[AstroNet] Free-Floating Planets May Be More Common Than Stars

[Sam Rametse]

http://science.nasa.gov/science-news/science-at-nasa/2011/18may_orphanplanets/

May 18, 2011:  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 are probably outcasts from
developing planetary systems and, moreover, they could be twice as
numerous as the stars themselves.

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

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 planets are located at an average
approximate distance of 10,000 to 20,000 light years from Earth.
Free-Floating Planets (concept, 550px)
This artist's concept illustrates a Jupiter-like planet alone in the dark
of space, floating freely without a parent star. [larger image] [video]

This could be just the tip of the iceberg.  The team estimates there are
about twice as many free-floating Jupiter-mass planets 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, 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.
Free-Floating Planets (lens, 200px)
A video from JPL describes the microlensing technique astronomers used to
detect the orphan 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 be in orbit around distant 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.

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


Sam Rametse
Hartebeesthoek Radio Astronomy observatory (NRF)
Science Awareness programme
tel: 012 301 3228/100
Fax: 012 301 3300/28
http://www.hartrao.ac.za/
"The universe is not required to be in perfect harmony with human ambition."
- Carl Sagan