SMALL PLANETS DON'T NEED STARS WITH HEAVY METAL

MOFFETT FIELD, Calif. -- The formation of small worlds like Earth
previously was thought to occur mostly around stars rich in heavy
elements such as iron and silicon. However, new ground-based
observations, combined with data collected by NASA's Kepler space
telescope, shows small planets form around stars with a wide range of
heavy element content and suggests they may be widespread in our
galaxy.

A research team led by Lars A. Buchhave, an astrophysicist at the
Niels Bohr Institute and the Centre for Star and Planet Formation at
the University of Copenhagen, studied the elemental composition of
more than 150 stars harboring 226 planet candidates smaller than
Neptune.

"I wanted to investigate whether small planets needed a special
environment in order to form, like the giant gas planets, which we
know preferentially develop in environments with a high content of
heavy elements," said Buchhave. "This study shows that small planets
do not discriminate and form around stars with a wide range of heavy
metal content, including stars with only 25 percent of the sun's
metallicity."

Astronomers refer to all chemical elements heavier than hydrogen and
helium as metals. They define metallicity as the metal content of
heavier elements in a star. Stars with a higher fraction of heavy
elements than the sun are considered metal-rich. Stars with a lower
fraction of heavy elements are considered metal-poor.

Planets are created in disks of gas and dust around new stars. Planets
like Earth are composed almost entirely of elements such as iron,
oxygen, silicon and magnesium. The metallicity of a star mirrors the
metal content of the planet-forming disk. Astronomers have
hypothesized that large quantities of heavy elements in the disk
would lead to more efficient planet formation. It has long been noted
that giant planets with short orbital periods tend to be associated
with metal-rich stars.

Unlike gas giants, the occurrence of smaller planets is not strongly
dependent on the heavy element content of their host stars. Planets
up to four times the size of Earth can form around stars with a wide
range of heavy element content, including stars with a lower
metallicity than the sun. The findings are described in a new study
published in the journal Nature.

"Kepler has identified thousands of planet candidates, making it
possible to study big-picture questions like the one posed by Lars.
Does nature require special environments to form Earth-size planets?"
said Natalie Batalha, Kepler mission scientist at NASA's Ames
Research Center at Moffett Field, Calif. "The data suggest that small
planets may form around stars with a wide range of metallicities --
that nature is opportunistic and prolific, finding pathways we might
otherwise have thought difficult."

The ground-based spectroscopic observations for this study were made
at the Nordic Optical Telescope on La Palma in the Canary Islands;
Fred Lawrence Whipple Observatory on Mt. Hopkins in Ariz.; McDonald
Observatory at the University of Texas at Austin; and W.M. Keck
Observatory atop Mauna Kea in Hawaii.

Launched in March 2009, Kepler searches for planets by continuously
monitoring more than 150,000 stars, looking for telltale dips in
their brightness caused by passing, or transiting, planets. At least
three transits are required to verify a signal as a planet. Follow-up
observations from ground-based telescopes are also needed to confirm
a candidate as a planet.

Ames manages Kepler's ground system development, mission operations
and science data analysis. NASA's Jet Propulsion Laboratory in
Pasadena, Calif., managed the Kepler mission development.

Ball Aerospace & Technologies Corp. in Boulder, Colo., developed the
Kepler flight system and supports mission operations with the
Laboratory for Atmospheric and Space Physics at the University of
Colorado in Boulder.

The Space Telescope Science Institute in Baltimore archives hosts and
distributes Kepler science data. Kepler is NASA's 10th Discovery
Mission and is funded by NASA's Science Mission Directorate at the
agency's headquarters in Washington.