SMALLER AND SMALLER: The newfound planet Kepler 10 c, just over twice Earth's diameter, is visible in the foreground of this artist's conception; its smaller neighbor Kepler 10 b is a dark speck on the face of the star. Image: NASA/Ames/JPL-Caltech/T. Pyle
BOSTON—Score one for the little guys. After years of obscurity in the corners of distant planetary systems smaller exoplanets are finally shuffling into the spotlight.
NASA's Kepler spacecraft, built to seek out planets in orbit around faraway stars, has since 2009 been monitoring a vast field of stars to see what kind of planets might be found there. Earlier this year scientists working on the mission announced that they had confirmed 15 exoplanets in Kepler's field of view and identified an astounding 1,200 or so additional planetary candidates, which are probable planets that await independent validation. Prior to that announcement, only about 500 extrasolar worlds had been discovered since planet searches first began to bear fruit in the 1990s.
One of the most fascinating aspects of the Kepler discovery was the small sizes of the planetary candidates. For years the list of known extrasolar planets had been dominated by massive worlds comparable to or larger than Jupiter, the biggest planet in our solar system. Such massive worlds are the easiest to find, whereas more diminutive planets, of roughly Earth or Neptune size, are much more difficult to spot. But Kepler was designed to be sensitive to those smaller worlds, even the temperate, rocky worlds that might be habitable—and it has not disappointed. The spacecraft is showing that smaller planets are common—more common, in fact, than their larger brethren. At least that is how things look in the inner regions of planetary systems, where Kepler's data is currently the strongest.
"There are some Jupiters, there are some Saturns," University of California, Berkeley, astronomer Geoff Marcy said here at the semiannual meeting of the American Astronomical Society (AAS). "But there are far more of the smaller and smaller planets going down to about two Earth diameters."
Kepler looks for the subtle but recurrent dimming of a star that often indicates the presence of an orbiting planet passing in front of its parent star and blocking a tiny fraction of its light. Planets that orbit close to their star, and therefore complete a full orbital revolution quickly, show up more often in Kepler's data. Marcy and his colleagues have focused on planets with orbital periods of 50 days or less because those planets have already revealed themselves to Kepler multiple times. In terms of orbital distances that translates to planets orbiting within about one fourth the distance Earth keeps from the sun.
Marcy and his colleagues have used Kepler's data set to extrapolate how often planets of different sizes appear around stars, taking into account the biases that make Kepler spot some planets, yet miss others. (For every planet that orbits its star in just such a way that it crosses Kepler's line of sight, there might be five to 20 other worlds that are not so favorably aligned.) The team found that planets of just a few times Earth's diameter are quite common around stars of the sun's spectral type. "If you take a sample of G-type, main-sequence stars, 8 percent of them will have two- to 2.8-Earth-radii planets with orbital periods of less than 50 days," Marcy said.
A potentially habitable rocky world would have to orbit somewhat farther from its star, preferably in a temperate Earth-like orbit of 365 days or so. Kepler will need more time to investigate those longer-period planets thoroughly, but the trends are favorable for the existence of small planets in more temperate orbits. Within Kepler's short-period planetary candidates, the number of smaller planets increases as the distance from the star increases. "There are apparently more and more small planets, at least edging out toward 50 days," Marcy said.
As for the existence of many of the even smaller planets, those Earth-size worlds that astronomers have long sought, the jury is still out. "We see a very rapid rise as we go to smaller objects, then a precipitous fall as we go to Earth-size and smaller planets," Kepler principal investigator Bill Borucki of NASA Ames Research Center said at the meeting. "We don't know what that represents." It could be that the small dimming signals produced by such tiny worlds are simply hard to find in the noise of the data. Both Borucki and Marcy cautioned that it was too early to make any inferences on the frequency of Earth-size planets, which are expected to take more time to identify.
Kepler's potential for spotting relatively small worlds was highlighted by the announcement at the AAS meeting of a newfound planet from the mission. Francois Fressin of the Harvard–Smithsonian Center for Astrophysics said that he and his colleagues have vetted a candidate planet, now known as Kepler 10 c, which has a diameter 2.2 times that of Earth. Kepler 10 c resides in a planetary system with the even smaller Kepler 10 b, which was announced in January. At the time of that announcement Kepler 10 c was an intriguing hint that could not be validated. But Fressin and his colleagues used complementary observations from NASA's Spitzer Space Telescope as well as a supercomputer program that simulates confounding astronomical phenomena that might mimic the signal of a planet to conclude that there is only a one in 60,000 chance that Kepler 10 c is not a planet.