This week at the First Kepler Science Conference at the Ames Research Park NASA scientists announced that the Kepler Space Telescope had potentially discovered an exoplanet smaller than the Earth. At the moment the team are reluctant to say that they are able to confirm this discovery, but a paper is being prepared for the journal Nature, which only publishes major discoveries that have not been documented elsewhere, which suggests that they have serious data to support their claim.
The new planet is in the Kepler-20 system, formerly known as KOI-70 (Kepler Object of Interest-70), which now has three confirmed planets, Kepler-20b, Kepler-20c and Kepler-20d and two unconfirmed planets, KOI-70.04 and KOI-70.05; the star is now referred to as Kepler-20A (letters are capitalized to indicate stars), but the unconfirmed objects retain the old designaton for the system until they are confirmed. All five orbit closer to their star than Mercury does to the sun. The three confirmed planets have been observed to cause a regular dimming of the star as they transit (pass in front of) it, and to cause perturbations in its rotation (wobbles) due to their mass. The two unconfirmed planets have been observed to transit the star, but are two small to cause any visible movement in the star, and will so need to be confirmed by another method. The smaller planets can be detected when they transit the star because they are so close to it; a small close planet causes as much dimming as a larger more distant one.
An artist's impression of the Kepler-20 system.
It is KOI-70.04 that is potentially smaller than the Earth, about 80-90% of the size of our planet. It is the second closest object to the star, at a orbiting every 6.1 days at a distance of about 9.6 million kilometers or 6.4% of the distance from Earth to the Sun. It is estimated that this would give the planet a surface temperature of 600°C.
The three larger planets have masses of 8.7 times that of the Earth (Kepler-20b), 16.1 times the mass of the Earth (Kepler-20c) and somewhere under 20.1 times the mass of the Earth (Kepler-20d). All three have radiuses between 1.9 and 3 times that of the Earth.
No further information is available at this time; no data on KOI-70.05 has been released, although it is presumably the closest object to the star.
See also NASA announces the discovery of Kepler-22b, The Atmosphere of GJ1214b and Exolanets on Sciency Thoughts YouTube.