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Extrasolar Planets

Page history last edited by PBworks 13 years, 11 months ago



"An extrasolar planet, or exoplanet, is a planet which orbits a star other than the Sun, and therefore belongs to a planetary system other than the solar system. As of August 2006, over 200 extrasolar planets have been discovered"


Image: Some stars known to have at least one orbiting planet.


For centuries, extrasolar planets were a subject of speculation. Astronomers generally supposed that some existed, but it was a mystery how common they were and how similar they were to the planets of our own solar system. The first confirmed detections were finally made in the 1990s. Since 2002, more than twenty have been discovered every year. It is now estimated that at least 10% of sunlike stars have planets, and the true fraction may be much higher. The discovery of extrasolar planets raises the question of whether some might support extraterrestrial life.



Planets Observed

Observational Selection

Due to the current detection methods employed, the vast majority of exoplanets found so far have:

  • High masses: Ninety percent of them have more than 10 times the mass of Earth. Many are considerably more massive than Jupiter.
  • Close orbit: Many exoplanets orbit much closer around their parent star than any planet in our own Solar System orbits around the Sun. "Hot Jupiters" came as a surprise to many, but it is now clear that most exoplanets (or at least, most high-mass exoplanets) have much larger orbits.


It appears plausible that in most exoplanetary systems around sunlike stars, the largest planets are comparable in size to Jupiter or Saturn. Also, the fact that astronomers have found several planets only a few times more massive than Earth, despite the great difficulty of detecting them, indicates that such planets are fairly common.

It also appears plausible that in most exoplanetary systems, there are one or two giant planets with orbits comparable in size to those of Jupiter and Saturn in our own Solar System.


General Observations

  • Eccentricity: Most known exoplanets have quite eccentric orbits. The prevalence of elliptical orbits is a major puzzle, since current theories of planetary formation strongly suggest planets should form with circular (non-eccentric) orbits. This is also an indication that our own Solar System may be unusual, since almost all of its planets do all follow basically circular orbits. (The only two planets in our own Solar System that do have substantial eccentricities, Pluto and 2003 UB313, are sometimes considered to be Kuiper-belt objects rather than genuine planets.)



  • Planetary Composition
  • Geochemistry
  • Atmospheric Chemistry
  • Probability of having satellites
  • Habitability




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