can see that the orbital plane is tilted to the left the arriving course is more northerly than the departing one.

Some people found the first version unclear. While devising it I must have tried at least a dozen viewpoints, each involving a re-drawing, before deciding on one that seemed clear and did not obscure certain details. Perhaps I’m too accustomed to my own idiom. Jan Kok suggested a view with the body’s orbital plane flat on – parallel with – the paper, or screen. That would be a view from the north pole of the body’s orbit. It might suit a picture for a single body, but the ecliptic plane would be strangely tilted also, it could not serve for a picture in which another body is added for comparison, or a picture containing several comets. I’ve found that a view from latitude 15° north of the ecliptic works generally for these pictures, with just the longitude varied. It’s like walking – but not flying – around a model to peer at it from various directions.

One commenter mentioned the “MOID” given in various sources. That is the minimum orbital intersection distance: the smallest distance between the orbits – not between the bodies. When Oumuamua was closest south of a point in Earth’s orbit, Earth had not yet reached that point.

It seems hard to find any source that mentions the date of the closest approach my calculation now spits out Oct. 14. I would be more confident, of course, if Jean Meeus or Aldo Vitagliano, or Gareth Williams of the Minor Planet Center, were doing the calculating.

Yes, I should write ‘Oumuamua. The initial mark, which you can call an apostrophe or inverted comma or quotation mark or, perhaps most handily, quotemark, will probably get dropped in circumstances such as computer listings. The quotemark is not a letter in English orthography – it doesn’t represent a segmental phoneme, like p or a – but it is used in transcribing certain unfamiliar consonants from other languages. For the language of Hawaii, which its natives would prefer us to spell as Hawai’i, it represents the “glottal stop” (plosive opening of the glottis, or slit between the vocal cords in the larynx, as in a cough). Hawai’ian has, at least by some linguists’ analysis, the record smallest number of segmental phonemes: 5 vowels and only 8 consonants (including ‘). For comparison, English has around forty, varying with method of analysis, and Khoisan languages of southern Africa have around a hundred, about half being consonants of the “click” or double-articulation type.

The single quotemark (which European scribes invented several centuries later than the double quotemark) is a bit awkward when used as a letter. If we type a “straight” quotemark, our smart software will turn it into the 6-shaped form if it’s at the beginning of a word (taking it for the beginning of a quotation), and into the 9-shaped form if it is between letters (taking it for an apostrophe). So the ‘ at the beginning of ‘Oumuamua may stay straight or become 6-shaped depending on what the software does. For transcribing Arabic or other Semitic languages, the convention is to use the 6-shaped quotemark for the pharyngeal consonant (at the beginning of ‘Abd or ‘Ali) and the 9-shape for the glottal stop, as in al-`Islam. So that conflicts with the usage for Hawaiian.

Not to mention that French, for instance, uses an entirely different form of quotemarks.

But enough of the pharynx and larynx and back to ‘Oumuamua.

It was discovered on October 19 by Robert Weryk, in an image of Oct. 18 from Pan-STARRS (the Panoramic Survey Telescope and Rapid Response System) on Mount Haleakala in Hawaii. Measured positions and the calculations from them showed that it was in a highly eccentric orbit, as are long-period comets, so it was announced, on Oct. 25, as C/2017 U1 (PANSTARRS). “U1” meant that it was the first comet discovery in the second half of October.

Variations in its faint point of light allowed deduction that it is tumbling, rather than spinning, and is a pencil of reddish rock, 250 yards long and 40 wide. This reminded science fiction fans of Arthur C. Clarke’s Rendezvous with Rama, and there was a wish to dub the object “Rama.” This, I think, would have led to chronic confusion between the real object, the fictional one, and the Hindu god. Radio telescopes have searched for artificial signals but found none, so far.

A better idea was the Hawaiian word, which is said to mean “scout” and is composed of ‘ou, “reach out,” and mua, “forward,” with reduplication. (Reduplication for emphasis is a regular feature of Hawaiian and some other languages, and an occasional one in English -“bye-bye” “Dorset-Dorset,” a website created by my friend Ian Dicks.)

Deep images did not show any haze like that given off by comets. And the eccentricity of the orbit proved to be significantly higher than any ever known: nearly 1.2.

What does this mean? Eccentricity is the number that expresses the shape of the curve. A perfect circle has eccentricity 0. Bodies that stay around the Sun travel ellipses, with eccentricity between 0 and 1: the planets have moderate eccentricities, up to 0.2 (Mercury) Comet Halley is in a long ellipse with eccentricity almost 0.97. Eccentricity of 1 means a parabola, which like the circle is a limiting case and no body could actually stay in it: it can’t decide between closing on itself or continuing to widen. Beyond the parabola, with eccentricities above 1, are the hyperbolas, which open out infinitely.

What we call the long-period or non-periodic comets are generally treated as having parabolic orbits: they are so long that it’s difficult to tell the difference, but they are thought to be in enormous ellipses with outer ends still in the solar system. A few comets have been found to be in slightly hyperbolic paths, but that is because they have been perturbed into them by passing among the planets. If a comet stays on a hyperbola, it is on its way out of the solar system.

Oumuamua is the first observed body that must have arrived along a hyperbola. It couldn’t have been perturbed into it by any planet, even a remote unknown one it was traveling far too fast. It could only have arrived from outside the solar system. The Sun became the focus of the hyperbola, and after whipping around it Oumeamea is on its way along the other arm of the hyperbola, back out to the stars in a different direction.

When discovered it had already passed that perihelion and passed its nearest to us, was hurtling away, and was of such obvious interest that the Minor Planet Circular sent to the world’s observatories asked for observations urgently.

Since it is not a comet of the solar system, the designation became A/2017 U1. The “A” prefix is for objects that are mistakenly taken to be comets but are really solar-system minor bodies of other kinds, usually, asteroids. This prefix had been set up for this kind of situation, but had never yet been used. Plenty of “apparently asteroidal” discoveries have shown cometary “activity” and been reclassified as comets, but no comets had had to be reclassified as asteroids.

Oumuamua may be an asteroid, but it is not a member of the solar system’s fleet of asteroids. An email conversation “between the IAU [International Astronomical Union] General Secretary, the IAU Division F President, the co-chairs of the IAU Working Group on Small Body Nomenclature and the Minor Planet Center” led on November 6 to the creation of a new class, “I,” for interstellar objects. This first of the kind becomes 1I/ U1 ?Oumuamua.

You will notice two other awkwardnesses resulting from our alphanumeric system, which is handier than Egyptian hieroglyphs or Roman numerals but not a creation of fully intelligently design. Capital I and lowercase l and numeral 1 (and capital O and numeral 0, and numerals 5 and 6) are similar enough to cause millions of hours per year of hesitation and confusion. As Anthony Barreiro suggested, it would have been better to use E/, for “extra-solar.” It isn’t reserved the prefixes in use for comets are C/, P/, D/, A/, and X/ (for non-periodic or not yet known, periodic, disappeared, really an asteroid, and insufficiently known).

And: at least some keyboards have not just one key for the single quotemark, but a second, which gives a backward-sloping mark like a grave accent, so that you can force the 6-shape where the 9-shape would otherwise appear. But not all fonts or software understand this, so they find it to be an unknown character and turn it into a question-mark. Not the worst thing to turn it into: the International Phonetic Alphabet character for the glottal stop looks like the question mark without a dot.

The Minor Planet Electronic Circular announcing the decision included the sentence “A solution has been proposed that solves the problem.” I caught this for my collection of examples of redundancy. But is that unfair? I suppose there are solutions that do not solve. Like, we may unkindly say, this one.

It may not matter that we’re stuck with 1I/ U1 ?Oumeamea, since in ordinary discourse we may drop the first six characters, and since we may not live to see 2I.

It is amazing that a rock wandering interstellar space aimed so close to our Sun. This is where the rareness lies. Stars are around 8 light-years apart in our region of the galaxy (it’s not a number that can be strictly given see the Astronomical Companion section on “Nearest Stars”). That’s around 40,000,000,000,000 times the length of Oumuamua. Comets are known to have been diverted into hyperbolic orbits, and other fragments have probably been ejected during the evolution of the solar system, so it is likely that Oumuamua was ejected from another star system and that there are trillions more like it. They have a fair chance of passing through a star’s outer Oort Cloud, which may reach out to around 3 light-years and is sparsely inhabited by proto-comets if this has happened for the Sun’s Oort Cloud, it would have been unobservable by us and would have negligible effect on the rock’s path. Oumuamua is just one that happened to aim almost directly at the Sun. Not quite directly, nor as nearly as at Mercury’s orbit, but near enough to be pulled by the Sun’s gravity inward into what became the hyperbola with the Sun at the focus, and thus sent out in an entirely new direction.

By mid December, Oumuamua was 2.5 astronomical units from the Sun (as far as the main belt of asteroids), beyond the reach of the largest telescopes. Some elaborate ways of getting a space probe to catch up with it, before it gets utterly too far away, have been proposed, making use of light-sails or gravitational wells.

Here is a chart of how Oumeamea’s path on the sky as it arrived would have appeared, if we could have seen it.

As with any comet approaching from far out, the path makes apparent loops, gradually widening, because viewed from Earth circling the Sun.

And here is the rapid path across the sky in 2017 – red from discovery onward – and the departure.

The receding path, like the approaching, goes into annual loops, but otherwise they are not symmetrical. This November, Oumuamua crossed the asterism known as the Circlet, otherwise the western of the two “fishes” of Pisces, and then seemed to make a sharp northeastward turn. This was because it was moving directly away while Earth was curving away around its own orbit.

Plainly, Oumuamua came from a direction in Lyra, about 5 degrees south of Vega. And the direction to which it is now heading is in Pegasus. I calculate that the spherical angle between these points in Lyra and Pegasus is about 68°. And that does appear to be the angle between the two arms of the hyperbola in the space picture.

What is interesting is the direction from which Oumuamua came. On average the stars must be moving in circular orbits around the galaxy’s center, and for us that means a direction in the Milky Way at galactic latitude 0°, longitude 180°, in the constellation Cygnus, not far from the supergiant star Deneb. But individual stars have their individual orbits, slowly changing under the influence of each other, and the Sun’s is carrying it slightly aside from the general direction, toward what is called the apex of the Sun’s way. This is near Vega, though actually just across the border from Lyra into Hercules. (See the Astronomical Companion, beginning of the “Outrush” section.)

So does this mean that Oumuamua was coming to meet us, sent from somewhere in almost exactly the opposite direction relative to the galaxy? No, I think its speed from the apex direction toward us means that, relative to the galaxy, it was moving forward in the same direction as us and some of the stars around us but with slightly slower speed. Imagine flipping a ball off the back of a moving truck. It would keep rolling forward along the highway, but would be run over by another truck. This may be a helpful though not precise analogy.

There is fascinating speculation about which star system Oumuamua may have been ejected from – maybe not Vega, which a million years ago was in a different position relative to us – how long ago this might have been, whether it could have been billions or years ago – many revolutions around the entire galaxy – because of the small chance of centering another star system. Watch this space, so to speak. Well, not this space, but the space of professional astrodynamics. Having at last almost hit but not been captured by a star, Oumuamua is out along a different path. Will this too be just a variation on the general orbit around the galaxy, or will it eject Oumuamua from the galaxy?

In the new year, Oumuamua will reach the Great Square of Pegasus, the winged horse, on which it is destined to ride away into, as far as we are concerned, infinity. If those who believe it is a cylindrical starship like Rama are right, it is wiring back to you a Vegan wish for a happy Earthyear.


An Interstellar Object's Masquerade

Some of the smaller inhabitants of our own Solar System are naughty little objects that teasingly refuse to be designated as one thing or another. But, what is true for our Sun and its family is apparently also true for the children of other stars, and a recent invader from a stellar system beyond our own has made this very clear. Oumuamua is the first known visitor from a distant stellar system to travel through our own Solar System, and it is certainly a mischievous little refugee from interstellar space. First assumed to be a comet, it was then determined to be an interstellar asteroid because it did not display gas emission or a dusty environment in observations--and without these hallmarks of cometary behavior, it was reclassified as the first interstellar asteroid known. But this strange story has a delightful twist, because on June 27, 2018, a team of astronomers reported that "Oumuamua is an inactive comet, and not an asteroid as previously thought."

Formally designated iI/2017 U1, Oumuamua was discovered by Dr. Robert Weryk on October 19, 2017. Dr. Weryk. who is of the University of Hawaii, used the Pan-STARRS telescope located at Haleakala Observatory in Hawaii to make his discovery 40 days after Oumuamua had made its closest approach to our Sun. When this mysterious object was first spotted, it was approximately 21,000,000 miles--or about 0.22 astronomical units (AU) from Earth--or 85 times as far away as the Moon. One AU is the equivalent of Earth's average distance from our Star, which is about 93,000,000 miles. At the time of its discovery, Oumuamua was already zipping away from our Sun. On November 6, 2017, Oumuamua was designated as the first of a new class of interstellar objects.

Oumuamua is of a dark red hue, which makes it similar in color to objects in the outer regions of our Solar System. It is also small, being only approximately 800 feet x 100 feet in size. Because Oumuamua displayed no signs of a thrashing comet tail, despite its close approach to the heat of our fiery, brilliant Star, and because it also displayed both a significant elongation and rotation rate, it was originally considered to be a metal-rich rock with a relatively high density. It also tumbled around chaotically, instead of smoothly rotating. In addition, it was traveling so swiftly relative to our Sun that there was no possibility it had been born in our own Solar System. This swift speed also indicates that Oumuamua cannot be snared into an orbit around our Sun. This means that it will eventually flee from our Solar System and again become a traveling denizen of the space between stars. Oumuamua's system of origin, and the amount of time it has spent traveling through interstellar space, remain mesmerizing mysteries.

Oumuamua is the word for "scout from the distant past" in Hawaiian. Alas, what this zippy little refugee from the family of an alien star actually is--an asteroid or a comet--has presented a complicated problem, and has proven to be difficult to determine. Soon after Oumuamua was first discovered, astronomers from all over the world attempted to discover its true identity.

The first tantalizing clue, revealing Oumuamua's real nature, has to do with its trajectory. Follow-up observations conducted by astronomers using the Canada-France-Hawaii Telescope (CFH), the European Space Agency's (ESA's) Optical Ground Station telescope in Tenerife, Canary Islands, and other telescopes around the world, have helped astronomers determine the mysterious past and elusive identity of this weird wanderer. Unlike any asteroid or comet ever seen before, this new and baffling object zipped past the Sun, approaching from "above" the plane of the planets on a highly inclined orbit. It was also certainly moving fast enough at the impressive clip of 70,800 miles per hour (as of June 1, 2018) to escape the gravitational grip of our Star.

Comets, Asteroids, And The Mysterious Identity of Oumuamua

In our own Solar System, comets are the lingering remnants of the multitude of icy planetesimals that built up the quartet of giant, gaseous planets inhabiting the outer regions of our Sun's domain--Jupiter, Saturn, Uranus and Neptune. The ancient planetesimals were the "seeds" from which the planets ultimately emerged. Asteroids, that are mostly found in the Main Asteroid Belt between Mars and Jupiter, are the rocky and metallic objects that built up the four inner, solid planets--Mercury, Venus, Earth and Mars. Because comets dwell in our Solar System's deep freeze, in a remote twilight region where our Sun's golden light and warmth can barely reach, they preserve in their frozen hearts the original elements that gave birth to our Solar System about 4.56 billion years ago.

Comets come screeching inward towards our Star from three regions in our Solar System's outer limits: the Kuiper Belt, Scattered Disk, and Oort Cloud. Recent studies from the mid-1990s have shown that the Kuiper Belt is dynamically stable, and that comets from this region actually originate in the Scattered Disk. The Scattered Disk is a dynamically active domain that probably formed as the result of the outward migration of Neptune, in the early days of our Solar System. The icy objects that bounce around within the Kuiper Belt, along with the frozen occupants of the Scattered Disk, are collectively called trans-Neptunian objects.

The very remote Oort Cloud is a thousand times farther away than the Kuiper Belt and, unlike the Kuiper Belt, it is not flat. The Oort Cloud is actually an enormous shell composed of icy objects that surrounds our entire Solar System--and it reaches halfway to the nearest star beyond our Sun.

Small and dark, Oumuamua is the first known interstellar object to have invaded our Solar System, and it appears to have come from about the same direction as the star Vega in the constellation Lyra. The direction of the incoming motion of Oumuamua indicates that it comes from the most likely direction that alien objects would take when entering our Solar System from interstellar space. Soon after its discovery, Oumuamua was playfully compared to the fictional alien spacecraft Rama because of its interstellar origin. Both the real and the fictional objects are unusually elongated and limited in size. However, Oumuamua's reddish color and fluctuating brightness at first suggested that it is an asteroid.

On October 26, 2017, two earlier observations of Oumuamua, derived from the Catalina Sky Survey, were found that were dated October 14th and 17th 2017. A two week observation verified a strongly hyperbolic trajectory. Indeed, Oumuamua has a hyperbolic excess velocity of about 58,900 miles--its speed relative to our Sun when in interstellar space. The Catalina Sky Survey's purpose is to discover comets and asteroids. It is conducted at the Steward Observatory's Catalina Station located near Tucson, Arizona.

By the middle of November 2017, astronomers were convinced that Oumuamua was a migrating interstellar object. Based on observations made over a period of 34 days, Oumuamua's orbital eccentricity of 1.20 was determined--the highest ever seen. An eccentricity higher than 1.0 indicates that an object exceeds our Sun's escape velocity, and is therefore not bound to our Solar System. Indeed, Oumuamua's eccentricity is so high that it could not have been caused by an encounter with any of our Solar System's planets, either known or as yet undiscovered. This is because even undiscovered planets--if any exist beyond Neptune--could not explain Oumuamua's trajectory. Any hypothetical undiscovered planet would have to be located very far from our Star and, therefore, would have to be traveling very slowly, according to Kepler's law of planetary motion. Encounters with such an undiscovered planet could not speed up Oumuamua's movement to the value observed. This strongly indicates that Oumuamua can only be an interstellar vagabond, entering our Solar System from the space between stars.

Oumuamua migrated into our Solar System from above the plane of the ecliptic, and the powerful pull of our Star's gravity caused it to speed up until it had attained a maximum speed of 196,200 miles per hour as it traveled below the ecliptic on September 6, 2017. It then made a sharp upward turn when it was at its closest approach to our Sun (perihelion). On September 9, 2017, Oumuamua's was closest to our Sun at a distance of 23,700,000 miles--or approximately 17% closer than Mercury's closest approach to our Star. The mysterious little vagabond is now screeching away from our Sun--in the direction of Pegasus--at an angle of 66 degrees from the direction of its approach.

Oumuamua, on the outward leg of its long and treacherous travels through our Solar System, passed below Earth on October 14, 2017 at a distance of about 15,020,000 miles from our planet, and then shot back above the ecliptic on October 16, 2017, passing above the orbit of Mars on November 1, 2017. It passed above Jupiter's orbit in May 2018, and is currently scheduled to pass above Saturn's orbit in January 2019, and Neptune's orbit in 2022. As it escapes from the gravitational grip of our Solar System, Oumuamua will be approximately right ascension 23h51m and declination +24degrees 45, in Pegassus. It will continue to slow down until it has reached a speed of 26.33 kilometers relative to our Sun. This is the same speed it had before its approach to our Solar System. It will take this little wanderer about 20,000 years to free itself from our Solar System's gravitational grasp entirely.

Oumuamua's place of origin seems to be from the star Vega. Vega (Alpha Lyrae) is the brightest star in the constellation Lyra, which is the 5th brightest star in the sky, as well as the second brightest star in the northern celestial hemisphere after Arturus. Vega is relatively close to our Solar System, at a mere 25 light-years from our Sun. It is only about 1/10th the size of the Sun, but it is 2.1 times as massive. Both our Star and Vega are considered to be approaching stellar mid-life. Because Vega is more massive than our Sun, even though it is younger, it has a shorter "life" span, and so reaches it's stellar mid-life at a younger age. The more massive the star, the shorter its "life" on the hydrogen burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution.

Taking into account Vega's proper motion, it would have taken Oumuamua 600,000 years to reach our Solar System from its original birthplace in the family of Vega. However, as a nearby star, Vega was not in the same part of the sky at that time. For this reason, astronomers have calculated that one hundred years ago, this interstellar vagabond was about 561 AU from our Star and traveling through space at a speed that is very close to the mean motion of material in our Milky Way Galaxy in the general neighborhood of our Star. This is also known as the local standard of rest. This particular velocity profile also suggests an extrasolar origin. However, it also apparently rules out the closest dozen stars. Indeed, the strong correlation between Oumuamua's velocity and the local standard of rest may suggest that it has circulated our Galaxy several times and therefore may have actually been born in an entirely different part of the Milky Way.

For this reason, it is not known how long this mysterious little traveler has been speeding its way through the space between stars. Some astronomers propose that our Solar System is probably the first star system that Oumuamua has visited during its long journey, after being unceremoniously evicted from its parent-star's birth system long ago. Indeed, this event potentially occurred several billion years ago. In addition, astronomers have suggested that this bewitching, bewildering little object may have been hurled out of a stellar system located in one of the local kinematic associations of bright young stars, such as Carina or Columba specifically, within a range of approximately 100 parsecs. Both the Carina and Columba associations are now very far away in the sky from the Lyra Constellation, the direction Oumuamua took when it first migrated into our Solar System. In addition, there is another interesting possibility, proposed by other astronomers, suggesting that Oumuamua was hurled out from a white dwarf system and that its constituent volatiles were lost when its star evolved into a swollen red giant.

A white dwarf is a dense little stellar corpse left behind by a sun-like star after it has run out of nuclear fuel, and has perished as a result. A white dwarf is really the progenitor star's very dense core, and it is usually surrounded by a breathtakingly beautiful planetary nebula--the candy-colored shimmering gases that were once the outer layers of the erstwhile small progenitor star. Before a small star like our Sun perishes to evolve into a white dwarf, it swells to monstrous proportions, and sports a red hue. This type of swollen red star is called a red giant, and our own Sun is destined to evolve into just such an enormous dying star. When our Sun goes red giant, it will first incinerate Mercury, then Venus and, after that, possibly Earth. As our dying Star continues to balloon in size, its heat will also move outwards. In the end, before our dying Sun--in its red giant phase--leaves its relic core behind in the form of a white dwarf, it will convert Pluto, its large moon Charon, and other currently frozen denizens of the distant Kuiper Belt into tropical havens.

Approximately 1.3 million years ago, Oumuamua may have wandered within a distance of 0.52 light-years of the nearby star dubbed TYC 4742-1027-1. However, its velocity is too high to have been born from that star system, and it likely just passed through the system's own Oort Cloud, composed of icy comet nuclei, at a swift speed of 230,000 miles-per-hour.

One especially interesting theory suggests that Oumuamua may be a fragment from a tidally disrupted planet. This particular scenario explains very well its elongated shape and "refractory" composition. Oumuamua probably contains nickel-iron, as well as other metals. This makes little Oumuamua a rare treasure of an unusual object, much less abundant than other extrasolar bodies that have been characterized as either "dusty snowballs" or asteroids.

An Interstellar Masquerade

At first, astronomers assumed Oumuamua was a comet. This is because current understanding of planet formation predicts that interstellar comets are much more abundant than interstellar asteroids. However, because initially astronomers detected no evidence of gas emission or a dusty environment--characteristic of comets--Oumuamua was determined to be an interstellar asteroid. Without these tattle-tale characteristics of comets, astronomers concluded that it could not be a comet, and had to be an asteroid.

But this weird tale of a mysterious object, dancing through the space between stars, has a strange twist. This is because, after the initial discovery observations with Pan-STARRS, a team of astronomers led by Dr. Marco Micheli of the ESA's SSA-NEO Coordination Centre, and Dr. Karen Meech of the University of Hawaii's Institute of Astronomy continued to make high precision measurements of the object and its position using a number of ground-based facilities like CFHT, as well as the Hubble Space Telescope (HST). The final images were obtained using HST in January 2018. This was before Oumuamua had grown too faint to be observed as it streaked away from Earth on its way out of our Solar System.

But, contrary to their expectations, the team of astronomers found that Oumuamua was not following the trajectory predicted if only our Sun's gravity and the planets were influencing its path. "Unexpectedly, we found that Oumuamua was not slowing down as much as it should have due to just gravitational forces," Dr. Micheli noted in a June 27, 2018 University of Hawaii Press Release. Dr. Micheli is the lead author of the paper reporting the team's findings, published in the June 27, 2018 issue of the journal Nature.

So, what could be causing the curious behavior of this weird wanderer from interstellar space?

Careful analysis eliminated a range of possible influences--for example, radiation pressure or thermal effects from our Sun, or even interaction with our Sun's solar wind. Less likely scenarios include a collision with another body, or the possibility that Oumuamua is really a duo of separate objects, loosely bound together.

Comets contain ices that can sublimate. This means that they can experience a sea change from a solid to a gas when warmed by our Sun. This process drags out dust from the comet's surface, forming a fuzzy "atmosphere", and sometimes a tail. The release of gas pressure at both different times and locations can push the comet slightly off course compared with the expected path it would take if only gravitational influences were playing a role.

"Thanks to the high quality of the observations we were able to characterize the direction and magnitude of the non-gravitational perturbation, which behaves the same way as comet outgassing," commented Dr. Davide Farnocchia in the June 27, 2018 University of Hawaii Press Release. Dr. Famocchia is of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.

The astronomers have not as yet spotted dusty material or chemical signatures that would normally indicate a comet, even in the deepest images obtained from the European Southern Observatory's (ESO's) Very Large Telescope (VLT), HST, and the Gemini South Telescope. "Oumuamua is a small object--no more than a mile long--and it could have been releasing a small amount of relatively large dust for it to have escaped detection. To really understand Oumuamua we need to send a space probe to it. This is actually possible, but it would be very expensive and take a long time to get there, so it isn't practical this time. We just have to be ready for the next one," commented Dr. Meech in the same University of Hawaii Press Release.

"It was relatively surprising that Oumuamua first appeared as an asteroid--given that we expect interstellar comets should be far more abundant, so we have at least solved that particular puzzle. It is still a tiny and weird object that is not behaving like a typical comet, but our results certainly lean towards it being a comet and not an asteroid after all," explained Dr. Oliver Hainaut to the press on June 27, 2018. Dr. Hainaut is of the ESO.

Because Oumuamua is so small and faint, today's observations do not provide all the information astronomers need to determine important aspects of the comet's surface. Dr. Ken Chambers from Pan-STARRS noted that "When Oumuamua was discovered the astronomy community gathered as much data as possible, but ultimately the object was just not visible long enough to answer all our questions. With Pan-STARRS monitoring the skies, we hope to discover more Oumuamua-like objects in the future and begin to answer the really interesting questions about this class of objects."

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.


What is the angle between the planes of Oumuamua's hyperbola and the Milky Way Galaxy? - Astronomy

Are the orientations of our solar system and others in our galactic disc "in-line" with the disc or are they oriented in all different directions? What determines their orientation?

They're oriented in all different directions. The size of a solar system is so much smaller than the size of the Galaxy, that the Galaxy's structure has no impact on the orientation of a solar system. What determines their orientations is the direction of the angular momentum that the system had when it formed, and that's pretty much random.

Our own solar system is tipped by about 63 degrees with respect to the plane of the galaxy. You can see that on this infrared picture taken by the IRAS satellite. The picture is a little tricky to interpret because, like many maps of the Earth, it's an Aitoff projection, which means that the entire sky has been flattened onto an ellipse. But you should be able to see that the angle between the bright horizontal band (the Milky Way's disk) and the blue haze (dust in the plane of the solar system) crosses at an angle of something like 60 degrees.

This page was last updated June 28, 2015.

About the Author

Christopher Springob

Chris studies the large scale structure of the universe using the peculiar velocities of galaxies. He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.


The Solar System May Have A Second Plane, Caused By The Galaxy's Gravitational Field

All the planets in the Solar System orbit close to a flat plane, as do most asteroids. Comets have a much wider distribution, but astronomers debate whether this is random or if there is a pattern to the cometary distributions. A new theory proposes that comets align with a second plane, a product of the Milky Way's gravitational field.

Observations of proto-star systems and astrophysical models agree that the planets originated in a disk nearly aligned with the Sun’s equator. Since then, Jupiter has acted as a planetary sheepdog, its mighty gravity keeping its smaller counterparts on what is known as the ecliptic. Cometary orbits are much more varied, a product of encounters with objects with enough gravity to change their course. However, Dr. Arika Higuchi of the Japanese University of Occupational and Environmental Health argues such disruption should leave a comet’s aphelion, or furthest point from the Sun, close to the ecliptic.

Yet many comets we see have aphelia nowhere near the ecliptic. Like many before her, Higuchi concluded there are too many comets with non-ecliptic aphelia to be explained by random forces. Higuchi noted that the Milky Way’s gravitational field exerts a force within the Solar System – small compared to that of the Sun and larger planets but omnipresent and possibly capable of influencing cometary orbits. This would particularly apply to those that spend most of their time remote from other forces.

In the Astronomical Journal, Higuchi reveals the presence of what she calls a “second ecliptic” caused by the misalignment between the Solar System’s main ecliptic and the Milky Way’s disk. This plane is at an angle of 120 degrees compared to the main ecliptic plane.

Initially, Higuchi assumes, the second ecliptic would have been empty. However, her modeling shows over time that disrupted comets would start to congregate. Even a passing star can disrupt a comet's orbit, and over billions of years, many have Higuchi, notes. However, without some more lasting influence, what we see should be effectively random, aside from those still on their initial, ecliptic-aligned orbits.

NASA’s Small-Body Database reveals there is indeed two clusters of cometary aphelia, as Higuchi’s work predicts. Some have previously wondered if such a cluster could be an indicator of some unknown object, an even more distant counterpart of Planet X. However, Higuchi argues, the second peak is more consistent with the influence of the galactic gravitational field. Nevertheless, the match is imperfect.

“The sharp peaks are not exactly at the ecliptic or [second] ecliptic planes, but near them,” Higuchi said in a statement. Among comets from the outer edges of the Solar System, approximately equal numbers lie at the peak near the ecliptic and near the second ecliptic, with a much smaller number scattered randomly through the remaining space.

Higuchi believes some other factor has pulled the bulk of cometary aphelia slightly away from either plane and intends to keep searching for what this might be.

All the planets in the Solar System orbit close to a flat plane, as do most asteroids. Comets have a much wider distribution, but astronomers debate whether this is random or if there is a pattern to the cometary distributions. A new theory proposes that comets align with a second plane, a product of the Milky Way's gravitational field.

Observations of proto-star systems and astrophysical models agree that the planets originated in a disk nearly aligned with the Sun’s equator. Since then, Jupiter has acted as a planetary sheepdog, its mighty gravity keeping its smaller counterparts on what is known as the ecliptic. Cometary orbits are much more varied, a product of encounters with objects with enough gravity to change their course. However, Dr. Arika Higuchi of the Japanese University of Occupational and Environmental Health argues such disruption should leave a comet’s aphelion, or furthest point from the Sun, close to the ecliptic.

Yet many comets we see have aphelia nowhere near the ecliptic. Like many before her, Higuchi concluded there are too many comets with non-ecliptic aphelia to be explained by random forces. Higuchi noted that the Milky Way’s gravitational field exerts a force within the Solar System – small compared to that of the Sun and larger planets but omnipresent and possibly capable of influencing cometary orbits. This would particularly apply to those that spend most of their time remote from other forces.

In the Astronomical Journal, Higuchi reveals the presence of what she calls a “second ecliptic” caused by the misalignment between the Solar System’s main ecliptic and the Milky Way’s disk. This plane is at an angle of 120 degrees compared to the main ecliptic plane.

Initially, Higuchi assumes, the second ecliptic would have been empty. However, her modeling shows over time that disrupted comets would start to congregate. Even a passing star can disrupt a comet's orbit, and over billions of years, many have Higuchi, notes. However, without some more lasting influence, what we see should be effectively random, aside from those still on their initial, ecliptic-aligned orbits.

NASA’s Small-Body Database reveals there is indeed two clusters of cometary aphelia, as Higuchi’s work predicts. Some have previously wondered if such a cluster could be an indicator of some unknown object, an even more distant counterpart of Planet X. However, Higuchi argues, the second peak is more consistent with the influence of the galactic gravitational field. Nevertheless, the match is imperfect.

“The sharp peaks are not exactly at the ecliptic or [second] ecliptic planes, but near them,” Higuchi said in a statement. Among comets from the outer edges of the Solar System, approximately equal numbers lie at the peak near the ecliptic and near the second ecliptic, with a much smaller number scattered randomly through the remaining space.

Higuchi believes some other factor has pulled the bulk of cometary aphelia slightly away from either plane and intends to keep searching for what this might be.


What is the angle between the planes of Oumuamua's hyperbola and the Milky Way Galaxy? - Astronomy

Discovery
The first known interstellar asteroid, 1I/2017 U1 ‘Oumuamua, was discovered Oct. 19, 2017 by the University of Hawaii’s Pan-STARRS1 telescope, funded by NASA’s Near-Earth Object Observations (NEOO) Program, which finds and tracks asteroids and comets in Earth’s neighborhood. While originally classified as a comet, observations revealed no signs of cometary activity after it slingshotted past the Sun on Sept. 9, 2017 at a blistering speed of 196,000 miles per hour (87.3 kilometers per second).

Overview
The first confirmed object from another star to visit our solar system, this interstellar interloper appears to be a rocky, cigar-shaped object with a somewhat reddish hue. The asteroid, named ‘Oumuamua by its discoverers, is up to one-quarter mile (400 meters) long and highly-elongated perhaps 10 times as long as it is wide. That aspect ratio is greater than that of any asteroid or comet observed in our solar system to date. While its elongated shape is quite surprising, and unlike asteroids seen in our solar system, it may provide new clues into how other solar systems formed.

The observations suggest this unusual object had been wandering through the Milky Way, unattached to any star system, for hundreds of millions of years before its chance encounter with our star system.

“For decades we’ve theorized that such interstellar objects are out there, and now – for the first time – we have direct evidence they exist,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington, in November 2017.

Immediately after its discovery, telescopes around the world, including ESO’s Very Large Telescope in Chile, were called into action to measure the object’s orbit, brightness and color. Urgency for viewing from ground-based telescopes was vital to get the best data.

Combining the images from the FORS instrument on the ESO telescope using four different filters with those of other large telescopes, a team of astronomers led by Karen Meech of the Institute for Astronomy in Hawaii found that ‘Oumuamua varies in brightness by a factor of 10 as it spins on its axis every 7.3 hours. No known asteroid or comet from our solar system varies so widely in brightness, with such a large ratio between length and width. The most elongated objects we have seen to date are no more than three times longer than they are wide.

“This unusually big variation in brightness means that the object is highly elongated: about ten times as long as it is wide, with a complex, convoluted shape,” said Meech. “We also found that it had a reddish color, similar to objects in the outer solar system, and confirmed that it is completely inert, without the faintest hint of dust around it.”

These properties suggest that ‘Oumuamua is dense, composed of rock and possibly metals, has no water or ice, and that its surface was reddened due to the effects of irradiation from cosmic rays over hundreds of millions of years.

A few large ground-based telescopes continued to track the fading asteroid as it receded from our planet. Two of NASA’s space telescopes (Hubble and Spitzer) tracked the object traveling about 85,700 miles per hour (38.3 kilometers per second) relative to the Sun. Its outbound path is about 20 degrees above the plane of planets that orbit the Sun. The object passed Mars’s orbit around Nov. 1 and will pass Jupiter’s orbit in May of 2018. It will travel beyond Saturn’s orbit in January 2019 as it leaves our solar system, ‘Oumuamua will head for the constellation Pegasus.

Preliminary orbital calculations suggest that the object came from the approximate direction of the bright star Vega, in the northern constellation of Lyra. However, it took so long for the interstellar object to make the journey – even at the speed of about 59,000 miles per hour (26.4 kilometers per second) — that Vega was not near that position when the asteroid was there about 300,000 years ago.

Astronomers estimate that an interstellar asteroid similar to ‘Oumuamua passes through the inner solar system about once per year, but they are faint and hard to spot and have been missed until now. It is only recently that survey telescopes, such as Pan-STARRS1, are powerful enough to have a chance to discover them.

“What a fascinating discovery this is!” said Paul Chodas, manager of the Center for Near-Earth Object Studies at NASA’s Jet Propulsion Laboratory, Pasadena, California. “It’s a strange visitor from a faraway star system, shaped like nothing we’ve ever seen in our own solar system neighborhood.”

How Oumuamua Got its Name
The object was officially named interstellar asteroid 1I/2017 U1 by the International Astronomical Union (IAU), which is responsible for granting official names to bodies in the solar system and beyond. In addition to the technical name, the Pan-STARRS team dubbed it ‘Oumuamua (pronounced oh MOO-uh MOO-uh), which is Hawaiian for “a messenger from afar arriving first.”

This artist’s impression shows the first interstellar asteroid: `Oumuamua. This unique object was discovered on 19 October 2017 by the Pan-STARRS 1 telescope in Hawai`i. Subsequent observations from ESO’s Very Large Telescope in Chile and other observatories around the world show that it was travelling through space for millions of years before its chance encounter with our star system. `Oumuamua seems to be a dark red highly-elongated metallic or rocky object, about 400 metres long, and is unlike anything normally found in the Solar System.

Interstellar Space

Interstellar space is the physical space within a galaxy beyond the influence of each star on the plasma

Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet. As minor planets in the outer Solar System were discovered and found to have volatile-based surfaces that resemble those of comets, they were often distinguished from asteroids of the asteroid belt. In this article, the term “asteroid” refers to the minor planets of the inner Solar System including those co-orbital with Jupiter.

There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun’s solar nebula that never grew large enough to become planets. The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter trojans). However, other orbital families exist with significant populations, including the near-Earth objects. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, and S-type. These were named after and are generally identified with carbon-rich, metallic, and silicate (stony) compositions, respectively. The size of asteroids varies greatly, the largest is almost 1,000 km (625 mi) across.

Asteroids are differentiated from comets and meteoroids. In the case of comets, the difference is one of composition: while asteroids are mainly composed of mineral and rock, comets are composed of dust and ice. In addition, asteroids formed closer to the sun, preventing the development of the aforementioned cometary ice. The difference between asteroids and meteoroids is mainly one of size: meteoroids have a diameter of less than one meter, whereas asteroids have a diameter of greater than one meter. Finally, meteoroids can be composed of either cometary or asteroidal materials.

Interstellar Asteroid FAQs

Why is the discovery of asteroid 1I/2017 U1 significant/important?

The discovery of interstellar object 1I/2017 U1 is the first detection of a celestial object in our solar system that originated from another solar system, and the shape of the object itself looks very different than any asteroid or comet we’ve seen in our own solar system.

Was this a surprise?

Not entirely. The discovery of an interstellar object has been anticipated for decades. While it is a historic discovery, the existence of interstellar objects is not a surprise. What is a surprise, however, is the fact that the first detected interstellar object is an asteroid—not a comet as most scientists had expected. Even more surprising is the strange, highly-elongated shape of 1I/2017 U1. None of the known asteroids or comets in our own solar system have such an elongated shape.

Why was this discovery anticipated by scientists?

Theories of how our own solar system was formed tell us that a large percentage of the original planetesimals in the early solar system were ejected into interstellar space through encounters with the giant planet Jupiter. The asteroids and comets that remain in our solar system today are only a small fraction of the original population: the rest of the material was dispersed by young Jupiter, either spewed out into interstellar space or sent crashing into the Sun. Planetary systems that formed around other stars likely evolved in the same way, with each Jupiter-sized planet ejecting its own systems’ asteroids and comets into interstellar space. The space between the stars probably has billions and billions of planetesimals roaming around independently. Scientists understood that inevitably, some of these small bodies would enter our own solar system. This interstellar visit by 1I/2017 U1 reinforces our models of how planetary systems form.

If this discovery was expected, why are scientists still surprised?

Scientists are surprised because they expected an interstellar comet, not an interstellar asteroid. Their models indicate that the material that our own solar system ejected into interstellar space after formation was mostly cometary. Evidence of that comes from the types of objects seen in the Oort Cloud, a vast spherical cloud around our solar system thought to consist of the planetesimals that were nearly ejected from our solar system, but not quite. The vast majority of objects that dive into the inner solar system from the Oort Cloud are comets, and only a very few are asteroids. The very high ratio of comets to asteroids is thought to also apply to objects ejected from our solar system. Scientists assumed the same high ratio of comets to asteroids also hold for material ejected from other solar systems, but the asteroidal nature of 1I/2017 U1 suggests that that may not be the case.

Why are scientists surprised and excited about the shape of 1I/2017 U1?

Scientists have never seen an asteroid as elongated as 1I/2017 U1 in our solar system—not even half this elongated. The most elongated asteroids we see in our own solar system have axis ratios of no more than 3:1. Scientists will have to come up with new theories explaining how such an elongated object as 1I/2017 U1 could form, and how it could have enough strength to hold itself together in one long piece. It is quite possible that entirely different conditions around this object’s parent star gave this object a composition and shape that are not possible in our own solar system.

Are you really sure this object came from outside our solar system?

Yes. The trajectory of this object has been tracked carefully since it was discovered, and its motion follows a hyperbolic path around the Sun. Basically, the high speed at which 1I/2017 U1 is traveling through the solar system cannot be due to acceleration from the Sun’s gravity alone. This object must have approached our solar system already with considerable initial speed. It is simply traveling too fast to have originated in our solar system. The object’s high speed also means that the Sun’s gravity cannot slow it down enough to keep it bound to our solar system. The object will leave, and end up with about the same speed with which it entered only its direction will have changed.

What do we know about the object’s size and shape?

The object is believed to be at least a quarter-mile (400 meters) long, but it’s the highly-elongated shape that has scientists excited and perplexed. The shape is revealed by high-precision measurements of the object’s brightness. The dramatic cyclical variations in brightness indicate that the object is cigar-shaped, with a length roughly ten times longer than the width. The asteroid spins once every 7.3 hours. Think of a pencil spinning on a table that cycles from being brightest when it its full length is visible, to faintest when its long axis or point is facing toward us. No known asteroid or comet from our solar system varies so widely in brightness, and therefore has such a large ratio between length and width. The most elongated objects we have seen up to now have been no more than three times longer than they are wide. The 10:1 elongation ratio is simply not found for any of the objects within our solar system.

What do we know about the object’s composition?

Regarding its composition, observations suggest this object is similar to many asteroids found in our solar system – dense, possibly rocky or even metallic. The object’s surface is somewhat reddish due to effects of irradiation from cosmic rays over millions of years.

How do we know this is a natural object?

From our theories of planetary formation, we expect that as each planetary system forms, it ejects countless small asteroids and comets into interstellar space. We are quite sure that our own solar system did that, and other star systems must have done the same. We think this object is a product of that process in some faraway star system. While its elongated shape is quite surprising, and unlike asteroids seen in our solar system, it may provide us with new clues into how other solar systems formed – some that could be quite different from our own. For the unique insights it might provide, we are excited about studying 1I/2017 U1 further, until it gets too far away for observations.

How fast was this object going when it was nearest the Sun?

The object was traveling fastest when it was nearest the Sun on Sep. 9, 2017. Its peak speed was about 196,000 miles per hour or 87.4 kilometers per second. After that, as it headed away from the Sun, the Sun’s gravity started to slow it down somewhat.

Will it ever return to our solar system?

No, the object is now headed away from the Sun, outbound back into interstellar space, never to return.

Where is this object now (Nov. 20, 2017) and how fast is it going at this time?

The object is currently about 183 million miles (295 million kilometers) from the Sun, traveling outbound above the region between the orbits of Mars and Jupiter at about 89,000 miles per hour, or 40 kilometers per second. It is currently about 124 million miles (200 million kilometers) from Earth.

How fast did this object travel through interstellar space?

This object’s cruising velocity through interstellar space was 59,000 miles per hour (26.3 kilometers per second). That was its speed as it approached our solar system, and the speed it will have after it exits our solar system. At that speed, interstellar object 1I/2017 U1 will cover one light year in about 11,000 years.

It is interesting to note that 1I/2017 U1 is traveling faster than any spacecraft ever launched – including the two Voyagers and New Horizons.

When was 1I/2017 U1 discovered?

The object was discovered on Oct. 19, 2017 by the NASA-funded Pan-STARRS1 telescope which surveys the sky searching for asteroids that could pose an impact hazard to Earth. Rob Weryk, a post-doc researcher at the University of Hawaii’s Institute of Astronomy, was first to identify the object.

Interstellar object 1I/2017 U1 has another name, doesn’t it?

The team from the Pan-STARRS observatory that was the first to detect the interstellar visitor has chosen the name ‘Oumuamua for their discovery. The name is of Hawaiian origin and means “a messenger from afar arriving first.”

When did you realize it was interstellar?

Astronomers on the University of Hawaii team and elsewhere tracked interstellar object 1I/2017 U1 for several days after its discovery before its trajectory was known well enough to confirm that it was, indeed, interstellar in origin. The official confirmation and announcement of this object was made on Oct. 25.

Where did it come from?

1I/2017 U1’s trajectory indicates it came from the general direction of the constellation Lyra. At the speed it was going, it must have taken at least a few hundred thousand years since it was near even the nearest star in that direction, and it may well have been traveling much longer than that. Scientists don’t know the motions of the stars well enough to say where they were located that long ago. We simply don’t know which star system this object came from.

Where is it going?

Interstellar object 1I/2017 U1 is on an outbound trajectory. It will pass above Neptune’s orbit in 2022. As it leaves our solar system it is headed towards the constellation Pegasus.

What was its path through our solar system?

This object approached our solar system from above the plane of the ecliptic. It passed closest to the Sun on Sept. 9, 2017, well within Mercury’s orbit. Pulled by the Sun’s gravity, the object made a sharp turn under our solar system, and then started its outbound leg, passing under Earth’s orbit on Oct. 14 at a distance of about 15 million miles (24 million kilometers). It passed above Mars’ orbit around Nov. 1, and will pass above Jupiter’s orbit in May of 2018. 1I/2017 U1 will pass above Saturn’s orbit in January of 2019. At present, it is traveling about 85,000 miles per hour (138,000 kilometers per hour) relative to the Sun. On this outbound leg, the object is once again traveling above the ecliptic plane, but it is moving at a much shallower angle than on its inbound leg.

What is NASA doing now?

A few large ground-based telescopes continue to track this object, even though it is rapidly fading as it recedes from our planet. Two of NASA’s space telescopes (Hubble and Spitzer) are attempting to observe the fading object the week of Nov. 20. Observations from large ground-based telescopes will continue until the object becomes too faint to be detected, sometime next month. NASA’s Center for Near-Earth Object Studies (CNEOS) at the Jet Propulsion Laboratory continues to take all available tracking measurements to refine the trajectory of 1I/2017 U1 until it becomes too faint to observe on its journey out of our solar system.

How long can this interstellar object be observed?

Interstellar object 1I/20167 U1 will be too far and too faint to be detected even in the largest telescopes after mid-December.

If interstellar space is littered with billions and billions of asteroids and comets, why have we never seen one before?

Scientists think that interstellar objects pass through our solar system all the time, but most are too small and too far away from Earth to be detected. 1I/2017 U1 was large enough and by chance passed close enough to Earth to be detected by Pan-STARRS. The discovery may also have been made possible by enhancements to the detection capabilities of the Pan-STARRS near-Earth object search program in recent years.

Will we find more interstellar objects in the future, and if so, how often?

Yes, scientists expect to find more interstellar objects, especially when next-generation asteroid search programs come online. They estimate that an interstellar object similar to 1I/2017 U1 passes inside the orbit of the Earth several times a year, but up until now they have been too faint and hard to detect. Recent upgrades to survey telescopes such as Pan-STARRS increase the chances of finding these objects, and those odds will increase even more when next-generation survey telescopes begin operations.


Little Galaxies

Little Galaxies is a weekly meeting where members of CCAPP, OSU Astronomy, and OSU Physics come together to discuss papers and recent developments in astronomy and astrophysics relating to many aspects of "little galaxies", in an informal setting. Please check the page for details regarding the next meeting.

Email Annika (peter.33 [at] osu.edu) for a Zoom link if you would like to join us!

Weekly on Mondays 3:00pm-4:00pm in the Price Place (PRB M2005) via Zoom

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Special guest: Devontae Baxter

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NO Meeting 3:00 pm, Monday, January 18, 2021 -- Martin Luther King Jr Day

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3:00 pm, Monday, December 14, 2020 on Zoom (last meeting in 2020)


Cometary Alignments and the Galactic Tide

A second ecliptic? What an interesting notion, referred to in a new paper from Arika Higuchi as an ‘empty ecliptic,’ constituting a second alignment plane for the Solar System. This is lively stuff, examined in a new paper in the Astronomical Journal that focuses on the aphelia of long-period comets, the points where they are farthest from the Sun in their orbit. The solutions arrived at through the paper’s dense mathematics show that the aphelia fall close to one or the other of the ecliptic planes, and offer insights into comet formation.

Higuchi (University of Occupational and Environmental Health, Japan) has previously been a part of the National Astronomical Observatory of Japan’s RISE project, RISE standing for Research of Interior Structure and Evolution of solar system bodies. Her work on the orbital evolution of planetesimals goes back at least to 2007 in a paper on the formation of the Oort Cloud, considering the effects of interactions with the ‘galactic tide,’ a reference to the influence of the gravitational field of the galaxy on Solar System objects analyzed through the equations governing orbital motion. The new paper is an extension of the 2007 work, one that derives “the analytical solutions to the Galactic longitude and latitude of the direction of aphelion.”

We know that long-period comets are not confined to the ecliptic, but models of the Solar System’s formation suggest that they formed on the ecliptic and were subsequently scattered through gravitational interactions into the orbits we see today. What Higuchi finds is that even given scattering interactions with the gas giant planets, the cometary aphelia should remain near the ecliptic. That they do not necessarily do so calls for an explanation that the author finds in the influence of the Milky Way’s gravitational field. It turns out that the aphelia of long-period comets, when this influence is taken into account, tend to collect around two planes.

The first is, as you would expect, the ecliptic with which we are all familiar. The second is the ‘empty ecliptic,’ a plane inclined with respect to the disk of the Milky Way by about 60 degrees, just as the ecliptic itself is inclined by 60 degrees, but in the opposite direction. Here we’re cautioned to be careful with the label, because the empty ecliptic is ‘empty’ only in the early days of the system. Over time, it comes to be populated with scattered comets, a population that has implications for how we go about finding long-period comets in the future.

In the passage from the paper that follows, L and B refer to the galactic longitude and latitude of the direction of aphelion. q refers to the perihelion distance, while i refers to inclination with respect to the ecliptic plane:

The concentration of long-period comets from the Oort cloud on the ecliptic and empty ecliptic planes is an observational evidence that the Oort cloud comets were planetesimals initially on the ecliptic plane. We expect the concentrations even when we consider the effect of passing stars. Perturbations from passing stars change the conserved quantities and may break the relation between q, B, and L more or less however, it takes a much longer time to change the eccentricity vector (i.e., L and B) than to change i (Higuchi & Kokubo 2015). Therefore, we suggest that observers, including the space mission Comet Interceptor, focus on the ecliptic plane and/or the empty ecliptic plane to find dynamically new comets.

Image: Artist’s impression of the distribution of long-period comets. The converging lines represent the paths of the comets. The ecliptic plane is shown in yellow and the empty ecliptic is shown in blue. The background grid represents the plane of the Galactic disk. (Credit: NAOJ).

Higuchi cross-checked her mathematical results against numerical computations run largely at NAOJ’s PC Cluster at the Center for Computational Astrophysics. She is able to show that the analytical and computational results she derives square with the data for long-period comets listed in NASA’s Small Body Database at JPL, identifying the two peaks expected near the ecliptic and the empty ecliptic. This favors the hypothesis that long-period comets originally formed on the ecliptic. We can use upcoming surveys to refine these results. Says the author:

“The sharp peaks are not exactly at the ecliptic or empty ecliptic planes, but near them. An investigation of the distribution of observed small bodies has to include many factors. Detailed examination of the distribution of long-period comets will be our future work. The all-sky survey project known as the Legacy Survey of Space and Time (LSST) will provide valuable information for this study.”

What an interesting result. We consider long-period comets from the Oort Cloud as forming on the ecliptic plane just as the planets did, but now we move to the view that their orbital evolution must be examined not just in terms of interactions with large objects in the early system, but also with the gravitational tide of the galaxy. You’ll recall that the aphelia of various objects in the Solar System have been considered in terms of possible perturbers within the system, including hitherto undiscovered planets. Their potential for unlocking long-period comet distribution in useful ways is one I had never considered until I ran into Higuchi’s work this morning.

The paper is Higuchi, “Anisotropy of Long-period Comets Explained by Their Formation Process,” Astronomical Journal Vol. 160, No. 3 (26 August 2020). Abstract / preprint. The 2007 paper is Higuchi et al., “Orbital Evolution of Planetesimals due to the Galactic Tide: Formation of the Comet Cloud,” Astronomical Journal Vol. 134, No. 4 (29 August 2020). Abstract.

Comments on this entry are closed.

Very interesting article but after reading it and looking at the charting, there is one aspect that would heavily influence the comets. The other stars that pass thru the Oort cloud, a good charting of how and when the nearby suns pass thru the Oort cloud may show that such alignment planes would be destroyed over the eons.

Another paper presents a important possibility that we may not need to go chasing after interstellar objects passing thru the solar system. They may have already been captured by the Sun!

Capture of interstellar objects: a source of long-period comets.

“We simulate the passage through the Sun-Jupiter system of interstellar objects (ISOs) similar to 1I/‘Oumuamua or 2I/Borisov. Capture of such objects is rare and overwhelmingly from low incoming speeds onto orbits akin to those of known long-period comets. This suggests that some of these comets could be of extra-solar origin, in particular inactive ones.
Assuming ISOs follow the local stellar velocity distribution, we infer a volume capture rate of 0.051 au3 yr−1.
Current estimates for orbital lifetimes and space densities then imply steadystate captured populations of ∼ 10 2 comets and ∼ 10 5 ‘Oumuamua-like rocks, of which 0.033% are within 6 au at any time.

Complex math indeed. I hope that this result is not an artifact of the methodology.

Regarding the result. A quick read of the Wikipedia entry for the Oort cloud suggests that there is an inner torous with a boundary perhaps 20,000 AU out, and then the more spherical outer cloud from 20,000 AU outwards.

The analysis is for long period comets that originate in one or other of these clouds. Does the origin of the comet from either of these clouds matter? Why is there both an ecliptic peak and an anti-ecliptic peak (180 degrees offset)? Is this some gravitational effect of the galactic plane?

Figure 7 is unnecessarily confusing. It lumps together all bodies with varying eccentricities which seems to obscure the analysis. Just eyeballing this graphic does not give one a sense that the ecliptic and empty-ecliptic represent the peaks in distributions.

Would the effects of galactic gravitation vary with the sun’s orbit through the Milky Way? Does the primary ecliptic have an orientation in space beyond the Milky Way (which would configure the orbit of Mercury to disdant galaxies) or would the primary galaactic plane be “tidally locked” to the galactic center (with the orbit of Mercury configured to the galactic center)?

This paper separates the astronomers from the dilettantes … pity I’m on the wrong side. Everything goes pretty well right up to …
“For some cases, equi-Hamiltonian curves circulate with ω and for other cases they librate around ω = 90◦ or 270◦. This libration is essentially the von Zeipel-Lidov-Kozai mechanism”
I found an article ( https://arxiv.org/abs/1911.03984 ), and there’s even a Wikipedia entry. I understand the orbital parameters ω and Ω can be scrambled as a comet shoots almost straight at the Sun, with slight jostling from bystanders. But how this leads to the aphelion of a comet being reflected through the Galactic plane? That could use some explanation.

Tunguska explosion in 1908 caused by asteroid grazing Earth

A new theory explains the mysterious explosion in Siberia, scientists say, suggesting Earth barely escaped a far greater catastrophe.


What is the angle between the planes of Oumuamua's hyperbola and the Milky Way Galaxy? - Astronomy

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