Astronomers Spot Evidence of Colliding Planet Embryos in Famous Star Cluster

November 14, 2007
For Immediate Release

Contact:
Peter Michaud
Gemini Observatory, Hilo, HI
Desk: 1-808-974-2510
Cell: 1-808-937-0845
e-mail: pmichaud@xxxxxxxxxx

Stuart Wolpert
UCLA
Desk: 1-310-206-0511
swolpert@xxxxxxxxxxxxxxxx


ASTRONOMERS SPOT EVIDENCE FOR COLLIDING PLANET EMBRYOS
IN FAMOUS STAR CLUSTER

NOTE: Publication-quality images and artwork available at:
www.gemini.edu/pleiadesdust

Astronomers have found evidence for the formation of young
rocky planets around the star HD 23514 located in the well-known
Pleiades (Seven Sisters) star cluster that is easily visible in
the current evening sky.

Using an infrared sensitive camera (MICHELLE) on the Gemini North
Telescope, Joseph Rhee of UCLA and his collaborators have measured
heat from hot dust surrounding a 100 million year old star in the
bright star cluster. The star has properties very much like our Sun
except that it is 45 times younger and is orbited by hundreds of
thousands of times more dust than our Sun. The star is also one of
the very few solar-type stars known to be orbited by warm dust
particles.

These warm emissions betray catastrophic collisions in an evolving
young planetary system around an adolescent-age solar type star.
The emission appears to originate from dust located in the
terrestrial planet zone between about 1/4 to two astronomical units
(AUs) from the parent star HD 23514, a region corresponding to
the orbits of Mercury and Mars in our solar system.

Rhee and team members Inseok Song of the Spitzer Science
Center and Benjamin Zuckerman of UCLA interpret the presence
of so much hot dust as a result of colliding planetary embryos
leading to the conclusion that a recent collision occurred between
relatively large rocky bodies. According to Zuckerman, this is
thought to be similar to the encounter that produced the Earth-
Moon system more than four billions ago. "Indeed, the collision
that generated the Moon sent a comparable mass of debris into
interplanetary orbits as is now observed in HD 23514," said
Zuckerman.

The astronomers analyzing the emission from countless
microscopic dust particles propose that the most likely explanation
is they were pulverized in the violent collision of planets or
"planetary embryos." Song calls the dust particles the "building
blocks of planets," which accumulate into comets and small
asteroid-size bodies, and then clump together to form planetary
embryos, and finally full-fledged planets. "In the process of
creating rocky, terrestrial planets, some objects collide and grow
into planets, while others shatter into dust; we are seeing that
dust," Song said.

These new observations indicate that rocky terrestrial planets,
perhaps like the Earth, Mars or Venus, appear to be forming or to
have recently formed. "This is the first clear evidence for planet
formation in the Pleiades, and the results we are presenting
strongly suggest that terrestrial planets like those in our solar
system are quite common," said Joseph Rhee, UCLA postdoctoral
scholar in astronomy, and lead author of the research.

Astronomers report the findings in an upcoming issue of the
Astrophysical Journal, published by the American Astronomical
Society.

The Pleiades star cluster, in the constellation of Taurus, is easily
visible to the naked eye at this time of the year. The cluster is
well-
known in many cultures, and is cited in the Bible, noted Rhee:
"Can you bind the beautiful Pleiades? Can you loose the cords of
Orion?" ( Job 38:31).

Although referred to as the seven sisters, "the cluster actually
contains some 1,400 stars," said Inseok Song, a staff scientist at
Caltech's Spitzer Science Center, former astronomer with the
Gemini Observatory, and a co-author of the research. Located
about 400 light years away, the Pleiades is one of the closest star
clusters to Earth.

Background Information:

-- The research team used the Gemini North telescope on Mauna
Kea on the island of Hawaii to measure the heat radiation coming
from the dust; "this heat emerges at infrared wavelengths, just as
the heat from our bodies does," Song said. "The Gemini data were
crucial" in establishing the amount and location of dust around the
star, he said.

-- The Pleiades star (HD 23514) is the second star around which
Song and Zuckerman have recently found evidence of terrestrial
planet formation. They and colleagues reported in the journal
Nature in July 2005 that a sun-like star, known as BD+20 307,
located 300 light-years from Earth in the direction of the
constellation Aries, is surrounded by a shocking one million times
more dust than is orbiting around our sun. According to
Zuckerman HD 23514, "_has hundreds of thousands of time as
much dust as around our sun."

-- While the sun is 4.5 billion years old, both HD 23514 and
BD+20-307 are "adolescents," about 100 million and 400 million
years old, respectively, Rhee said. Based on the age of the two
stars and the dynamics of the orbiting dust particles, the
astronomers deduce that most adolescent-age sun-like stars are
likely to be building terrestrial-like planets via recurring violent
collisions of massive objects, but the cosmic debris from only a
small percentage can be seen at any one time _ currently, only
stars HD23514 and BD+20 307 are known.

-- HD23514 and BD+20 307 are by far the dustiest not-so-young
stars in the sky, Song said; "nothing else is even close." Very
young stars, 10 million years old or younger, may have this much
dust around them, as a remnant of the star formation process.
However, by the time a star is 100 million years old, the dust has
dissipated because the dust particles get blown away or dragged
onto the star, or the particles clump together to form much larger
objects.

-- In parts of Polynesia and Hawai'i, the Pleiades were used to help
determine the start of the new year (the New Year celebration is
called Makahiki in Hawaii). In Hawai'i the cluster is known as
Makali'i.

-- "In Greek mythology, the Pleiades represented Seven Sisters; to
the Vikings, the Pleiades was Freyja's hens," Rhee said. In Bronze
Age Europe, the Celts and others associated the Pleiades with
mourning and funerals because the cluster rose in the eastern night
sky between the autumn equinox and the winter solstice, which
was a festival devoted to the remembrance of the dead. The ancient
Aztecs of Mexico and Central America based their calendar upon
the Pleiades.

-- The results are based on mid- and far- infrared observations made
with the Gemini 8-meter Frederick C. Gillett Telescope (Gemini
North) and space-based infrared observatories: Infrared
Astronomical Satellite (IRAS), Infrared Space Observatory (ISO),
and Spitzer Space Telescope.

-- UCLA is California's largest university, with an enrollment of
nearly 37,000 undergraduate and graduate students. The UCLA
College of Letters and Science and the university's 11 professional
schools feature renowned faculty and offer more than 300 degree
programs and majors. UCLA is a national and international leader
in the breadth and quality of its academic, research, health care,
cultural, continuing education and athletic programs. Four alumni
and five faculty have been awarded the Nobel Prize.

-- The Gemini Observatory is an international collaboration with
two identical 8-meter telescopes. The Frederick C. Gillett Gemini
Telescope is located at Mauna Kea, Hawai'i (Gemini North) and
the other telescope at Cerro Pachon in central Chile (Gemini
South), and hence provide full coverage of both hemispheres of the
sky. Both telescopes incorporate new technologies that allow large,
relatively thin mirrors under active control to collect and focus
both optical and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in
each partner country with state-of-the-art astronomical facilities
that allocate observing time in proportion to each country's
contribution. In addition to financial support, each country also
contributes significant scientific and technical resources. The
national research agencies that form the Gemini partnership include:
the US National Science Foundation (NSF), the UK Science and
Technology
Facilities Council (STFC), the Canadian National Research Council
(NRC),
the Chilean Comision Nacional de Investigacion Cientifica y
Tecnologica
(CONICYT),
the Australian Research Council (ARC), the Argentinean Consejo
Nacional de
Investigaciones Cientificas y Tecnicas (CONICET) and the
Brazilian Conselho Nacional de Desenvolvimento Cientifico e
Tecnologico
CNPq). The observatory is managed by the Association of Universities
for
Research
in Astronomy, Inc. (AURA) under a cooperative agreement with
the NSF. The NSF also serves as the executive agency for the
international partnership.
.

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