LONG BEACH, CALIFORNIA—Small planets like our own are extremely abundant in the universe, according to new results presented here today at the 221st meeting of the American Astronomical Society. Indeed, says planetary scientist John Johnson of the California Institute of Technology in Pasadena, our galaxy harbors at least a hundred billion planets, many of them Earth-sized.
The findings come thanks to NASA's Kepler space telescope, which has been notching up an increasing tally of exoplanets—worlds orbiting stars other than our own sun—ever since it was launched in 2009. "The total number of exoplanet candidates is now 2740," says Christopher Burke of the SETI Institute in Mountain View, California, including 461 new ones unveiled at the meeting. The number of Earth-sized candidates has increased by 43% since Kepler's previous catalog was published about a year ago.
Kepler keeps an eye on the brightness of some 150,000 stars. If an orbiting planet passes in front of the star, the telescope sees a small, periodic dip in the star's brightness. Follow-up observations with ground-based telescopes have so far confirmed that 105 Kepler candidates are true planets. Many of them reside in multiplanet systems, although most orbit much closer to their parent stars than the planets in our own solar system do.
Because Kepler is observing so many stars, Johnson says, "Its true power is in statistics." For instance, the Kepler results imply that planetary systems around puny red dwarf stars are the most common type in the galaxy. Also, a rigorous statistical analysis by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, shows that almost all sunlike stars have planetary systems orbiting them and that at least one in six has an Earth-sized planet. "We expect most Earth-sized planets to be rocky," he says. (If they were gaseous, they would have evaporated.)
Most such planets, however, would be very close to their parent stars, so they're too hot to host life. According to Fressin, "it's still too early to say how many Earth-sized planets orbit their parent star in the so-called habitable zone," where balmy temperatures allow the existence of liquid water on a planet's surface.
Meanwhile, Kepler is slowly starting to find them. At the meeting, Burke announced an exoplanet candidate just 50% larger than Earth in the habitable zone of a sunlike star. However, "there's still a lot of uncertainty here," he says. "It will probably take another couple of years before we are able to confirm the existence of a true Earth analog."
While Kepler hunts for planets orbiting other stars, billions of rogue planets roam interstellar space, says Nathan Kaib of Northwestern University in Evanston, Illinois. Computer simulations that Kaib presented at the meeting reveal that planets orbiting a star that has a distant stellar companion run a high risk of being ejected into space through gravitational disturbances. Because most stars are part of a binary system, "this is not an uncommon occurrence," Kaib says.