By ANDY FLEMING
The signature of the smallest known planet, KOI-2124.01, near its habitable zone. This plot of intensity versus time shows how the light from the star dims ever-so slightly as the exoplanet candidate passes across the star as seen from the Earth. (Credit: NASA/Kepler; Batalha).
There are currently 861 exoplanets (planets around other stars) according to the official exoplanet encyclopaedia website. This list includes only the objects that have been confirmed as exoplanets; most of them have many of their physical parameters reasonably well determined, for example their masses and orbits. Many more objects have been spotted as potential, or “candidate,” exoplanets, but additional observations and analyses are needed to verify their reality as exoplanets.
How many? Writing in the latest issue of the Astrophysical Journal Supplement, a team of fourteen Harvard-Smithsonian Centre for Astrophysics astronomers, along with fifty-two of their colleagues, announced the figures. In the first sixteen months of operation, the Kepler spacecraft, which began operations in May, 2009, has found over 2300 candidate exoplanets in its observations (which examined over 190,000 stars). The new figure includes updates to earlier exoplanet candidate estimates and is based not only on additional observations but also on new data analysis techniques. Applied to the earlier datasets, these new techniques are better able to identify smaller candidates.
The huge new compilation significantly boosts the number of potential Earth-sized planets. More than 91% of the new candidates are smaller than Neptune; the number of candidates smaller than two Earth-radii jumped by 201%, with 624 new candidates. The fraction of stars hosting systems with more than one planet has also grown, from 17% to 20% of the total number of stars with planets. The new results are tantalizing in one other respect: the possibility that Earth-sized planets in their habitable zone (where water can remain liquid) will soon be discovered.