Wednesday, October 21, 2015

White Dwarf Spotted Tearing Apart a Terrestrial Exoplanet, Consuming the Comet-like World

The Death Star of the movie Star Wars may be fictional, but planetary destruction is real. Astronomers announced today that they have spotted a large, rocky object disintegrating in its death spiral around a distant white dwarf star. The discovery also confirms a long-standing theory behind the source of white dwarf "pollution" by metals.

"This is something no human has seen before," says lead author Andrew Vanderburg of the Harvard-Smithsonian Center for Astrophysics (CfA). "We're watching a solar system get destroyed."

The evidence for this unique system came from NASA's Kepler K2 mission, which monitors stars for a dip in brightness that occurs when an orbiting body crosses the star. The data revealed a regular dip every 4.5 hours, which places the object in an orbit about 520,000 miles from the white dwarf (about twice the distance from the Earth to the Moon). It is the first planetary object to be seen transiting a white dwarf.

Vanderburg and his colleagues made additional observations using a number of ground-based facilities: the 1.2-meter and MINERVA telescopes at Whipple Observatory, the MMT, MEarth-South, and Keck.

Combining all the data, they found signs of several additional chunks of material, all in orbits between 4.5 and 5 hours. The main transit was particularly prominent, dimming the star by 40 percent. The transit signal also showed a comet-like pattern. Both features suggest the presence of an extended cloud of dust surrounding the fragment. The total amount of material is estimated to be about the mass of Ceres, a Texas-sized object that is the largest main-belt asteroid in our solar system.

The white dwarf star is located about 570 light-years from Earth in the constellation Virgo. When a Sun-like star reaches the end of its life, it swells into a red giant and sloughs off its outer layers. The hot, Earth-sized core that remains is a white dwarf star, and generally consists of carbon and oxygen with a thin hydrogen or helium shell.

Sometimes, though, astronomers find a white dwarf that shows signs of heavier elements like silicon and iron in its light spectrum. This is a mystery because a white dwarf's strong gravity should quickly submerge these metals.

"It's like panning for gold - the heavy stuff sinks to the bottom. These metals should sink into the white dwarf's interior where we can't see them," explains Harvard co-author John Johnson (CfA).

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