[meteorite-list] Astronomers Discover Dusty Remains of Two Terrestrial Planets

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 3 Oct 2008 15:19:23 -0700 (PDT)
Message-ID: <200810032219.PAA17290_at_zagami.jpl.nasa.gov>


Worlds in collision
Astronomers discover dusty remains of two terrestrial planets
By Stuart Wolpert
September 23, 2008

Two terrestrial planets orbiting a mature sun-like star some 300
light-years from Earth recently suffered a violent collision,
astronomers at UCLA, Tennessee State University and the California
Institute of Technology will report in a December issue of the
Astrophysical Journal, the premier journal of astronomy and astrophysics.
"It's as if Earth and Venus collided with each other," said Benjamin
Zuckerman, UCLA professor of physics and astronomy and a co-author on
the paper. "Astronomers have never seen anything like this before.
Apparently, major catastrophic collisions can take place in a fully
mature planetary system."
"If any life was present on either planet, the massive collision would
have wiped out everything in a matter of minutes - the ultimate
extinction event," said co-author Gregory Henry, an astronomer at
Tennessee State University (TSU). "A massive disk of infrared-emitting
dust circling the star provides silent testimony to this sad fate."
Zuckerman, Henry and Michael Muno, an astronomer at Caltech at the time
of the research, were studying a star known as BD+20 307, which is
surrounded by a shocking 1 million times more dust than is orbiting our
sun. The star is located in the constellation Aries. The astronomers
gathered X-ray data using the orbiting Chandra X-ray Observatory and
brightness data from one of TSU's automated telescopes in southern
Arizona, hoping to measure the age of the star.
"We expected to find that BD+20 307 was relatively young, a few hundred
million years old at most, with the massive dust ring signaling the
final stages in the formation of the star's planetary system," Muno said.
Those expectations were shown to be premature, however, when Carnegie
Institution of Washington astronomer Alycia Weinberger announced in the
May 20, 2008, issue of the Astrophysical Journal that BD+20 307 is
actually a close binary star - two stars orbiting around their common
center of mass.
"That discovery radically revised the interpretation of the data and
transformed the star into a unique and intriguing system," said TSU
astronomer Francis Fekel who, along with TSU's Michael Williamson, was
asked to provide additional spectroscopic data from another TSU
automated telescope in Arizona to assist in comprehending this
exceptional binary system.
The new spectroscopic data confirmed that BD+20 307 is composed of two
stars, both very similar in mass, temperature and size to our own sun.
They orbit about their common center of mass every 3.42 days.
"The patterns of element abundances in the stars show that they are much
older than a few hundred million years, as originally thought," Fekel
said. "Instead, the binary system appears to have an age of several
billion years, comparable to our solar system."
"The planetary collision in BD+20 307 was not observed directly but
rather was inferred from the extraordinary quantity of dust particles
that orbit the binary pair at about the same distance as Earth and Venus
are from our sun," Henry said. "If this dust does indeed point to the
presence of terrestrial planets, then this represents the first known
example of planets of any mass in orbit around a close binary star."
Zuckerman and colleagues first reported in the journal Nature in July
2005 that BD+20 307, then still thought to be a single star, was
surrounded by more warm orbiting dust than any other sun-like star known
to astronomers. The dust is orbiting the binary system very closely,
where Earth-like planets are most likely to be and where dust typically
cannot survive long. Small dust particles get pushed away by stellar
radiation, while larger pieces get reduced to dust in collisions within
the disk and are then whisked away. Thus, the dust-forming collision
near BD+20 307 must have taken place rather recently, probably within
the past few hundred thousand years and perhaps much more recently, the
astronomers said.
"This poses two very interesting questions," Fekel said. "How do
planetary orbits become destabilized in such an old, mature system, and
could such a collision happen in our own solar system?"
"The stability of planetary orbits in our own solar system has been
considered for nearly two decades by astronomer Jacques Laskar in France
and, more recently, by Konstantin Batygin and Greg Laughlin in the
U.S.A.," Henry noted. "Their computer models predict planetary motions
into the distant future and they find a small probability for collisions
of Mercury with Earth or Venus sometime in the next billion years or
more. The small probability of this happening may be related to the
rarity of very dusty planetary systems like BD+20 307."
"There is no question, however," Zuckerman said, "that major collisions
have occurred in our solar system's past. Many astronomers believe our
moon was formed from the grazing collision of two planetary embryos -
the young Earth and a body about the size of Mars - a crash that created
tremendous debris, some of which condensed to form the moon and some of
which went into orbit around the young sun. By contrast with the massive
crash in the BD+20 307 system, the collision of an asteroid with Earth
65 million years ago, the most favored explanation for the final demise
of the dinosaurs, was a mere pipsqueak."
In their 1932 novel "When Worlds Collide," science fiction writers
Philip Wylie and Edwin Balmer envisioned the destruction of Earth by a
collision with a planet of a passing star. The 1951 classic movie based
on the novel began a long line of adventure stories of space rocks
apocalyptically plowing into Earth.
"But," Zuckerman noted, "there is no evidence near BD+20 307 of any such
passing star."
This research is federally funded by the National Science Foundation and
NASA and also by Tennessee State University and the state of Tennessee,
through its Centers of Excellence program.
The Astrophysical Journal is published by the American Astronomical Society.
For more information on TSU's Automated Astronomy Group, visit
Tennessee State University is a comprehensive, urban, coeducational
land-grant university, offering 45 bachelor's degrees, 24 master's
degrees and six doctoral programs. Its diverse student population
totals approximately 9,000 students, who come from 46 states and 45
countries. TSU has been listed for 11 consecutive years in U.S. News &
World Report's "Guide to America's Best Colleges."
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.

  Media Contacts

Stuart Wolpert,
swolpert at support.ucla.edu <mailto:swolpert at support.ucla.edu>
Received on Fri 03 Oct 2008 06:19:23 PM PDT

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