Formation of the Solar System
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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This clip shows the late stages in building
the terrestrial planets.
They are seen accreting material in the form of large bodies crashing into them. We zoom
in on one, see the cloud of intense collisional fragments clear, and then witness a series
of spectacular explosions as objects impact the surface. These planets continued to
grow for tens of millions of years.
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However, the excitement was not over. 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. |
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This diagram shows what is happening in more detail. (From The Essential Cosmic Perspective, by Bennett et al.) |
Another approach 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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We are finding lots of other planets in these ways - more than 200 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 process that caused 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. The closest we have gotten is 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).
Below is an example. Fomalhaut is about 200 million years old, so the system of debris
shown in the images is from recent collisions that produced the cloud of dust we now
see spread in orbit around the star. (from K. Stapelfeldt, Spitzer
Science Center, Caltech/JPL/NASA)
An even more impressive disk orbits the nearby 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.
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. Intriguingly, though, there is very little debris disk activity around other stars around the time of the Late Heavy Bombardment, suggesting that such events are both rare and (as we might expect) occur at many different times in the systems that have them.
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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