Deep in the sun’s broiling core, hydrogen atoms vibrate with energy. They collide and combine at high speeds, pumping out helium and enough heat to force temperatures up to 27 million degrees Fahrenheit. But billions of years from now, the sun will burn through all of its nuclear fuel and shrivel into a stellar corpse known as a white dwarf.
Debate has been swirling about what will happen to the planets when our sun dies. Now, observations from NASA’s Kepler spacecraft back up theories that the fading star will consume Earth and any other rocky bodies that remain in its deadly orbit.
Since its launch in 2009, the Kepler space telescope has been gazing at a patch of stars, watching for blips in their otherwise steady glow that signal an orbiting body. When Andrew Vanderburg observed a telltale dip around the orbit of a white dwarf star, dubbed WD 1145+017, he was ecstatic. No one had yet found a planet zipping around one of these stellar remnants.
But unlike the expected regularity of an orbiting planet, the star had an asymmetric flicker, with its light first sharply decreasing and then slowly fading away, says Vanderburg. The scientists soon realized they were observing the long dusty streaks trailing behind a disintegrating rocky body—a planet in the process of being consumed.
“The white dwarf was ripping it apart by its extreme gravity and turning it into dust,” says Vanderburg, a graduate student at the Harvard-Smithsonian Center for Astrophysics.
When stars like our sun run out of hydrogen fuel, they start to cool and expand into red giants. When this happens to our sun, it will puff up enough to engulf Mercury, Venus and possibly Earth, explains Vanderburg, although there is a chance our planet will get scorched but survive intact.
As the star’s fuel reaches its end, “it eventually gives up,” says Vanderburg. At this point, the core of the star collapses in on itself, releasing its outer layer in a spectacular farewell nebula. The remnant core becomes a dense white dwarf with staggeringly strong gravity. The white dwarf Sirius B, for instance, has a surface gravity 350,000 times that of Earth.
Scientists have speculated that this altered gravitational pull would set the orbits of any remaining planets awry. Collisions would smash up the rocky orbs and catapult their pieces into the white dwarf. And any planets that simply wandered too close to the dead star would get drawn in and shredded apart.
Astronomers have seen signs of this grisly fate in observations of "polluted" white dwarfs. The star's intense gravity should pull any heavy elements down to its core in a matter of a million years or so, leaving a pure outer layer of hydrogen and helium.
But the atmospheres of roughly one-third of the known white dwarfs contain a sprinkling of silicon, aluminum, iron and magnesium—a chemical mix common to rocky planets.
As they report this week in Nature, Vanderburg and his colleagues found this same elemental pollution on WD 1145+017. Even more exciting, they can see the signs of disintegrating rocky bodies as they pass in front of the star—the first time anyone has spotted this catastrophic destruction in action.
The discovery supports the notion that even if Earth and Mars survive the sun's bloated red giant phase, they will likely be consumed when our star becomes a white dwarf.
There are other ways to explain the observation, though, says Martin A. Barstow, president of the Royal Astronomical Society in the U.K. It's unclear whether the disintegrating stuff belonged to one planet about the size of our moon or a cluster of asteroids. It's also possible a smudge of debris dispersing on the white dwarf's surface caused the fluctuating dip in starlight.
Even so, “it’s a key result,” says Barstow, who studies white dwarf composition.
He hopes that this study marks the beginning of more data to come. Catching a dying star while it consumes a rocky planet had been seen as an unlikely feat. “I’m still a bit nervous—it is such a nice experiment—there isn’t a flaw,” Barstow laughs.