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.
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