[meteorite-list] When Worlds Collide: Have Astronomers Observed the Aftermath of a Distant Planetary Collision?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 10 Jan 2008 15:09:33 -0800 (PST)
Message-ID: <200801102309.PAA14958_at_zagami.jpl.nasa.gov>

Public Affairs Office
Harvard-Smithsonian Center for Astrophysics
Cambridge, Massachusetts

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463

For Release: Wednesday, January 09, 2008 11:00:00 AM EST

Release No.: 2008-01

When Worlds Collide: Have Astronomers Observed the Aftermath of a Distant
Planetary Collision?

Austin, TX -- Astronomers announced today that a mystery object orbiting a
star 170 light-years from Earth might have formed from the collision and
merger of two protoplanets. The object, known as 2M1207B, has puzzled
astronomers since its discovery because it seems to fall outside the
spectrum of physical possibility. Its temperature, luminosity, age, and
location do not match up with any theory.

"This is a strange enough object that it needs a strange explanation," said
Eric Mamajek of the Harvard-Smithsonian Center for Astrophysics (CfA).

The announcement was made in a press conference at the 211th meeting of the
American Astronomical Society.

2M1207B orbits a 25-Jupiter-mass brown dwarf called 2M1207A seen in the
direction of the constellation Centaurus. Computer models show that 2M1207A
is very young, only about 8 million years old; therefore its companion
should also be 8 million years old. At that age, it should have cooled to a
temperature of less than 1300 degrees Fahrenheit (1000 Kelvin). However,
observations show that 2M1207B is actually about 2400 degrees F (1600 K).
The extra heat might be the result of a protoplanetary collision.

"Most, if not all, planets in our solar system were hit early in their
history. A collision created Earth's moon and knocked Uranus on its side,"
explained Mamajek. "It's quite likely that major collisions happen in other
young planetary systems, too."

Given its temperature, astronomers would expect a certain luminosity for
2M1207B, but it is 10 times fainter than expected. In 2006, astronomers
suggested that it is obscured by a dusty, edge-on disk. Mamajek and his
colleague, Michael Meyer of the University of Arizona, propose an
alternative explanation: 2M1207B is small, only about the size of Saturn,
and therefore has a smaller-than-expected surface area radiating energy.

They derive a radius of 31,000 miles (50,000 km) for 2M1207B, compared to
37,000 miles (60,000 km) for Saturn. Given typical densities for giant
planets, this would give 2M1207B a mass about 80 times Earth (or one-fourth
Jupiter). The only plausible way for such a small object to be so hot
millions of years after it formed is if it suffered a recent, titanic
collision that heated it.

The planets in our solar system assembled from dust, rock, and gas,
gradually growing larger over millions of years. But sometimes, two
planet-sized objects collided catastrophically. For example, the Moon formed
when an object about half the size of Mars hit the proto-Earth. If planet
formation works the same way in other star systems, then 2M1207B might be
the product of a collision between a Saturn-sized gas giant and a planet
about three times the size of Earth. The two smacked into each other and
stuck, forming one larger world still boiling from the heat generated in the
collision.

"The Earth was hit by something one-tenth its mass, and it's likely that
other planets in our solar system were too, including Venus and Uranus,"
explained Meyer. "If that one-tenth scale holds in other planetary systems,
then we could be seeing the aftermath of a collision between a 72 Earth-mass
gas giant and an 8 Earth-mass planet, even though such collisions are very
unlikely."

Mamajek also points out that the collision theory is reasonable from a
timescale point of view. A 2400-degree, Saturn-sized object would radiate
its heat away over about 100,000 years. If the system were billions of years
old, it is unlikely that we would be looking at the right time, but since
the system is young, the chances are much better that we would catch it
shortly after the collision while the hot aftermath is still observable.

The collision hypothesis makes several predictions that astronomers can
test. Chief among them is a low surface gravity (which depends on a planet's
mass and radius). To check this prediction, astronomers will need to get a
better spectrum of 2M1207B -- a challenge since it is very faint and very
close to the brown dwarf 2M1207A. Others are checking the dusty disk theory
by looking for signs of polarization in the light from 2M1207B. More answers
should be forthcoming within a year or two.

Mamajek emphasized that while a planet collision may not be the correct
explanation for the weirdness of 2M1207B, examples of colliding planets are
likely to be found by the next generation of ground-based telescopes.

"Hot, post-collision planets might be a whole new class of objects we will
see with the Giant Magellan Telescope."

"Even if we're wrong, I wouldn't be surprised if someone finds a clear-cut
case in the next 10 years," Mamajek added.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

[NOTE: An image supporting this release is available at
http://cfa-www.harvard.edu/press/2008/pr200801_images.html ]
Received on Thu 10 Jan 2008 06:09:33 PM PST