U.Texas Astronomer Explores Planet Formation Around Our Galaxy'sSmallest, Most Abundant Stars (Forwarded)

McDonald Observatory
University of Texas
Fort Davis, Texas

Contact: Rebecca Johnson
ph: 512-475-6763
fax: 512-471-5060

Scientist Contacts:

Jacob Bean, (512) 471-3466

Michael Endl, (512) 471-7336

Fritz Benedict, (512) 471-3448

13 December 2006

University of Texas Astronomer Explores Planet Formation Around Our
Galaxy'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. Bean's 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 Sun's 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 Sun's 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 star's
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, Bean's 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.

Bean's 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.

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