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9.24 Properties of Extrasolar Planets

The various exoplanet detection techniques have lead to over 4800 (as of mid-2015) planets and planet candidates around stars other than the Sun being studied by astronomers. Any census is a moving target as this research field is dynamic, with dozens of groups around the world searching for and characterizing extra solar planets. It goes without saying that it's also a highly competitive field; careers can be made with a single discovery! The totals of, by detection technique, are shown in the following list:

• Confirmed transit detections - 1200.

• Confirmed radial velocity detections - 540.

• Confirmed imaging detections - 40.

• Confirmed microlensing detections - 30.

• Candidate transit detections - 3600.

The outsized influence of Kepler on these statistics is clear. Of the 1850 confirmed extra solar planets by any technique since 1995, two thirds have come from Kepler since 2009. Another very exciting statistic is 300, the number of confirmed and candidate extra solar planets in the habitable zones of their host stars. This assumes a temperature at the planet distance of 180 to 310 Kelvin and stellar flux at the surface between a quarter and two times the flux of the Sun at the Earth's surface.

This collection of planets has led to a number of preliminary conclusions about planet characteristics. In a rapidly moving field like this, it is worth being cautious, because we are still far from knowing the "true population" of extra solar planets. Every selection method has biases and limitations. Transits require a particular orientation so only deliver a small fraction of all planets, and the method is biased toward large planets close in (hot Jupiters) since they give the largest signal. Radial velocity or Doppler detection gives a signal that depends on orientation, and the method is not quite sensitive enough to detect Earths. Direct imaging is biased toward planets that are large and far from their stars. Microlensing has the fewest biases as a method, but it returns few planets so the statistics are poor. This all sounds bad, but with a well-designed survey and a well-understood detection method, these selection effect and biases can be quantified and corrected for.

Since a large majority of these planets are due to Kepler,which uses the transit technique,it is convenient to categorize planets by radius. The planet radius is what determines how much fainter the star gets when a planet transits. Astronomers have used several different category breakdowns for planet radius, but one used by the Kepler team is shown in the following list of the fraction of stars with at least one planet in the following size ranges:


• 0.8-1.25 Earth radii (Earth clone) - 17%.

• 1.25-2 Earth radii (super-Earth) - 21%.

• 2-4 Earth radii (mini-Neptune) - 20%.

• 4-6 Earth radii (large Neptune) - 2%.

• > 6 Earth radii (Gas Giant) - 3%/.


These statistics are tentative because the surveys are still underway and the data is not yet fully analyzed. In astronomy, it’s a truism that you only detect the things you are able to detect. Which means that measuring the true or underlying population of extra solar planets involves a good understanding of limitations of the surveys and the detection methods.