Astronomy 105G Lecture Notes, 30 Apr. 2004

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Planet Formation

The solar nebula had a fairly uniform composition of 98% hydrogen and helium, 2% "other stuff." How did the planets end up with such different compositions? This happened through the process of condensation, which is the formation of a solid or liquid from a gas (think of water condensing on the outside of your cold drink on a summer day). The critical point here is that different materials condense at different temperatures.




Near the Sun, T ~ 1500 K. The only materials that could condense there must have high melting points. These are the metals (iron, nickel, aluminum). Farther out from the Sun, the temperatures were slightly cooler: silicates (rock) could condense. In the outer region of the solar nebula, temperatures were much colder: water, methane, and ammonia ices could condense. [The light gases like hydrogen and helium never condensed, so they stayed in the gaseous form.]




The temperature differences between the inner and outer solar nebula determined what condensates were available for planet formation (as the building blocks, or "seeds").




There are four main processes at work for planet formation:

  1. Condensation: solid flakes of material form

  2. Accretion: flakes grow by particles sticking together after gentle collisions (like building a snowman). This resulted in the formation of "planetesimals" (pieces of planets). This step also determined the size as well as the composition of the planets. For example, in the inner solar system, there was not a lot of rocky and metallic material, thus the terrestrial planets could not grow very large.

  3. Nebular Capture: When the planetesimals grew, they became more massive, and therefore their gravity became stronger. The more massive objects captured surrounding H and He gas. This resulted in the growth of the giant planets. As the giant planets captured more gas, the same processes that we saw in the solar nebula took place: heating, spinning, and flattening. This resulted in a "mini solar system" around the giant planets, out of which the satellites and rings condensed. [As you go outward from Jupiter, Io is more dense and Callisto is less dense.]

  4. Solar WInd: the solar wind was even stronger shortly after the Sun formed; it swept away all of the leftover gas out of our solar system. We are now left with the planets!




Immediately following the planet formation, there was a period known as early bombardment. The left-over planetesimals collided with other bodies in the early solar system. The solar system formed about 4.6 billion years ago, and the early bombardment period was about the first few hundred million years. This resulted in a large number of impact craters on the solid bodies. Moon and Mercury preserved that period in history with their old surfaces. Venus, Earth, and Mars have undergone more erosion, thus there is less of a record of the early bombardment on those surfaces. These impactors also may have delivered low-density materials (such as WATER) to the inner solar system, resulting in our large abundance of water on Earth today. Jupiter may have played a role in gravitationally "flinging" the comets towards the inner solar system.




The concept of impacts is used to explain a lot of the exceptions seen in the solar system today. Impacts can explain:







If all of the above information is true, would you expect most (or many) stars to have planetary systems?




Yes, but they are difficult to detect. However, there are two pieces of observational evidence that lead us to believe that we are on the right track to understanding planet formation.

  1. We can see disks of maeterial around other stars (out of which planets will presumably form).

  2. Planets around other stars HAVE been discovered recently!




Discovery of Extrasolar Planets

Within the last decade, the study of planets orbiting stars other than our Sun has exploded. The first "extrasolar planet" was discovered in 1995, and as of Wednesday (4/28/04) there are 123 known planets around other stars. So far, all of these planets have been detected indirectly, meaning that astronomers observe their effects on their parent stars rather than detecting signals from the planets directly.




The detection of planets around other stars is difficult for several reasons.

  1. Planets themselves are intrinsically faint and do not emit a lot of their own radiation
  2. They are extremely far away from us, and thus appear faint.
  3. A planet's signal is overwhelmed by the signal from its parent star.




There are 4 basic detection methods for finding extrasolar planets. They are very nicely described here in an interactive tutorial. Briefly, the four methods are:

  1. Doppler method: the slight redshift and blueshift of the star's spectrum due to the presence of the planet is measured; this is also known as the radial velocity method


    Image courtesy of the California and Carnegie Search for Extrasolar Planets web page (http://exoplanets.org/doppframe.html)




  2. Astrometry method: the physical back-and-forth motion of the star due to the presence of the planet is measured
  3. Transit method: the amount of radiation received from a star decreases slightly when a planet passes in front of it and blocks a small portion of the starlight
  4. Direct detection: using techniques to block out the signal from the star, the planet can be studied directly with imaging and spectroscopy (this has not been successful yet!)




All extrasolar planets except one have been discovered using the Doppler method; the exception was discovered using the transit method.




Current Census of Extrasolar Planets

The first two discovery techniques described above are most sensitive to large planets that are fairly close to the parent star; this would induce the greatest "wobble" on the parent star, and therefore would be the easiest to detect. It may not be surprising, then, that most of the planets discovered so far fit this description.





Image courtesy of the California and Carnegie Search for Extrasolar Planets web page (http://exoplanets.org/massradiiframe.html)




Things to note about the above figure:




How do these discoveries compare to what we already have learned about planetary formation?




This field is very young (less than 10 years old), but there is a lot of activity and excitement about the discovery of planets in other solar systems. It is one of the most significant discoveries in modern astronomy; we have gone from being the only solar system we knew of to being one of over a hundred! Yet none of the new systems discovered resembles our own. Is this an observational effect (meaning we are not sensitive to small, Earth-like planets orbiting stars near 1 AU), or are we truly unique? Only time will tell. There are plans for several spacecraft missions in the next few decades to search for more planets - including terrestrial planets. You can read more about those missions here.