Adding to its already long roster of firsts, NASA's Kepler spacecraft has found the three smallest extrasolar planets ever detected — all of them smaller than Earth, and the most diminutive no larger than Mars. The newly discovered trio forms a miniature planetary system orbiting a cool, dim red dwarf star called KOI-961.
Because they are so close to their star, the three exoplanets are too hot to support life. But unlike most previously known exoplanets, the vast majority of which are Jupiter-scale gas giants, all three are thought to be rocky worlds like Earth and the other worlds of the inner Solar System. And because red dwarfs are the most common type of star in the Milky Way, the finding suggests that the Galaxy may be teeming with rocky planets — with at least some residing in the 'habitable region' around those stars, where the temperature would be just right for water to remain liquid and life might have got a foothold.
“We need to get a census of how many rocky planets are out there to understand how Earth formed, and how common life may be in the Milky Way,” notes Sara Seager, an astronomer at the Massachusetts Institute of Technology in Cambridge who was not part of the latest study. The discovery bodes well for fulfilling that quest, she adds.
“The suggestion — but not proof — is that rocky planets are common and diverse, and that our Solar System is not some cosmic quirk composed of weirdo worlds,” agrees study collaborator Geoffrey Marcy of the University of California, Berkeley.
The Kepler findings were presented on 11 January at the semi-annual meeting of the American Astronomical Society (AAS) in Austin, Texas, by John Johnson and Philip Muirhead of the California Institute of Technology in Pasadena. A paper describing the discovery has been accepted by the Astrophysical Journal.
Kepler hunts for planets by monitoring a field of some 150,000 stars in search of a subtle, periodic dimming — a sign that an orbiting body is passing, or transiting, in front of a star, blocking a tiny fraction of light. Most of the stars Kepler examines are similar in mass to the Sun, but some are considerably smaller — including one, a red dwarf star some 40 parsecs (130 light years) away from Earth, that was among the stars flagged as a candidate for possessing one or more planets.
Dubbed KOI-961, for “Kepler Object of Interest”, the red dwarf came under close scrutiny by the researchers. Muirhead realized early on that the Kepler mission team, which concentrates on finding planets around Sun-like stars, had overerestimated the brightness and size of the much smaller red dwarfs, which are notoriously difficult to model. On average, the red dwarfs' diameters turned out to be only about half the values listed in Kepler's official star catalog. And for KOI-961, in particular, the diameter had to be revised downwards even further: its actual size is only about one-sixth that of the Sun.
The revised estimate for the diameter of KOI-961 was crucial to discovering the latest trio, because the size of any transiting planets found by Kepler are measured only relative to the size of their parent star. The tinier the star, the tinier the transiting planet that can be detected. An essential clue came from a fortuitous e-mail that Johnson received last September from Kevin Apps, an amateur astronomer in Horley, England. Apps, a co-author on the Astrophysical Journal paper, spends his spare time studying the data on extrasolar planets and nearby stars, and he has an encyclopaedic knowledge of their properties. He alerted the rest of the team that KOI-961 bore an uncanny resemblance in colour and temperature to a well-known dwarf called Barnard’s Star. Follow-up observations with ground-based telescopes confirmed that the two stars were virtual twins. And because the size of Barnard’s Star has been very precisely measured, thanks to its location only 1.9 parsecs (6 light years) from Earth, Johnson’s team could gauge the diameter of KOI-961 — and therefore its transiting planets — with high precision.