Formation of the Solar System
As with similar stars, the very young Sun was therefore surrounded by gas
and dust leftover from its formation. Through a series of collisions between the gas
molecules and dust particles, this material became organized in the form of a circumstellar disk. The circumstellar disk is where the planets formed.
 (animation from L. Close,
http://athene.as.arizona.edu/~lclose/teaching/a202/lect4.html) |
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This movie shows
the forming system
of planets. We rocket through a molecular cloud, penetrating the cold cloud core where
the Sun is being born. As we approach, we see the disk of material orbiting the protostar, the end point of the animation just above. It begins to glow bright red as
energy is released by its contraction under gravity. Gas clouds come and go above the disk
and then a wind starts from the young Sun and clears excess gas from the disk, leaving the
young planetary system. Not shown, eventually the star blows away the excess gas and some
of the dust to become visible.
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At an early stage (less than a million
years old), when the young Sun was still surrounded by the dense disk of
both gas and dust, the gas giant planets Jupiter and Saturn formed. The
simulation below illustrates the growth of instabilities in a disk and the
eventual formation of planets. (from Ken Rice, (http://faculty.ucr.edu/~krice/)
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Once the gas had been ejected from the
system, the possibilities for giant planets forming were over.
"Terrestrial" planets (like the earth) can take longer to
form. Planet embryos formed in the disk within a few million years and continued to grow
through multiple violent collisions for millions of years after the gas had left the system. (from
Chris Butler, http://www.lpi.usra.edu/science/hahn/web/)
Animation below from G. J. Taylor, http://www.psrd.hawaii.edu/Nov06/hit-and-run.html
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As this process continued, young terrestrial planets formed in the disk but still collided frequently, and comets fell into the young Sun at a high rate. (top and bottom pictures from Don Dixon) |
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Around other stars, some of the planetesimals that did not stick together to form planets still had a big influence on their systems. The giant planets had to plow through swarms of them, and they slowed the planets a bit like lots and lots of bugs hitting the windshield of your car would slow it down. These planetesimals got thrown into eccentric orbits or ejected from the systems, but the giant planets migrated inward, often to orbits very close to the stars Fortunately, this did not happen in the Solar System. from http://casa.colorado.edu/~raymonsn/graphics.html Raymond, Mandell & Sigurdsson (2006, Science, 313, 1413-1416), Sean Raymond |
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However, the excitement was not over for the Solar System. Around 700 million years after the formation of the system, Jupiter and Saturn got into orbits where the period of Saturn was exactly twice that of Jupiter. This "orbital resonance" caused strong interactions because of the continuously repeating gravitational effects from these two massive planets.* The outer planets were shifted into their current orbits as a result. At the same time, the overall gravitational field of the system kept changing in ways that resulted in the ejection of most of the smaller bodies that had not yet been captured into planets. As these bodies shifted orbits, there was a period of high impact rates, called the Late Heavy Bombardment. In this computer simulation of the process, the Sun is in the center, the circles are the orbits of Jupiter, Saturn, Uranus, and Neptune, and the green dots are the smaller bodies. The system appears to be stable as Saturn and Jupiter migrate slowly toward the 2:1 resonance, but when they reach it Uranus and Neptune are scattered into their current orbits and most of the smaller bodies are very quickly thrown out of the system - it is a cataclysm! (The event occurs a little late compared with the solar system in this particular simulation.) (from http://www.psrd.hawaii.edu/Aug06/cataclysmDynamics.html)
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| Planetary systems appear to form very frequently | "There are countless suns and countless Earths all rotating around
their suns in exactly the same way as the seven planets of our system. We see only the
suns because they are the largest bodies and are luminous, but their planets remain
invisible to us because they are smaller and non-luminous." - Giordano Bruno, 1548 - 1600, in De L'Infinito Universo E Mondi |
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We are finding evidence for massive planets around many stars from Doppler shifts indicating something unseen orbiting the star. This animation is based on a real system (from Sylvain G. Korzennik, http://cfa-www.harvard.edu/afoe/orbits/). If you watch closely, you can see a small movement of the star around the common center of mass of it and the massive planet orbiting it. The resulting Doppler shift of the stellar lines is shown in the graph at the bottom. The net effect is just over + 50 m/s, about + 0.00002%. It is just possible to detect such a tiny shift in the wavelengths of the spectral lines. An earth-sized planet would produce shifts more than a hundred times smaller, less than we can measure. Also, a large planet too far from the star would produce too slow a recoil for us to have detected it. These systems must be examples where a gas giant planet formed far from the sky and migrated inward as described above. |
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This diagram shows what is happening in more detail. (From The Essential Cosmic Perspective, by Bennett et al.) |
Another approach to finding planets is to look for the small reduction in the light from a star when a planet passes between us and it -- a transit. This requires that the planet orbit be lined up just so, but we do know of about three dozen examples. One is when Mercury or Venus pass between us and the sun:
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Here is an example, the transit of Venus in June, 2004 (Venus is to
the lower left on the solar disk). (from Astronomy Magazine). To
see more, try this link
http://www.solarviews.com/cap/sun/noaavenustransit.htm
Of course, for other stars we only see the slight reduction of the light. (from
http://eo.ucar.edu/staff/dward/sao/exoplanets/methods.htm)
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The Kepler satellite was built to look for transiting planets (and some were found by the earlier CoROT mission also). Here is an example - a planet that takes out nearly 1% of the light of the star when it passes in front of it. This planet orbits its star in a little less than 5 days, and has a mass about 40% as large as that of Jupiter. |
We are finding lots of other planets in these ways - more than 500 are now known. However, the systems are strangely different from our own - they have giant planets like Jupiter orbiting as close or closer to their stars than Mercury! These planets have to have formed in orbits far from the stars, like those of Jupiter and Saturn, and then migrated inward by the same kind of processes that caused the inward orbital migration we showed above, or the Late Heavy Bombardment. As many as 10% of stars like the Sun have such planets, so the process must be common. Why didn't this happen in the Solar System (with potentially disastrous consequences for Earth)? It is proposed that we were saved by the accident of forming two massive planets close to each other, and that the orbital resonance that caused the Late Heavy Bombardment also stabilized Jupiter and Saturn's orbits out where they are to this day.
Actually seeing normal planets orbiting even the nearest stars is much more difficult than observing Doppler recoils or transits, both because the planets are so faint, and because they tend to be lost in the glare from the star itself. Until recently, the closest we had gotten was to find, in a few cases, brown dwarfs in orbit that are more massive than we would normally accept as planets. The brown dwarf systems may be more similar to double stars than planetary systems in the way they formed. Still, they are tantalizing suggestions of planetary systems.
For example, one of the first known older brown dwarfs is called Gl229B. It orbits a nearby star.
None of these systems let us look at how ones like ours
evolved; all of them are too different from ours, and we see them at some random
late time in their evolution. We need a different approach to learn about the
evolution of systems like ours.
Fortunately, there are also many examples
of stars surrounded by circumstellar disks of debris. The dust and small grains in these
disks will either be blown quickly away from the star or will fall into it in only about a
million years. Therefore, the debris has to be renewed - we think this happens when small
planets, typically on the scale of large asteroids in the solar system - collide with each
other (from Robert Hurt, SSC).
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able to take pictures of planets orbiting stars with debris disks.
An example is to the right. Fomalhaut is about 200 million years old. The
narrow ring is a system of debris from recent collisions that produced a
cloud of dust we now see spread in orbit around the star. The sharp inner
edge is maintained by a massive planet, whose orbital motion can be seen in
the inset to the lower right.
(From NASA, ESA, P.
Kalas et al.
http://apod.nasa.gov/apod/ap081114.html)
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| The debris disk was first discovered because of the infrared emission from its heated dust. Here are images at 450 microns ((James Clerk Maxwell Telescope) showing pretty much the same ring as seen by HST, but at lower resolution); and at 70 and 24 microns (Spitzer Telescope). At 70 microns, we see the side of the ring closer to the star heated to a higher temperature; this offset of the ring can be seen in the HST image above. At 24 microns, the ring is filled in with warm dust that is not revealed by HST. The structure is a bit like that of the Solar System, with a cold ring like the Kuiper Belt filled with warm material like the asteroid belt. |
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| A second example is HR 8799. Three massive planets were discovered by Marois and others, while Su, Rieke, and others imaged the huge debris system. This star is much younger than Fomalhaut, perhaps 30 million years old. It is not thought that the three planets can stay in stable orbits and that one of them may be ejected from the system. They are also stirring up the small bodies in the debris disk causing a lot of collisions so the disk is very bright; small, weakly bound dust grains are on very eccentric orbits extending to 1000 AU from the star, while tiny grains are being ejected altogether through impacts with photons from the star. (concept by G. Rieke) |
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An even more impressive disk orbits the nearby very young star, beta Pictoris - however, because this star is only 10 - 20 million years old, astronomers debate whether the disk is left over from its formation or is due to a recent planet collision. Nonetheless, it too has a massive planet, as can be seen below.
We know of about 300 stars with debris disks, indicating planet systems actively evolving (and colliding) around them. The intense debris disk stage appears to last about 100 million years, after which most planetary systems seem to have "settled down" and have a lower rate of collisions and debris generation. This time period matches pretty well the theoretical estimates for the time required for our Solar System to have settled down. Like the three examples above, perhaps all of these stars harbor planetary systems, but the rest are too faint for us to image yet.
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The big question is whether many Earth-like planets exist, since they
are potential sites for life and perhaps even civilizations Ambitious programs to
probe this question have fallen victim to budget woes in NASA, so we have to
wait for a definitive answer. Meanwhile, we can probe this question by studying our own
planetary system.Here is an artist's idea of what it might look like to be on a small moon orbiting an Earth-like planet. (by David Hardy, http://www.hardyart.demon.co.uk/html/main.html)
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Starburst Candy |
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Mechanical model of the solar system, or orrery, made in 1712 for the Earl of Orrery (hence the name).http://www.sciencemuseum.org.uk/on-line/treasure/objects/1952-73.asp |
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G. H. Rieke