Third rock. Seen in the foreground in this artist impression, the outermost of the three planets orbiting the red dwarf star KOI-961 is barely larger than Mars.


Kepler Spies Smallest Alien Worlds Yet

AUSTIN—NASA's Kepler space telescope has found its tiniest extrasolar planets yet. The three rocky worlds are smaller than Earth; the smallest one is barely larger than Mars. Together, they constitute the most compact planetary system ever seen—less than 5 million kilometers across. Moreover, the parent star, known as KOI-961, is a puny red dwarf, just 70% larger than the giant planet Jupiter. Indeed, says astronomer John Johnson of the California Institute of Technology in Pasadena, the KOI-961 system is more akin to Jupiter and its moons than to a sunlike star with orbiting planets.

Kepler finds distant planets by watching stars dim as the worlds pass in front of them, blocking part of their light. Kepler had already seen periodic brightness dips in the faint light of the red dwarf star, but astronomers had not yet been able to rule out other possible explanations. They also weren't sure about the exact size of the star, which must be known before they can calculate the sizes of the transiting planets from the amount of starlight they block.

But as Johnson explained here today at the 219th meeting of the American Astronomical Society, that all changed thanks to British amateur astronomer Kevin Apps, who is working closely with the Kepler science team. Apps noted that the precise colors of KOI-961, which is some 120 light-years away from Earth, are exactly like those of a much nearer red dwarf star known as Barnard's Star. That suggests that it also must have the same size as Barnard's Star, whose diameter has been measured quite precisely.

Using additional observations from the 10-meter Keck telescope on Mauna Kea, Hawaii, and from the 5-meter Hale Telescope on Palomar Mountain in California, Johnson and his team could also confirm that the observed brightness dips were indeed due to planets. A paper describing the new discovery has been accepted for publication in The Astrophysical Journal.

Although KOI-961 is a dim and relatively cool dwarf, the three rocky planets are too hot to sustain life because of their closeness to the star. The inner one orbits the star in a mere 10.8 hours. "I look forward to observing two successive transits of this planet in one single night," Johnson says. Future observations may yield more-precise information on the masses and hence the composition of the three planets.

According to a very rough statistical analysis, the new discovery suggests that up to one-third of all red dwarf stars in the Milky Way galaxy are accompanied by small, rocky planets, many of which might be in wider orbits. "Because red dwarfs themselves are so common," Johnson says, "the whole galaxy must be just swarming with little habitable planets around faint red dwarfs."

Commenting on the discovery of the remarkable system, Virginia Trimble of the University of California, Irvine, recalls a famous principle put forward by particle physicist Murray Gell-Mann: "Everything not forbidden is compulsory," Trimble says. "Any kind of system you can think of, if it doesn't violate the laws of nature, it probably exists somewhere out there. So as long as people think up new techniques, they will also find new types of planets. There will surely be lots of new, neat stuff in the coming years."