Astronomy

Which focus point does the sun occupy for each planet?

Which focus point does the sun occupy for each planet?


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I am aware that planets orbit in an elliptical fashion and that the sun occupies one of two focal points.

Let us say that the left focal point is f1 and the right f2.

Is the sun at the same focal point for each planet?

If not which ones are different?


I am aware that planets orbit in an elliptical fashion

Yes, or at least to a good approximation. Because the planets affect each other's motion to a small extent (and also because of relativistic effects) the motion is not quite elliptical.

and that the sun occupies one of two focal points.

Again, because of the complexities of the gravitational interactions of all the bodies, the Sun does not occupy a single position, but has a complex motion.

You need to read about Barycenter and have a look at this explanation of the complex motions this results in.

And in reality this all happens in more than two dimensions - the orbits are inclined at different angles as well.

Let us say that the left focal point is f1 and the right f2.

An ellipse does indeed have two foci, but the orbits of planets and the motion of the Sun are not quite ellipses anyway.

But even if we ignore these small deviations from an ellipse, each planet's orbit is in a different orientation and has different foci.

Is the sun at the same focal point for each planet?

So "no".


Ignoring inclination and by simplifying the orbits into Kepler's ellipses, you still couldn't determine left and right because the relation would be relative to a 360 degree plane, like hands on a clock.

Using this simplified approach, the Perihelion, the Sun, the center of the ellipse, the other Foci and the Aphelion are all in a straight line along the major axis. The Sun and the Perihelion point in the same direction relative to the "fixed" stars.

So, another way to ask your question is to ask where and at what angle do the planets perihelion happen and this is a little bit easier to look up because the perihelion is a real event, unlike the foci which a largely irrelevant mathematical representation. Each planet passes through it's perihelion once every orbit. It's not like clockwork, as there's variation from the other planets, but it's roughly in the same place, once every orbital year.

I would think a table of planet's perihelion would be fairly easy to look up in a table or chart, but surprisingly, I didn't see any, but individual planets can be found.

Earth is at Perihelion usually in the first week of January.

Mercury was at perihelion (according to this website) on March 23, 2017 and about every 88 days after that (about June 19 and Sept 15, 2017)

Using NASA-Eyes on the Solar System, a ballpark estimate puts Earth and Mercury's Perihelion are some 30-40 degrees apart, and Venus will be at Perihelion on Oct 3, 2017, and it's perihelion direction relative to the Sun is even closer, between Earth's and Mercuries, but that pattern ends with Mars. Mars' next Perihelion will be on September 16, 2018. it appears to be nearly 180 degrees opposite Mercury.

Jupiter is in a different direction as well. Directions can be anywhere on the 360 degree orbital plane (ignoring inclination).

It's worth noting that over long periods of time (or not that long for Mercury), The planet's perihelion moves. Earth's moves around a full circle in roughly 112,000 years. This is called Apsidal precession.

It's also worth noting that if you do apply Kepler ellipses, the location of the focal point relative to the center of the ellipse is a product of eccentricity and distance. The focal point and center of the ellipse are just mathematical points in empty space anyway. They have no real use, but if you calculated about where each planet's focal point is relative to the sun, they'd not only be in different directions relative to the sun, but they'd all be different distances from the sun too and with difference inclination. That would be a kind of fun, but entirely useless mathematical exercise if you wanted to do it.

I want to stress that none of this is astronomically useful. It's not relevant to observation or location of planets because Kepler's laws, while a brilliant leap forward at the time and accurate enough to overcome the geocentric model, they are incomplete and so, the foci are a product of a roughly accurate but incomplete model.


In addition to the other answers, that are right, a different doubt seem to arise from the wording of the question:

Let us say that the left focal point is f1 and the right f2.

Is the sun at the same focal point for each planet?

If the question is if the Sun is in f1 or f2, the answer is that it doesn't matter. Ellipses are symmetrical, and therefore both foci are equivalent and any naming of them is just conventional. Just by saying that planets move in elliptical orbits and the Sun occupies one focus, Kepler's first law is unambiguously established.

If you need to mention a particular focus, one is just the Sun position and the other one is often referred as the empty focus.


Motion in the Heavens: Stars, Sun, Moon, Planets

The purpose of this lecture is just to review the various motions observed in the heavens in the simplest, most straightforward way. We shall ignore for the moment refinements like tiny deviations from simple motion, but return to them later.

It is illuminating to see how these observed motions were understood in early times, and how we see them now. Of course, you know the Earth rotates and orbits around the Sun. However, I want you to be bilingual for this session: to be able to visualize also the ancient view of a fixed Earth, and rotating heavens, and be able to think from both points of view.

This is really largely an exercise in three-dimensional visualization--that's the hard part! But without some effort to see the big picture, you will not be able to appreciate some really nice things, like the phases of the moon, eclipses, and even just the seasons. You really need to have a clear picture of the Earth orbiting around the Sun and at the same time rotating about an axis tilted relative to the plane the orbit lies in, with the axis of rotation always pointing at the same star, and not changing its direction as the Earth goes around the Sun. Then you must add to your picture the Moon orbiting around the Earth once a month, the plane of its orbit tilted five degrees from the plane of the Earth's orbit around the Sun. Then we add in the planets.

Some of these topics are treated nicely in Theories of the World from Antiquity to the Copernican Revolution, by Michael J. Crowe, Dover.


What is SOHO's orbit?

SOHO is in orbit between the Earth and the Sun. It is about 150,703,456 kilometers (92 million miles) from the Sun and only about 1,528,483 Kilometers (1 million miles) from the Earth (three times farther than the moon). This orbit is around a mathematical point between the Earth and the Sun known as the Lagrange point or the L1 point. The L1 point is a point of equilibrium between the Earth's and Sun's gravitational field, that is to say that the pull is equal from both the Sun and the Earth. The L1 point is a point of unstable equilibrium (like a bowl round side up with a marble balanced on it). As a result, we have to compensate for perturbations due to the pull of the planets and the Earth's moon. Every few months we use a little fuel to fine tune our orbit and keep it from getting too far off track. This is known as "station keeping manoeuvres"

No spacecraft is actually orbiting at the L1 point. For SOHO there are two main reasons: the unstable orbit at the L1 point and facility of communication in a halo orbit. If SOHO was sitting directly at the L1 point, it would always be right in front of the Sun. The trouble is that the Sun is very noisy at radio wavelengths, which would make it very difficult to tune into the radio telemetry from the spacecraft. By putting it into a halo orbit, we can place it so that it's always a few degrees away from the Sun, making radio reception much easier.

Check out this website for more on SOHO's orbit.

Bowl and marble demo - stable and unstable equilibria


How far are the planets from the Sun?

Artist’s impression of the planets in our solar system, along with the Sun (at bottom). Credit: NASA

The eight planets in our solar system each occupy their own orbits around the Sun. They orbit the star in ellipses, which means their distance to the sun varies depending on where they are in their orbits. When they get closest to the Sun, it's called perihelion, and when it's farthest away, it's called aphelion.

So to talk about how far the planets are from the sun is a difficult question, not only because their distances constantly change, but also because the spans are so immense—making it hard for a human to grasp. For this reason, astronomers often use a term called astronomical unit, representing the distance from the Earth to the Sun.

The table below (first created by Universe Today founder Fraser Cain in 2008) shows all the planets and their distance to the Sun, as well as how close these planets get to Earth.

  • Closest: 46 million km / 29 million miles (.307 AU)
  • Furthest: 70 million km / 43 million miles (.466 AU)
  • Average: 57 million km / 35 million miles (.387 AU)
  • Closest to Mercury from Earth: 77.3 million km / 48 million miles
  • Closest: 107 million km / 66 million miles (.718 AU)
  • Furthest: 109 million km / 68 million miles (.728 AU)
  • Average: 108 million km / 67 million miles (.722 AU)
  • Closest to Venus from Earth: 40 million km / 25 million miles
  • Closest: 147 million km / 91 million miles (.98 AU)
  • Furthest: 152 million km / 94 million miles (1.1 AU)
  • Average: 150 million km / 93 million miles (1 AU)
From the Solar Dynamics Observatory: Planet Venus transiting the Sun in the 304 Anstrom wavelength at approx. 90,000 degrees Fahrenheit in July 2012. Credit: NASA/SDO
  • Closest: 205 million km / 127 million miles (1.38 AU)
  • Furthest: 249 million km / 155 million miles (1.66 AU)
  • Average: 228 million km / 142 million miles (1.52 AU)
  • Closest to Mars from Earth: 55 million km / 34 million miles
  • Closest: 741 million km /460 million miles (4.95 AU)
  • Furthest: 817 million km / 508 million miles (5.46 AU)
  • Average: 779 million km / 484 million miles (5.20 AU)
  • Closest to Jupiter from Earth: 588 million km / 346 million miles
  • Closest: 1.35 billion km / 839 million miles (9.05 AU)
  • Furthest: 1.51 billion km / 938 million miles (10.12 AU)
  • Average: 1.43 billion km / 889 million miles (9.58 AU)
  • Closest to Saturn from Earth: 1.2 billion km /746 million miles
  • Closest: 2.75 billion km / 1.71 billion miles (18.4 AU)
  • Furthest: 3.00 billion km / 1.86 billion miles (20.1 AU)
  • Average: 2.88 billion km / 1.79 billion miles (19.2 AU)
  • Closest to Uranus from Earth: 2.57 billion km / 1.6 billion miles
The “pale blue dot” of Earth as seen from Cassini on July 19, 2013. Credit: NASA / JPL-Caltech / Space Science Institute
  • Closest: 4.45 billion km /2.77 billion miles (29.8 AU)
  • Furthest: 4.55 billion km / 2.83 billion miles (30.4 AU)
  • Average: 4.50 billion km / 2.8 billion miles (30.1 AU)
  • Closest to Neptune from Earth: 4.3 billion km / 2.7 billion miles

As a special bonus, we'll include Pluto too, even though Pluto is not a planet anymore.

Artist’s impression of New Horizons’ encounter with Pluto and Charon. Credit: NASA/Thierry Lombry
  • Closest: 4.44 billion km / 2.76 billion miles (29.7 AU)
  • Furthest: 7.38 billion km / 4.59 billion miles (49.3 AU)
  • Average: 5.91 billion km / 3.67 billion miles (39.5 AU)
  • Closest to Pluto from Earth: 4.28 billion km / 2.66 billion miles

Many cities and countries have also installed scale models of the Solar System, such as:


Which focus point does the sun occupy for each planet? - Astronomy

Is it true that, as we follow the planets outward from the sun, the distances become about double each time? Does that mean that Venus is closer to Earth than Mars is?

Yes, it is true that there is somewhat of a pattern to the distances of the planets from the Sun. Venus is 1.8 times as far from the Sun as Mercury, and Earth is about 1.4 times as far from the sun as Venus. Mars is 1.5 times farther than Earth. This seems to be a pattern - each planet could be between 1.4 and 1.8 times farther from the sun than its "inside" neighbor. Then comes the problem - Jupiter is 3.4 times farther from the sun than Mars. This is where the pattern falls apart, although some say that the asteroid belt, which is in between Jupiter and Mars, could count as a substitute for a planet. Then Saturn is 1.8 times farther than Jupiter, Uranus is 2 times farther than Saturn, and Neptune is 1.6 times farther from the Sun than Uranus. Pluto doesn't fit this pattern at all. So there seems to be some sort of pattern to this, but there's no real theory that explains why the planets ended up at the distances they did, so it could also be a complete coincidence that they're somewhat evenly spaced.

So the "doubling" rule does work, but only approximately. This means that yes, the difference between the average orbital distance of Mars from the Sun to the average orbital distance of Earth from the Sun is greater (about 78 million km) than the difference between the Earth's average orbital distance from the Sun to Venus' average orbital distance from the Sun (41 million km). However, since the distance between the Earth and other planets depends not only on the size of their orbits but also on where they are in their orbits relative to each other, Venus is not always closer to Earth than Mars is.

This page was last updated on July 18, 2015.

About the Author

Cathy Jordan

Cathy got her Bachelors degree from Cornell in May 2003 and her Masters of Education in May 2005. She did research studying the wind patterns on Jupiter while at Cornell. She is now an 8th grade Earth Sciences teacher in Natick, MA.


Questions About Planets & Dwarf Planets

The Ask an Astronomer team's favorite links about Planets:

    - The history and current meaning of the term "planet".
  • The Nine Eight Planets: An excellent resource for information about each planet.
  • NASA's Planetary Photojournal has all the best images of planets.
  • Weekly Planet Roundup: Find out what planets are visible in the night sky, as part of Sky and Telescope Magazine'sSky at a Glance feature.
  • Celestia: A free, easy to use, downloadable 3D simulation of the solar system and nearby stars. Point and click to travel to any planet, moon, satellite or star. Awesome!
  • If you are interested in detailed calculations of the positions of the planets, please see: NASA/JPL Solar System Dynamics (particularly Keplerian Elements for Approximate Positions of the Major Planets and HORIZONS), the NREL Solar Position Algorithm, or How to Compute Planetary Positions.

How to ask a question?

If you have a question about another area of astronomy, find the topic you're interested in from the archive on the side bar or search using the below search form. If you still can't find what you are looking for, submit your question here.


Which focus point does the sun occupy for each planet? - Astronomy

We see angular motions on the sky, not 3-d motions.

For this lecture, we will focus on empirical description of how celestial objects move on the sky. Leave interpretation in terms of true 3-d positions and motions until later.

DAILY MOTION OF THE STARS

Constellations: Patterns of stars on the sky, help to identify particular stars. Not true 3-d groupings.

  • One bright star, Polaris, doesn't move. A.k.a. north star.
  • Other stars appear to move in perfectly circular arcs.
  • Circumpolar stars circle entirely above horizon, centered on Polaris.
  • Other northern stars circle partly below horizon. Rise in east, set in west.
  • Southern stars circle south pole instead of north pole. From northern hemisphere, mostly or entirely below horizon.
  • Height of a given star above horizon depends on observer's latitude on Earth.
  • Paths of stars depend on observer's latitude.

YEARLY MOTION OF THE STARS

  • Stars complete daily circle in 23 hours, 56 minutes.
  • After 24 hours (1 day), move 4 minutes into next circle.
  • 4 minutes per day = (1/15 hour)/(24 hour)

  • Stars fixed to a "celestial sphere," a giant imaginary sphere that encircles the Earth.
  • Celestial sphere rotates once per 23h56m.
  • Only see stars at night, so see different parts of sphere at different times of year.
  • (We now know that) this model is not physically accurate, but it is still useful as a description of celestial motions.
  • Positions of stars on celestial sphere can be described by coordinates called right ascension and declination, analogous to longitude and latitude.

USING THE STARS

  • Simple calendar: tell time of year from positions of stars at Sunset.
  • Night time clock: stars move at 15 degrees per hour.
  • Compass: Polaris always north.
  • More general navigation tool: Height of stars above horizon depends on observer's latitude. E.g., Polaris is overhead at north pole, near horizon at equator.

MOTION OF THE SUN

  • Rises in east, sets in west.
  • Moves in circle centered on north or south pole.
  • Daily cycle takes 24 hours, not 23h56m.
  • Thus, position against background stars changes over time.
  • Also, height of Sun above horizon changes with season.
  • Summer: day is long, Sun high in sky at noon. Path like northern star.
  • Winter: day is short, Sun low in sky at noon. Path like southern star.
  • Sun moves nearly with celestial sphere, but position on sphere changes over year.
  • Moves along a great circle called the ecliptic, inclined 23.5 degrees relative to equator.
  • Constellations along ecliptic are called the "Zodiac" signs. Constellation opposite Sun depends on time of year.
  • Behaves like northern star half the year, like southern star the other half.
  • Completes circle every 365.24 days. One year is almost, but not quite, 365 days.

MOTION OF THE MOON

  • Completes circuit once a month, not once a year.
  • Specifically: returns to same position against stars every 27.3 days.
  • Slightly different path, inclined 5 degrees to ecliptic.
  • Goes through phases: new, 1st quarter, full, 3rd quarter, new.

MOTION OF PLANETS

To naked eye, planets look like stars, but they move around in the sky.
Greeks called them "wandering stars" (asterai planetai).


The Sun in Astrology, The Zodiac

The Sun, the giver of life, represents our conscious mind in Astrology. It represents our will to live and our creative life force.

Just as the planets revolve around the Sun in our solar system, we derive our life purpose from the Sun in our natal charts. The Sun is our ego. It is also our “adult”– the part of us that censors our “inner child,” that reasons things out, and makes final decisions. The Sun is our basic identity, representing self-realization.

When you are asked, “Who are you?,” and you’ve passed your basic statistics and occupation, your answers will likely embody a description of your Sun. The Sun also represents our overall vitality. The Sun directs us and can be considered “the boss” of our chart.

The Sun () is so important in the chart, that the happiest people on this earth are those who identify (without over-identifying) with the Sun’s expression. Though one might think that the traits of their Sun would come easily to them, the truth is, the Sun shows what we are learning to be. It is very important to remember that the Sun represents reason as opposed to instinct.

With respect to the other luminary (the Moon), the Sun reflects the present or the “here and now,” while the Moon infuses the past into our lives through the feelings.

Grant Lewi referred to the Sun as indicative of “the psychological bias which will dominate your actions.” He went on to say, “You may think, dream, imagine, hope to be a thousand things, according to your Moon and your other planets: but the Sun is what you are, and to be your best self in terms of your Sun is to cause your energies to work along the path in which they will have maximum help from planetary vibrations.” (1)

When we are “acting out” our Sun, we are purposeful, directed, proud, and creative. On the negative side, we can be haughty, overly willful, self-centered, and judgmental.

In the chart, the position of the Sun by zodiac sign represents the native’s life purpose and the style in which they leave their mark in the world.

By house, the Sun’s position shows where our personalities shine. The areas of life associated with that house reveal the types of experiences that contribute to our sense of individuality and that shape our sense of pride. These areas of life are ones in which we seek to express and focus our Sun sign qualities.

Note: When interpreting your chart, you will be particularly interested in both the Sun’s sign and the Sun’s house. For example, someone may have the Sun in Aries in the 2nd house, and if so, they would read both the interpretation for the Sun in Aries and the interpretation of the Sun in the second house. Another person may have the Sun in Aries in the 8th house and would read interpretations of the Sun in Aries and the Sun in the eighth house. While these two individuals share a Sun sign, they express their Aries Sun in different ways.

Take a closer look at the Sun’s glyph or symbol. It shows the circle of Spirit, indicating potentiality, brought to focus in the central point or dot. We may also see it as a symbol of wholeness or our inner center.


Sun in zodiac constellations, 2018

Ophiuchus the Serpent Bearer is not an astrological sign, but it is one of the 13 constellations of the zodiac. In 2018, the sun crosses into Ophiuchus on November 30. Image via www.ianridpath.com.

You might know that the real sun in the real sky does not appear in front of a constellation of the zodiac within the same range of dates you’ll see listed in astrological horoscopes. That’s because astrology and astronomy are different systems. Astrologers typically indicate the sun’s position with signs while astronomers use constellations. We were asked for:

… a list of the constellations that fall on the ecliptic with the exact degrees.

And we’ve located this information in Guy Ottewell’s Timetable of astronomical events. Below, you’ll find the dates for the sun’s entry into each zodiacal constellation during the year 2018, plus the sun’s ecliptic longitude – its position east of the March equinox point on the ecliptic – for each given date.

We are using the boundaries for the zodiacal constellations established by the International Astronomical Union in the 1930s.

The sun resides at a longitude of 0 o on the ecliptic at the March equinox. The sun is at 90 o ecliptic longitude at the June solstice, 180 o ecliptic longitude at the September equinox and 270 o ecliptic longitude on the December solstice. Image via Wikipedia

Date of sun’s entry into each zodiacal constellation (and corresponding ecliptic longitude):

Dec 18, 2017: Sun enters constellation Sagittarius (266.59 o )

Jan 19, 2018: Sun enters constellation Capricornus (299.71 o )

Feb 16, 2018: Sun enters constellation Aquarius (327.88 o )

Mar 12, 2018: Sun enters constellation Pisces (351.57 o )

Apr 19, 2018: Sun enters constellation Aries (29.08 o )

May 14, 2018: Sun enters constellation Taurus (53.46 o )

Jun 21, 2018: Sun enters constellation Gemini (90.43 o )

Jul 21, 2018: Sun enters constellation Cancer (118.25 o )

Aug 10, 2018: Sun enters constellation Leo (138.18 o )

Sep 17, 2018: Sun enters constellation Virgo (174.15 o )

Oct 31, 2018: Sun enters constellation Libra (217.80 o )

Nov 23, 2018: Sun enters constellation Scorpius (241.14 o )

Nov 30, 2018: Sun enters constellation Ophiuchus (248.03 o )

Dec 18, 2018: Sun enters constellation Sagittarius (266.60 o )

Earth-centered ecliptic coordinates as seen from outside the celestial sphere. Ecliptic longitude (red) is measured along the ecliptic from the vernal equinox at 0 o longitude. Ecliptic latitude (yellow) is measured perpendicular to the ecliptic. Image via Wikimedia Commons

Constellations of the zodiac:

Dates of sun’s entry into astrological signs versus astronomical constellations. Chart and more explanation at Guy’s Ottewell’s blog. Used with permission.

Bottom line: Sun-entry dates to zodiac constellations in 2018, using boundaries for constellations set by International Astronomical Union in 1930s.


Solar System

Earth's solar system is comprised of the Sun , nine major planets, some 100,000 asteroids larger than 0.6 mi (1 km) in diameter, and perhaps 1 trillion cometary nuclei. While the major planets lie within 40 Astronomical Units (AU) — the average distance of Earth to the Sun — the outermost boundary of the solar system stretches to 1 million AU, one-third the way to the nearest star. Cosmologists and Astronomers assert that the solar system was formed through the collapse of a spinning cloud of interstellar gas and dust.

The central object in the solar system is the Sun. It is the largest and most massive object in the solar system its diameter is 109 times that of Earth, and it is 333,000 times more massive. The extent of the solar system is determined by the gravitational attraction of the Sun. Indeed, the boundary of the solar system is defined as the surface within which the gravitational pull of the Sun dominates over that of the galaxy. Under this definition, the solar system extends outwards from the Sun to a distance of about 100,000 AU. The solar system is much larger, therefore, than the distance to the remotest known planet, Pluto, which orbits the Sun at a mean distance of 39.44 AU.

The Sun and the solar system are situated some 26,000 light years from the center of our galaxy. The Sun takes about 240 million years to complete one orbit about the galactic center.

Since its formation the Sun has completed about 19 such trips. As it orbits about the center of the galaxy, the Sun also moves in an oscillatory fashion above and below the galactic plane with a period of about 30 million years. During their periodic sojourns above and below the plane of the galaxy, the Sun and solar system suffer gravitational encounters with other stars and giant molecular clouds . These close encounters result in the loss of objects (essentially dormant cometary nuclei located in the outer Oort cloud) that are on, or near, the boundary of the solar system. These encounters also nudge some cometary nuclei toward the inner solar system where they may be observed as long-period comets .

The objects within our solar system demonstrate several essential dynamical characteristics. When viewed from above the Sun's North Pole, all of the planets orbit the Sun along near-circular orbits in a counterclockwise manner. The Sun also rotates in a counterclockwise direction. With respect to the Sun, therefore, the planets have prograde orbits. The major planets, asteroids, and short-period comets all move along orbits only slightly inclined to one another. For this reason, when viewed from Earth, the asteroids and planets all appear to move in the narrow zodiacal band of constellations. All of the major planets, with three exceptions, spin on their central axes in the same direction that they orbit the Sun. That is, the planets mostly spin in a prograde motion. The planets Venus, Uranus, and Pluto are the three exceptions, having retrograde (backwards) spins.

The distances at which the planets orbit the Sun increase geometrically, and it appears that each planet is roughly 64% further from the Sun than its nearest inner neighbor. The separation between successive planets increases dramatically beyond the orbit of Mars. While the inner, or terrestrial planets are typically separated by distances of about four-tenths of an AU, the outer, or Jovian planets are typically separated by 5 — 10 AU.

Although the asteroids and short-period comets satisfy, in a general sense, the same dynamical constraints as the major planets, we have to remember that such objects have undergone significant orbital evolution since the solar system formed. The asteroids, for example, have undergone many mutual collisions and fragmentation events, and the cometary nuclei have suffered from numerous gravitational perturbations from the planets. Long-period comets in particular have suffered considerable dynamical evolution, first to become members of the Oort cloud, and second to become comets visible in the inner solar system.

The compositional make-up of the various solar system bodies offers several important clues about the conditions under which they formed. The four interior planets — Mercury, Venus, Earth, and Mars — are classified as terrestrial and are composed of rocky material surrounding an iron-nickel metallic core. In contrast, Jupiter, Saturn, Neptune, and Uranus are classified as the "gas giants" and are large masses of hydrogen in gaseous, liquid, and solid form surrounding Earth-size rock and metal cores. Pluto fits neither of these categories, having an icy surface of frozen methane. Pluto more greatly resembles the satellites of the gas giants, which contain large fractions of icy material. This observation suggests that the initial conditions under which such ices might have formed only prevailed beyond the orbit of Jupiter.

In summary, any proposed theory for the formation of the solar system must explain both the dynamical and chemical properties of the objects in the solar system. It must also be sufficient flexibility to allow for distinctive features such as retrograde spin, and the chaotic migration of cometary orbits.

Astronomers almost universally assert that the best descriptive model for the formation of the solar system is the solar nebula hypothesis. The essential idea behind the solar nebula model is that the Sun and planets formed through the collapse of a rotating cloud of interstellar gas and dust. In this way, planet formation is postulated to be a natural consequence of star formation.

The solar nebula hypothesis is not a new scientific proposal. Indeed, the German philosopher Immanuel Kant first discussed the idea in 1755. Later, the French mathematician, Pierre Simon de Laplace (1749 – 1827) developed the model in his text, The System of the World, published in 1796.

The key postulate in the solar nebula hypothesis is that once a rotating interstellar gas cloud has commenced gravitational collapse, then the conservation of angular momentum will force the cloud to develop a massive, central condensation that is surrounded by a less massive flattened ring, or disk of material. The nebula hypothesis asserts that the Sun forms from the central condensation, and that the planets accumulate from the material in the disk. The solar nebula model naturally explains why the Sun is the most massive object in the solar system, and why the planets rotate about the Sun in the same sense, along nearly circular orbits and in essentially the same plane.

During the gravitational collapse of an interstellar cloud, the central regions become heated through the release of gravitational energy. This means that the young solar nebular is hot, and that the gas and (vaporized) dust in the central regions is well mixed. By constructing models to follow the gradual cooling of the solar nebula, scientists have been able to establish a chemical condensation sequence. Near to the central proto-sun, the nebular temperature will be very high, and consequently no solid matter can exist. Everything is in a gaseous form. Farther away from the central proto-sun, however, the temperature of the nebula falls off. At distances beyond 0.2 AU from the proto-sun, the temperature drops below 3,100 ° F (1,700 ° C). At this temperature, metals and oxides can begin to form. Still further out (at about 0.5 AU), the temperature will drop below 1,300 ° F (730 ° C), and silicate rocks can begin to form. Beyond about 5 AU from the protosun, the temperature of the nebula will be below − 100 ° F ( − 73 ° C), and ices can start to condense. The temperature and distance controlled sequence of chemical condensation in the solar nebula correctly predicts the basic chemical make-up of the planets.

Perhaps the most important issue to be resolved in future versions of the solar nebula model is that of the distribution of angular momentum. The problem for the solar nebula theory is that it predicts that most of the mass and angular momentum should be in the Sun. In other words, the Sun should spin much more rapidly than it does. A mechanism is therefore required to transport angular momentum away from the central proto-sun and redistribute it in the outer planetary disk. One proposed transport mechanism invokes the presence of a magnetic field in the nebula, while another mechanism proposed the existence of viscous stresses produced by turbulence in the nebular gas.

Precise dating of meteorites and lunar rock samples indicate that the solar system is 4.6 to 5.1 billion years old. The meteorites also indicate an age spread of about 20 million years, during which time the planets themselves formed.

The standard solar nebula model suggests that the planets were created through a multi-step process. The first important step is the coagulation and sedimentation of rock and ice grains in the mid-plain of the nebula. These grains and aggregates, 0.4 in (1 cm) to 3 ft (1 m) in size, continue to accumulate in the mid-plain of the nebula to produce a swarm of some 10 trillion larger bodies, called planetesimals, that are some 0.6 mi (1 km), or so in size. Finally, the planetesimals themselves accumulate into larger, self-gravitating bodies called proto-planets. The proto-planets were probably a few hundred kilometers in size. Finally, growth of proto-planet-sized objects results in the planets.

The final stages of planetary formation were decidedly violent — it is probable that a collision with a Mars-sized proto-planet produced Earth's Moon . Likewise, it is thought that the retrograde rotations of Venus and Uranus may have been caused by glancing proto-planetary impacts. The rocky and icy planetesimals not incorporated into the proto-planets now orbit the Sun as asteroids and cometary nuclei. The cometary nuclei that formed in the outer solar nebula were mostly ejected from the nebula by gravitational encounters with the large Jovian gas giants and now reside in the Oort cloud.

One problem that has still to be worked-out under the solar nebula hypothesis concerns the formation of Jupiter. The estimated accumulation time for Jupiter is about 100 million years, but it is now known that the solar nebula itself probably only survived for 100,000 to 10 million years. In other words, the accumulation process in the standard nebula model is too slow by at least a factor of 10 and maybe 100.

Of great importance to the study of solar systems was the discovery in 1999 of an entire solar system around another star. Although such systems should be plentiful and common in the cosmos, this was the first observation of another solar system. Forty-four light-years from Earth, three large planets were found circling the star Upsilon Andromedae. Astronomers suspect the planets are similar to Jupiter and Saturn — huge spheres of gas without a solid surface.

See also Astronomy Big Bang theory Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars Cosmology Dating methods Earth (planet) Earth, interior structure Geologic time Revolution and rotation



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