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Astronomers Detect Planets Without Solar Systems

hubble space telescope picture of galaxy

Astronomers 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 probably were ejected from developing planetary systems.

According to a statement released by NASA, 18 May 2011, 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. Also known as orphan planets, these objects are
difficult to spot, and had gone undetected until now. The 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 adds 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, Indiana. "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 others
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.

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