In
this artist's conception, a Ceres-like asteroid is slowly
disintegrating as it orbits a white dwarf star. Astronomers have spotted
telltales signs of such an object using data from the Kepler K2
mission. It is the first planetary object detected transiting a white
dwarf. Within about a million years the object will be destroyed,
leaving a thin dusting of metals on the surface of the white dwarf. Credit: Mark A. Garlick / markgarlick.com . High Resolution (jpg) - Low Resolution (jpg)
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).
Theorists speculated that white dwarfs showing evidence of heavy
metals became "polluted" when they consumed rocky planets or asteroids.
However, the evidence was often circumstantial. A fraction of polluted
white dwarfs showed signs of surrounding debris disks, but the origin of
the disks was uncertain. This system shows all three: a polluted white
dwarf, a surrounding debris disk, and at least one compact, rocky
object.
"We now have a 'smoking gun' linking white dwarf pollution to the destruction of rocky planets," says Vanderburg.
Questions remain about the origin of these rocky objects. The most likely scenario is that an existing planet's orbit became unstable and it was kicked inward.
What is certain is that the remaining objects will not last forever. They are being vaporized by the intense heat of the white dwarf. They also are orbiting very close to the tidal radius, or distance at which gravitational tides from the white dwarf can rip apart a rocky body. Within the next million years or so, all that will remain of these asteroidal bits is a thin metal dusting on top of an innocent-looking white dwarf star.
This discovery will be published in the Oct. 22, 2015, issue of the journal Nature.
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.
For more information, contact:
Christine Pulliam
Media Relations Manager
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu
