Planets as common as stars in Milky Way

Analysis suggests a galaxy riddled with worlds

When you turn an eye to the evening sky, there’s a good chance that many of the stars above have at least one planet.

An artist’s conception of the Milky Way, at greatly exaggerated scale, illustrates the findings of a new survey that finds evidence for an average of more than one planet per star in the galaxy. M. Kornmesser/ESO

Using six years of data from planet-finding surveys, an international team of researchers concludes that, on average, every star in the Milky Way is accompanied by 1.6 planets. That’s at least 100 billion planets, the scientists report January 12 in Nature.

The figure might seem enormous, but it doesn’t shock planet hunters. “I’m not surprised by this result,” says astrophysicist Wesley Traub of the Jet Propulsion Laboratory in Pasadena, Calif., who was not involved in the study. “This sounds reasonable. This sounds good.”

To make their estimate, the scientists used data that had been gathered from 2002 to 2007 by surveys looking for the temporary brightening in a distant star’s light caused by the gravity of a body passing in front of it. If that passing body is a star with planets, the system causes a predictable boost in the distant star’s light, revealing the presence of the closer planet.

Unlike other types of planet searches, this technique, called gravitational microlensing, works well for stars both near to Earth and far away. “If we want go out of our little box and see into the infinite universe, or in the galactic bulge, or far outside the galaxy — are there planets even there? — then microlensing is the way,” says Kailash Sahu, an astronomer and study coauthor from the Space Telescope Science Institute in Baltimore. And microlensing can more easily detect small planets in orbits far from their stars — though the new study considered only planets circling from half the distance of Earth to the sun out to the equivalent of Saturn’s orbit.

Both the eclipse method used by NASA’s Kepler team and radial velocity searches that measure stellar wobbles are more sensitive to planets tucked in close to their hosts. And those methods more easily find larger planets. Microlensing is the best way to estimate planet frequencies for a range of masses, from planets 10 times the mass of Jupiter to those more like Earth, says astronomer and study coauthor Arnaud Cassan of the Astrophysical Institute of Paris.

Some scientists note that the team based its estimate on a small number of planet detections, but say the small sample size was accurately accounted for. “Non-detections are just as important as detections to constrain the planet frequency,” Cassan says.

Along with identifying the galactic abundance of planets, Cassan and his colleagues also predicted the distribution of planets by mass. The team found that smaller planets are much more common than larger ones, a conclusion that matches the data pouring in from other planet searches.

“Low-mass planets are common as dirt,” says Scott Gaudi, an astronomer at Ohio State University in Columbus who is not a part of the study team. “That doesn’t mean that there aren’t stars that have no planets. There probably are. But on average, planets are pretty common.”

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