Free-Floating Planets May be More Common Than Stars

MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE 818-354-5011
http://www.jpl.nasa.gov

News release: 2011-147 May 18, 2011

Free-Floating Planets May be More Common Than Stars

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-147&cid=release_2011-147

PASADENA, Calif. -- Astronomers, including a NASA-funded team member, have discovered
a new class of Jupiter-sized planets floating alone in the dark of space, away from the light of a
star. The team believes these lone worlds were probably ejected from developing planetary
systems.

The discovery is based on a joint Japan-New Zealand survey that scanned the center of the Milky
Way galaxy during 2006 and 2007, revealing evidence for up to 10 free-floating planets roughly
the mass of Jupiter. The isolated orbs, also known as orphan planets, are difficult to spot, and had
gone undetected until now. The newfound planets are located at an average approximate
distance of 10,000 to 20,000 light-years from Earth.

"Although free-floating planets have been predicted, they finally have been detected, holding
major implications for planetary formation and evolution models," said Mario Perez, exoplanet
program scientist at NASA Headquarters in Washington.

The discovery indicates there are many more free-floating Jupiter-mass planets that can't be seen. The team estimates there are about twice as many of them as stars. In addition, these worlds are thought to be at least as common as planets that orbit stars. This would add up to hundreds of billions of lone planets in our Milky Way galaxy alone.

"Our survey is like a population census," said David Bennett, a NASA and National Science
Foundation-funded co-author of the study from the University of Notre Dame in South Bend,
Ind. "We sampled a portion of the galaxy, and based on these data, can estimate overall numbers
in the galaxy."

The study, led by Takahiro Sumi from Osaka University in Japan, appears in the May 19 issue of
the journal Nature.

The survey is not sensitive to planets smaller than Jupiter and Saturn, but theories suggest lower-
mass planets like Earth should be ejected from their stars more often. As a result, they are
thought to be more common than free-floating Jupiters.

Previous observations spotted a handful of free-floating, planet-like objects within star-forming
clusters, with masses three times that of Jupiter. But scientists suspect the gaseous bodies form
more like stars than planets. These small, dim orbs, called brown dwarfs, grow from collapsing
balls of gas and dust, but lack the mass to ignite their nuclear fuel and shine with starlight. It is
thought the smallest brown dwarfs are approximately the size of large planets.

On the other hand, it is likely that some planets are ejected from their early, turbulent solar
systems, due to close gravitational encounters with other planets or stars. Without a star to circle,
these planets would move through the galaxy as our sun and other stars do, in stable orbits
around the galaxy's center. The discovery of 10 free-floating Jupiters supports the ejection
scenario, though it's possible both mechanisms are at play.

"If free-floating planets formed like stars, then we would have expected to see only one or two
of them in our survey instead of 10," Bennett said. "Our results suggest that planetary systems
often become unstable, with planets being kicked out from their places of birth."

The observations cannot rule out the possibility that some of these planets may have very distant
orbits around stars, but other research indicates Jupiter-mass planets in such distant orbits are
rare.

The survey, the Microlensing Observations in Astrophysics (MOA), is named in part after a giant
wingless, extinct bird family from New Zealand called the moa. A 5.9-foot (1.8-meter) telescope
at Mount John University Observatory in New Zealand is used to regularly scan the copious stars
at the center of our galaxy for gravitational microlensing events. These occur when something,
such as a star or planet, passes in front of another, more distant star. The passing body's gravity
warps the light of the background star, causing it to magnify and brighten. Heftier passing
bodies, like massive stars, will warp the light of the background star to a greater extent, resulting
in brightening events that can last weeks. Small planet-size bodies will cause less of a distortion,
and brighten a star for only a few days or less.

A second microlensing survey group, the Optical Gravitational Lensing Experiment (OGLE),
contributed to this discovery using a 4.2-foot (1.3 meter) telescope in Chile. The OGLE group
also observed many of the same events, and their observations independently confirmed the
analysis of the MOA group.

NASA's Jet Propulsion Laboratory, Pasadena,Calif., manages NASA's Exoplanet Exploration
program office. JPL is a division of the California Institute of Technology in Pasadena.

More information about exoplanets and NASA's planet-finding program is at
http://planetquest.jpl.nasa.gov .

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

Trent Perrotto 202-358-0321
Headquarters, Washington
trent.j.perrotto@nasa.gov

-end-

To remove yourself from this mailing, please go to http://www.kintera.org/TR.asp?a=kjJULcPPIjIWKgJ&s=dnKOJMNlG9JAJLPrHnG&m=ehISJYMDIjKQI9L

To remove yourself from all mailings from NASA Jet Propulsion Laboratory, please go to http://www.kintera.org/TR.asp?a=enKIIUMrHdLLLZI&s=dnKOJMNlG9JAJLPrHnG&m=ehISJYMDIjKQI9L