SCIENCE - The Universe's Multiple-Planet Stars

"Stars With Multiple Planets Abound" by Govert Schilling, Science 5/23/2011

Planets like company. No less than one in three of all planets around other stars found by NASA's Kepler space telescope are in multiple-planet systems. What's more, the sheer number of those systems suggests that they are more tranquil places than our own solar system, say astronomers working with the telescope.

Since it was launched just over 2 years ago, Kepler has found 116 systems with two planets, 45 with three, eight with four, one with five, and one with six planets, astronomer David Latham of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, reported here today at the meeting of the American Astronomical Society. That's a total of 171 multiple-planet systems. "We thought we might see a few multiplanet systems," Latham says. "Instead, we found lots of them."

Kepler detects a planet when it passes in front of its parent star, as seen from Earth. Such transits produce minute, periodic dips in the brightness of the star. Kepler continuously monitors 165,000 stars to search for those transits.

The low expectation for multiple-planet systems was based on the fact that the eight planets in our own solar system do not orbit in exactly the same plane. For instance, the plane of Mercury's orbit is tilted by 7˚ relative to that of Earth. So if the solar system were viewed from a vast distance, if one planet produced transits, most others would not.

This means Kepler's multiplanet systems must be very flat, with orbital tilts of less than a degree. "Most likely, if our solar system didn't have large planets like Jupiter and Saturn to have stirred things up with their gravitational disturbances, it would be just as flat," Latham says. In the Kepler systems found so far, smaller planets such as Neptune are much more common. Latham says, "Systems with smaller planets probably had a much more sedate history."

Still, the planets do slightly disturb each other with their mutual gravity, pulling each other around a little bit and thus causing tiny variations in the exact timing of the transits. In fact, such transit timing variations provide information on the masses of the planets. "This is a completely new [tool in the] field that might become very important," says planetary scientist Sara Seager of the Massachusetts Institute of Technology in Cambridge.

Transit timing variations even show "indications" of the existence of nontransiting planets in some of the multiplanet systems, says Kepler principal investigator William Borucki of NASA's Ames Research Center in Moffet Field, California. "We're looking into it," Borucki says. Those unseen planets' orbits may have been tilted a bit by the perturbations of a Neptune-like planet in the system. But Ames theorist Jack Lissauer says that if no transits are observable, it will be much harder to pin down the orbits and masses of these planets.

Meanwhile, a thorough statistical analysis of the sizes of the planets Kepler has seen so far reveals "a very profound discovery," planet hunter Geoff Marcy of the University of California, Berkeley, reported at the meeting. "There are some Jupiters and some Saturns, but there are many more smaller planets out there," Marcy says, probably including a lot of them comparable in size to Earth.

The ultimate goal of the Kepler mission, which has an operational lifetime of 3½ years, is to find the abundance of Earth-like planets orbiting stars like our own sun at distances where liquid water—and possibly life—could exist on their surfaces. Borucki expects an additional release of Kepler data in June 2012 and doesn't want to speculate about what it might show. "We don't want to get premature information out," he says. "There's still a lot of analysis that needs to be done."