Dying sun caught tearing apart its own asteroids (Science)

The Kepler Space Telescope has detected disintegrating asteroids orbiting a white dwarf, the type of burned-out star our sun will become about 8 billion years from now. The discovery explains why some white dwarfs have heavy elements on their surfaces and also gives us a possible preview of Earth's grisly fate.
"It's really amazing," says Jay Holberg, an astronomer at the University of Arizona in Tucson, who was not involved in the discovery. "We've never seen this before for a white dwarf."
A typical white dwarf is nearly as massive as the sun but only slightly larger than Earth, so the star exerts a strong gravitational pull at its surface: Drop a rock from a height of 1 meter and it would hit the star at thousands of kilometers per hour. The strong gravitational force should also yank all elements heavier than helium beneath the star's surface, yet the surfaces of many white dwarfs nevertheless possess heavy elements, suggesting that asteroids deposit elements such as silicon and iron.
Now, for the first time, researchers have seen this scenario unfold. Andrew Vanderburg, an astronomer at the Harvard-Smithsonian Center for Astrophysics, was analyzing data from Kepler, which detects planets when they block the light of their sun. "One of the white dwarfs suddenly popped up with this really intriguing signature," Vanderburg says. As his team reports online today in Nature, the white dwarf, located in the constellation Virgo and named WD 1145+017, has at least one, and probably several, asteroids that are disintegrating. As a debris cloud from each asteroid passes between us and the star, Kepler detects a dimming of the star's light.
"It's fascinating," says astronomer Michael Jura of the University of California, Los Angeles, who was not part of the discovery team. "They've actually caught in the act the process of some asteroid breaking into pieces, being disrupted by the white dwarf host star."
Indeed, the star itself is the asteroids' enemy. Its gravity has torn them asunder, and its light is vaporizing their rock. The asteroids are so close to the star that they revolve in just 4.5 to 4.9 hours; Vanderburg estimates they are roughly the size of Ceres, the largest asteroid between the orbits of Mars and Jupiter.
Billions of years from now, our sun will expand into a red giant, engulfing and incinerating Mercury and possibly Venus and Earth. Then the red sun will eject its outer layers and expose its hot core, which will contract into a white dwarf. Even if the sun never engulfs Earth, the drama may destabilize orbits in the solar system so that asteroids crash into our world and grind it up. Thus, the newly discovered asteroids could conceivably be the wreckage of a planet that once resembled our own.

NASA's K2 Finds Dead Star Vaporizing a Mini 'Planet' (NASA & Nature)

Vanderburg et al.¹ analysed data from the white dwarf star WD 1145+017 taken by the Kepler space observatory and ground-based telescopes. The data revealed transit features (periodic attenuations of the stellar brightness) that are best explained by the passage in front of the star of one or several disintegrating minor planets. The authors performed simulations of this process in which a minor planet that orbits very close to the star loses mass in the form of dust particles that generate leading and trailing cometary tails (colours indicate dust density). The various phases of the transit (leading tail, minor planet core and trailing tail) induce attenuations of the stellar brightness of different magnitudes and durations. Adapted from Fig. S7 of ref. 1.


Scientists using NASA's repurposed Kepler space telescope, known as the K2 mission, have uncovered strong evidence of a tiny, rocky object being torn apart as it spirals around a white dwarf star. This discovery validates a long-held theory that white dwarfs are capable of cannibalizing possible remnant planets that have survived within its solar system.
"We are for the first time witnessing a miniature "planet" ripped apart by intense gravity, being vaporized by starlight and raining rocky material onto its star," said Andrew Vanderburg, graduate student at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and lead author of the paper published in Nature.
As stars like our sun age, they puff up into red giants and then gradually lose about half their mass, shrinking down to 1/100th of their original size to roughly the size of Earth. This dead, dense star remnant is called a white dwarf.
The devastated planetesimal, or cosmic object formed from dust, rock, and other materials, is estimated to be the size of a large asteroid, and is the first planetary object to be confirmed transiting a white dwarf. It orbits its white dwarf, WD 1145+017, once every 4.5 hours. This orbital period places it extremely close to the white dwarf and its searing heat and shearing gravitational force.
During its first observing campaign from May 30 to Aug. 21, 2014, K2 trained its gaze on a patch of sky in the constellation Virgo, measuring the minuscule change in brightness of the distant white dwarf. When an object transits, or passes in front of a star from the vantage point of the space telescope, a dip in starlight is recorded. The periodic dimming of starlight indicates the presence of an object in orbit about the star.
A research team led by Vanderburg found an unusual, but vaguely familiar pattern in the data. While there was a prominent dip in brightness occurring every 4.5 hours, blocking up to 40 percent of the white dwarf's light, the transit signal of the tiny planet did not exhibit the typical symmetric U-shaped pattern. It showed an asymmetric elongated slope pattern that would indicate the presence of a comet-like tail. Together these features indicated a ring of dusty debris circling the white dwarf, and what could be the signature of a small planet being vaporized.
"The eureka moment of discovery came on the last night of observation with a sudden realization of what was going around the white dwarf. The shape and changing depth of the transit were undeniable signatures," said Vanderburg.
In addition to the strangely shaped transits, Vanderburg and his team found signs of heavier elements polluting the atmosphere of WD 1145+017, as predicted by theory.
Due to intense gravity, white dwarfs are expected to have chemically pure surfaces, covered only with light elements of helium and hydrogen. For years, researchers have found evidence that some white dwarf atmospheres are polluted with traces of heavier elements such as calcium, silicon, magnesium and iron. Scientists have long suspected that the source of this pollution was an asteroid or a small planet being torn apart by the white dwarf's intense gravity.
Analysis of the star's atmospheric composition was conducted using observations made by the University of Arizona's MMT Observatory near Tucson.
"For the last decade we've suspected that white dwarf stars were feeding on the remains of rocky objects, and this result may be the smoking gun we're looking for," said Fergal Mullally, staff scientist of K2 at SETI and NASA's Ames Research Center in Moffett Field, California. "However, there's still a lot more work to be done figuring out the history of this system."
"This discovery highlights the power and serendipitous nature of K2. The science community has full access to K2 observations and is using these data to make a wide range of unique discoveries across the full range of astrophysics phenomena," said Steve Howell, K2 project scientist at Ames