Astronomers find tiny planet orbiting tiny star (Forwarded)

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University of Notre Dame
Notre Dame, Indiana

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David Bennett, 574-315-6621 or 574-631-8298

June 2, 2008

Astronomers find tiny planet orbiting tiny star
By William G. Gilroy

An international team of astronomers led by David Bennett of the
University of Notre Dame has discovered an extra-solar planet of about
three Earth masses orbiting a star with a mass so low that its core may
not be large enough to maintain nuclear reactions. The result was
presented Monday (June 2) at the American Astronomical Society annual
meeting in St. Louis.

The planet, referred to as MOA-2007-BLG-192Lb, establishes a record for
the lowest mass planet to orbit a normal star. The star,
MOA-2007-BLG-192L, is at a distance of 3,000 light years and the lowest
mass host star to have a companion with a planetary mass ratio. The mass
of the host is about 6 percent of the mass of the sun. Such a star is
called a brown dwarf, because this is slightly below the mass needed to
sustain nuclear reactions in the core. But the measurement uncertainty
also permits a host mass slightly above 8 percent of a solar mass, which
would make MOA-2007-BLG-192L a very low-mass hydrogen burning star.

"Our discovery indicates that even the lowest mass stars can host
planets," Bennett said. "No planets have previously been found to orbit
stars with masses less than about 20 percent of that of the sun, but this
finding suggests that we should expect very low-mass stars near the sun to
have planets with a mass similar to that of the Earth. This is of
particular interest because it may be possible use NASA's planned James
Webb Space Telescope to search for signs of life on Earth-mass planets
orbiting low-mass stars in the vicinity of the sun."

The discovery of the MOA-2007-BLG-192L star-planet system was made by the
Microlensing Observations in Astrophysics (MOA), which includes Bennett,
and the Optical Gravitational Lensing Experiment (OGLE) collaborations
using the gravitational microlensing method.

Gravitational microlensing takes advantage of the fact that light is bent
as the rays pass close to a massive object, like a star. The gravity from
the mass of the intervening object, or lens star, warps surrounding space
and acts like a giant magnifying glass. As predicted by Albert Einstein
and later confirmed, this phenomena causes an apparent brightening of the
light from a background "source" star. The effect is seen only if the
astronomer's telescope lies in almost perfect alignment with the source
star and the lens star.

Astronomers are then able to detect planets orbiting the lens star if the
light from the background star also is warped by one or more planets.

The primary challenge of the microlensing method is the precise alignments
needed for the planetary microlensing signals are quite rare and brief,
often lasting less than a day. This discovery was made possible by the new
MOA-II telescope at New Zealand's Mt. John Observatory, using the MOA-cam3
camera, which is able to image an area of sky 13 times larger than the
area of the full moon in a single image.

"The new MOA telescope-camera system allows us to monitor virtually all of
the known microlensing events for planetary signals," Bennett said. "We
would not have made this discovery without it."

The microlensing observations provided evidence that the host star has a
mass of about 6 percent of the mass of the sun. This was confirmed by high
angular resolution adaptive optics images with the Very Large Telescope
(VLT) at the European Southern Observatory in Chile. These images confirm
that the planetary host is either a brown dwarf or a very low-mass star.

The planet orbits its host star or brown dwarf with an orbital radius
similar to that of Venus. But the host is likely to be between 3,000 and 1
million times fainter than the sun, so the top of the planet's atmosphere
is likely to be colder than Pluto. However, the planet is likely to
maintain a massive atmosphere that would allow warmer temperatures at
lower altitudes. It is even possible that interior heating by radioactive
decays would be sufficient to make the surface as warm as the Earth, but
theory suggests that the surface may be completely covered by a very deep
ocean.

This result also supports the 1996 prediction by Bennett and Sun Hong Rhie
that the microlensing method should be sensitive to Earth-mass planets.

"I'll hazard a prediction that the first extra-solar Earth-mass planet
will be found by microlensing," Bennett said. "But we'll have to be very
quick to beat the radial velocity programs and NASA's Kepler mission,
which will be launched in early 2009."

A paper describing this result has been accepted for publication in the
Astrophysical Journal and is scheduled for publication in the Sept. 1
edition. Bennett's work is funded by the National Science Foundation and
the National Aeronautics and Space Administration.

In addition to Bennett, the MOA group is composed of astronomers from
Nagoya University, Konan University, Nagano National College of
Technology, and Tokyo Metropolitan College of Aeronautics in Japan, as
well as Massey University, the University of Auckland, Mt. John
Observatory, the University of Canterbury, and Victoria University in New
Zealand.

The OGLE group is comprised of astronomers from Warsaw University
Observatory in Poland, the Universidad de Concepción in Chile, and the
University of Cambridge in England. Additional collaborators who provided
the VLT data and analysis are from the Institut d'Astrophysique de Paris,
the Observatoire Midi-Pyrénées, and the Observatoire de Paris in France,
the European Southern Observatory in Chile, and Heidelberg University in
Germany.

More information on the discovery, including animations, can be found at
http://www.nsf.gov/news/news_summ.js...=NSF&from=news

Andrew Yee wrote:
An international team of astronomers led by David Bennett of the
University of Notre Dame has discovered an extra-solar planet of about
three Earth masses orbiting a star with a mass so low that its core may
not be large enough to maintain nuclear reactions. The result was
presented Monday (June 2) at the American Astronomical Society annual
meeting in St. Louis.

snip

This result also supports the 1996 prediction by Bennett and Sun Hong Rhie
that the microlensing method should be sensitive to Earth-mass planets.

"I'll hazard a prediction that the first extra-solar Earth-mass planet
will be found by microlensing," Bennett said. "But we'll have to be very
quick to beat the radial velocity programs and NASA's Kepler mission,
which will be launched in early 2009."

So is a 3 Earth mass planet still considered an Earth mass planet?

Yousuf Khan