NASA’s amazing Kepler planet-searching spacecraft has made a significant new discovery. Planet Kepler-10b is an inhospitable world but it is one of the smallest confirmed exoplanets yet found and may be the most Earth-like world discovered so far.
The Kepler satellite looks for planets passing between their parent star and Earth. Each time this happens a tiny fraction of the star’s light is blocked, so it appears to dim by a minute amount. To confirm this dimming is due to transiting planet and not some variability in the star’s output of light, the star must be carefully observed until the astronomers are sure it is dimming in a regular period. Once the planet’s existence is proven its diameter can be inferred by how much of the star’s light it blocks. Additionally, the star’s position can be observed with great precision, enabling astronomers to determine how the planet is pulling it to and fro on its parent star during its orbit. This data allows the planet’s mass to be calculated. For this newly-discovered world, these follow-up measurements were made at the Keck observatory in Hawaii. In this case, the astronomers were aided by the fortuitous fact that Kepler-10b’s parent star is relatively stable (its luminosity is pretty constant and it is relatively free from the local version of sunspots). This means that astronomers are much more certain of this planet’s properties than they are of other similar exoplanets such as COROT-7b.
The newly discovered planet is almost certainly a rocky world, not a “hot Jupiter”-type gas giant, and it is about 4.6 times as massive as Earth. Placed beside our homeworld, Kepler-10b would be about 40% wider than Earth. It orbits a G-class star, older but otherwise similar to our Sun, at a distance less than a twentieth as close as Mercury is to our own Sun (giving it a “year” of just over 20 hours). The star lies some 560 light years (172 parsecs) from our Solar System in the constellation Draco. Kepler has possibly observed a second planet (Kepler-10c) in this system but this remains to be confirmed. (UPDATE: Kepler-10c’s existence was confirmed in 2014, and it has been found to be the first known mega-earth.)
Kepler-10b is not an inviting place. Any atmosphere it once had has long since gone, stripped away by the fierce radiation and stellar winds of its star. Its surface gravity is an oppressive 2.3g. The daytime temperature is more than 2500°F (1370°C), such a temperature suggests its surface is largely molten. Possibly only occasional jagged islands of metal and rock break the monotony of its glowing lava oceans. Perhaps its permanently dark night is cool enough to allow the existence arid rocky plains. Scorched and rocky worlds like this are sometimes classed as chthonian planets (a term derived from the Greek word for “of the Earth”, often used to refer to gods and other supernatural beings from the Underworld).
This world is a significant discovery. Before this year is out we ought to see the first discoveries by Kepler of planets in the habitable zones of stars. By “habitable zone” (also known as the “Goldilocks Zone”) I mean the region around a star where conditions are just right to allow water to exist as a liquid (rather than solely as steam or ice). Planets in this region around a sunlike star would have a similar orbital period to our planet (about a year), so it takes longer to establish their periodicity. We have yet to find Earth’s twins out among the stars, but we are closing in.
Update added 12 January 2011: Paul Gilster at the excellent Centauri Dreams blog has pointed out that Kepler-10b is not in fact the smallest known exoplanet alter all:
“That honor goes to PSR B1257+12A, a ‘pulsar planet’ some 980 light years away in the constellation Virgo. That object is about twice as massive as our own Moon, a small planet indeed, but the same pulsar has yielded an even smaller candidate, PSR B1257+12 D, which is more or less the size of a Kuiper Belt object in our own Solar System.”