When it comes to extrasolar planets, smaller is better--at least for astronomers, who long to find worlds like Earth. A European team now has taken a step toward that goal by spotting the smallest planet yet found circling an ordinary star. The planet weighs in at 14 times Earth's mass, and although the astronomers have no direct evidence of the planet's composition, it may be the first known alien world made mainly of rock, rather than gas, in a planetary system like our sun's.
Most planet hunters watch for wobbles in the light from stars, which arise from the back-and-forth gravitational tugs of unseen companions. This method has revealed more than 120 extrasolar planets, most of which resemble the gas-giant Jupiter--318 times more massive than Earth. To find smaller planets, which exert barely perceptible tugs on their parent stars, astronomers are developing instruments to detect ever-tinier motions.
One such device is the sensitive HARPS spectrograph, recently installed on the European Southern Observatory's 3.6-meter telescope at La Silla, Chile. Starting in June, a team led by astronomer Nuno Santos of the University of Lisbon, Portugal, used HARPS to monitor a star called μ Arae, faintly visible to the eye. The star had a known Jupiter-size companion, but Santos and his colleagues found an extra tiny wobble on a regular cycle lasting 9.5 days. Their analysis points to a body with about the same mass as Uranus--about 40% of the mass of the previous lightest exoplanet. The team has submitted its findings to Astronomy & Astrophysics and announced them on 25 August at the EuroScience Open Forum meeting in Stockholm, Sweden.
The planet races around μ Arae at less than 1/10th of the distance between Earth and the sun. According to models of planetary formation, an object of that size is too small to have coalesced farther out as a gas giant and then migrated toward the star. Rather, Santos says, it probably formed near its present orbit by accumulating a hot core of rock and a relatively small gaseous atmosphere.
Because the planet does not cross directly in front of its star, the team cannot confirm its inferences by measuring the planet's radius, says planetary theorist Alan Boss of the Carnegie Institution of Washington in Washington, D.C. Still, Boss finds the logic convincing: "They seem to have found what I would call the top end of the range of terrestrial-type planets. This is a very encouraging sign that we will find a lot of lower-mass rocky planets in the next 10 years or so."