SMALL
PLANETS DON'T NEED STARS WITH HEAVY METAL CONTENT TO FORM
From
NASA
June
13, 2012
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 is
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 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.
[So
it would appear that Earth sized planets should be more common that previously
thought. For the emergence and development of life-as-we-know-it this can only
be a good thing.]
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