Key points: Steps in forming a planetary system; evidence for planet systems - Doppler recoil, transit, debris disk
The star 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. ![]() |
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This movie shows a forming system
of planets. We rocket through a molecular cloud, penetrating the cold cloud core where a
new star 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 star 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. (reload to restart lecture animations) |
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At an early stage (less than a million years old), when the young star is still surrounded by the dense disk of both gas and dust, gas giant planets like Jupiter and Saturn can form. Once the gas has been ejected from the system, the possibilities for such planets forming are over. |
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"Terrestrial" planets (like the earth) can take longer to
form. Planet embryos form in the disk within a few million years and continue to grow
through multiple violent collisions even after the gas has 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 continues, young terrestrial planets have formed in the disk but still collide frequently, and comets are fall into the central star at a high rate. (top and bottom pictures from Don Dixon) |
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This clip shows the late stages in building 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 can continue to
grow for a hundred million years or so. |
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 if 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, due to a massive planet in orbit around the star. 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. Despite the limitations in mass and planetary orbit, many stars appear to have detectable planets, perhaps as many as 10% of nearby stars! |
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This diagram shows what is happening. (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 two 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 ![]() |
Actually seeing normal planets orbiting even the nearest stars is currently beyond our capabilities, both because they would be so faint, and because they would 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.
We also have found 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 a recent collision that produced a huge 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.
The 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.
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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![]() 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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