Astronomers Find Three Smallest Planets Outside Solar System

ScienceDaily (Jan. 11, 2012) A team of astronomers led by scientists at the California Institute of Technology (Caltech) has discovered the three smallest confirmed planets ever detected outside our solar system. The three planets, which all orbit a single star, are smaller than Earth and appear to be rocky with a solid surface. Until now, astronomers have found at most only four other rocky planets, also called terrestrial planets, around other stars.

The trio of new planets is too close to the central star to be in its habitable zone -- the ring-shaped region around a star where the temperature is mild enough for liquid water, and possibly life, to exist. But the planets are the first rocky ones to be found orbiting a type of dim, small star called a red dwarf, the most common kind in the Milky Way. Their existence suggests that the galaxy could be teeming with similarly rocky planets -- and that there's a good chance that many are in the habitable zone.

The red dwarf, called KOI-961, was first flagged as a potential planetary system by the Kepler mission, a space telescope that looks for planets around sunlike stars by scanning the sky for stars that periodically dip in brightness -- the result of one or more planets passing in front of them. Although Kepler reported 900 potential planetary systems in February, only about 85 of those were red-dwarf systems. The fact that a relatively small sample of red dwarfs produced three terrestrial planets means that either the Caltech-led team was really lucky or, more likely, that these planets are commonly found around red dwarfs.

"When you combine that with the fact that these are some of the most numerous stars in the galaxy, you realize this type of system could be common," says Philip Muirhead, a postdoctoral scholar at Caltech. "There's no question that it's exciting." Muirhead is the lead author on the paper describing the discovery, which has been accepted by the Astrophysical Journal. The team will also present their results in two talks on January 11 at the meeting of the American Astronomical Society in Austin, Texas.

"Red dwarfs make up eight out of every ten stars in the galaxy," adds John Johnson, assistant professor of astronomy and one of the paper's coauthors. "That boosts the chances of other life being in the universe -- that's the ultimate result here. If these planets are as common as they appear -- and because red dwarfs themselves are so common -- then the whole galaxy must be just swarming with little habitable planets around faint red dwarfs."