“Rogue” Planets More Common Than Stars?

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Researchers have detected “rogue” planets that are drifting through interstellar space, unbound by any parent star. The Jupiter-sized planets were likely ejected from their host star system during the development of said system.

During a joint Japan-New Zealand survey which scanned the center of our Milky Way galaxy in 2006 and 2007, evidence was discovered for up to 10 free-floating planets roughly the mass of Jupiter. Located between 10,000 and 20,000 light-years from Earth, the “orphan” planets are relatively hard to spot, and were undetected until recently. “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.

Based on their discoveries, the team estimates that there may be twice as many of these “orphan” planets as stars. Additionally, the team believes these to be at least as common as ordinary planets that are gravitationally bound to a host star. Based on these estimates, there could be hundreds of billions of these planets that have been ejected from their host system. “Our survey is like a population census,” said David Bennett, a NASA and NSF-funded co-author of the study (University of Notre Dame). “We sampled a portion of the galaxy, and based on the data, can estimate overall numbers in the galaxy.”

Led by Takahiro Sumi from Osaka University in Japan, the results of the study will appear in the May 19 issue of the journal Nature. While many Jupiter-sized planets have been detected so far, the survey can in fact, detect planets much smaller than Jupiter or Saturn, and as such may be more common than “rogue” Jupiters.

In previous studies, free-floating planets had been found in star forming clusters, but researchers suspected they may be Brown Dwarfs, which are star-like bodies that lack enough mass to sustain nuclear fusion (converting Hydrogen to Helium, like our Sun.) What makes discerning these objects from planets difficult is that the difference between a high mass gas giant planet like Jupiter, and a low-mass Brown Dwarf is very fuzzy. “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 team cannot rule out the possibility that some of these planets may, in fact have a very distant orbit from their host star, but other research shows that high-mass planets at distant orbits are rare.

The survey, the Microlensing Observations in Astrophysics (MOA) uses A 1.8 meter telescope at Mount John University Observatory in New Zealand to search for gravitational microlensing events. Said events occur when something with a large amount of mass, such as a star, planet passes in front of another star. The passing body’s gravity bends the light of the background star, causing it to magnify and brighten. More massive bodies will bend the light more, resulting in an event that can last many weeks, whereas smaller bodies, like planets will only produce an event for a few days or even hours.

The Optical Gravitational Lensing Experiment (OGLE), also contributed to this discovery using a 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.

For more information about NASA’s exoplanet research, visit: http://planetquest.jpl.nasa.gov

Source(s):NASA/JPL Press Release, Nature.com

Ray Sanders is a Sci-Fi geek, astronomer and blogger. Currently researching variable stars at Arizona State University, he writes for Universe Today, The Planetary Society blog, and his own blog, Dear Astronomer