PIO Contact:
Rebecca Johnson, McDonald Observatory
The University of Texas at Austin
1-512-475-6763
rjohnson@astro.as.utexas.edu

FOR IMMEDIATE RELEASE
December 13, 2006

UNIVERSITY OF TEXAS ASTRONOMER EXPLORES PLANET FORMATION
PROCESS AROUND MILKY WAY'S SMALLEST, MOST ABUNDANT STARS

Austin, Texas - A study published in this week's edition of
Astrophysical Journal Letters, led by University of Texas at
Austin graduate student Jacob Bean with research scientists
Michael Endl and Fritz Benedict, brings new insight into how
planets form around the most populous stars in our Milky Way
galaxy. Bean¹s work shows that the chemical make-up of these "red
dwarf" stars with orbiting planets is different from most of
Sun-like stars that harbor planets -- and indicates that
astronomers must take chemical composition into account in their
planet searches around these stars.

Red dwarfs have lower mass than any other type of star, ranging
from just 8 per cent of the Sun¹s mass to as much as 60 per cent.
They also give off correspondingly less light, making them more
difficult to study. Despite their stature, though, red dwarfs are
the most numerous stars in the galaxy. Of the hundreds of
billions of stars in our Milky Way, at least 70 per cent are red
dwarfs. "This factor alone makes them a crucial sample for
determining the fraction of stars that are orbited by planets,"
Bean says.

Roughly 200 planets have been found around Sun-like stars. Most
of these planets are several times the size of Jupiter, the
largest planet in our solar system. In contrast, only three red
dwarf stars were known to have planets or planet-candidates at
the time of Bean's study: Gliese 876, Gliese 436, and Gliese 581
(a possible fourth was recently announced). Gliese 876 harbors
two Jupiter mass planets, with a third lower mass planet
suspected.

One interesting trend that has emerged from studies of the
Sun-like hosts to Jupiter-mass planets is the larger amount of
"metals" -- that is, elements heavier than hydrogen and helium --
in their atmospheres compared to the Sun¹s atmosphere. This
property is known as "metallicity."

The amount of heavy elements in a star's atmosphere is thought to
be a clue to the composition of the cloud of gas and dust from
which it -- and its planets -- formed.

Benedict recounts a bit of project history: "Our original
motivation was to determine the metallicity of red dwarfs in
binary stars to help disentangle a completely different problem.
Jacob early-on recognized the value of applying his techniques to
planet-bearing red dwarfs."

Planets probably grow faster in a proto-stellar cloud with higher
metallicity. "Just as rain drops need a speck of dust in the air
around which to form, the formation of planets is thought to be
assisted by a similar successful first step," Benedict says.
"More dust in the protoplanetary disk might increase the chances
for planet formation."

According to Bean, "the formation of high-mass planets like
Jupiter is also a time issue. A rocky core with sufficient mass
to gravitationally pull in a lot of gas must form before the star
switches on and its strong radiation pressure pushes the
remaining gas away." Apparently Sun-like stars have about a ten
percent chance of having a planet. The chance for red dwarfs
seems far less.

The purpose of Bean's study was to find out if the red dwarfs
with known planets also have high metallicity values. "It is
predicted that high-mass planets should be rarer around red
dwarfs because there should have been less material overall to
form the star and potential planets in the primordial cloud,"
Bean says. "That theory is supported by surveys discovering fewer
of these types of planets around red dwarf stars. But, the
dependence of high-mass planet formation on metallicity
complicates what would otherwise be a straightforward result. If
the red dwarfs being surveyed for planets have lower
metallicities than the Sun-like stars that are being surveyed,
that could also cause the discovery of fewer high-mass planets.
The purpose of this work was to try and disentangle the two
effects."

Bean used the 2.7-meter Harlan J. Smith and 9.2-meter
Hobby-Eberly telescopes at The University of Texas at Austin
McDonald Observatory in West Texas to study the compositions of
the three red dwarfs.

Analyzing the light from these tiny, dim stars is difficult, he
says, because of the low temperatures in their atmospheres -- the
region from which the light to be analyzed is coming. "Molecules
form in the star¹s atmosphere," Bean says, which "produce spectra
that are very complex -- a forest of lines all blended together."

Bean's work involves not only telescope observations, but
computer modeling as well. He studied and improved
computer-generated "low-temperature model atmospheres" for red
dwarf stars.

When he analyzed his telescope data with these improved models,
he found that these red dwarfs with planets contain significantly
fewer metals than Sun-like stars that harbor planets.

Current theory holds that red dwarfs have fewer high-mass planets
because the formation rate of high-mass planets depends on the
mass of the host star. The low-frequency of planet detections
around red dwarfs seems to support this theory. Now, however,
Bean¹s result shows that the effects of metallicity cannot be
ignored when testing planet formation theories around red dwarfs.
If planet searches are biased toward lower metallicity red
dwarfs, then that could account for the low numbers of high-mass
planets found around these stars.

Bean¹s result is a preliminary finding from an ongoing project to
determine the metallicities of all the red dwarfs included in the
McDonald Observatory Planet Search. Because three stars is not a
sufficient number upon which to establish a significant trend,
Bean will determine many more red dwarf metallicities.

His co-author, Michael Endl, has searched for planets around
about 100 red dwarfs to date. "Red dwarf stars represent very
interesting targets for planet hunters," Endl says. "For the red
dwarfs with the lowest masses, like Proxima Centauri, we are
sensitive to planets down to two Earth masses using the standard
radial velocity technique.

"There appears to be a paucity of giant planets detected around
red dwarfs, as compared to more Sun-like stars. This could mean
that gas giant planet formation is less efficient around low mass
stars. But it is possible that the majority of red dwarf giant
planets orbit their star at larger separation and still await
their discovery.

"Jabob¹s result is an important step toward a better
understanding of the planet formation history around the most
common stars in the galaxy," Endl says.

END

Scientists' Contact Information:

Jacob Bean
1-512-471-3466
bean@astro.as.utexas.edu

Michael Endl
1-512-471-7336
mike@astro.as.utexas.edu

Fritz Benedict
1-512-471-3448
fritz@astro.as.utexas.edu