Weird Object May Be Result of Colliding Protoplanets
Heat from a titanic whack explains the extra energy

Lori Stiles
Jan. 9, 2008


Something bizarre orbiting a young, failed star 170 light-years from Earth may be the progeny of two protoplanets that collided and merged, astronomers announced at the American Astronomical Society meeting in Austin, Texas, today.

Given its hotter-than-expected temperature, dim luminosity, young age and location, the orbiting object, known as 2M1207B, should be a physical impossibility, scientists say.

“This is a strange enough object that it needs a strange explanation,” Eric Mamajek of the Harvard-Smithsonian Center for Astrophysics said.

Mamajek and Michael Meyer of The University of Arizona propose that the object orbiting brown dwarf 2M1207A is small, about the size of Saturn. The "brown dwarf," or failed star, in the system is believed to be 25 times as massive as Jupiter and only about eight million years old.

The brown dwarf might be the outcome of a collision between a Saturn-sized gas giant and a planet about three times the size of Earth, Mamajek and Meyer suggest. The two smacked into each other and fused, forming one larger world still boiling from the heat generated in the titanic collision.

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

“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,” Meyer said. “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 eight Earth-mass planet.”

The collision theory is reasonable from a timescale point of view, Mamajek said. A 2400-degree Fahrenheit, 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 astronomers would be looking at the right time, but because the system is only eight million years old, chances are much better that they would catch it shortly after the collision, when they could still see the hot aftermath.

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. That's challenging because the object is very faint and very close to the brown dwarf 2M1207A.

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.”

Harvard and UA are members of an international consortium building the Giant Magellan Telescope, which is slated for completion in 2016 at a site in northern Chile. It will be composed of seven 8.4-meter primary mirrors arranged in a hexagonal pattern, giving it 4.5 times the collecting area of any current optical telescope. The UA Steward Observatory Mirror Lab cast the first GMT mirror in 2005.

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

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