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Zombie white dwarf star caught destroying an orbiting planet

It's the first time astronomers have found a planet around a white dwarf

Mark Garlick

More than 570 light years away in the constellation Virgo, a disintegrating planet orbits around a white dwarf — the leftovers of a yellow star after it died. The cause of the planet's demise is the zombie star itself; the white dwarf is extremely dense, and its enormous gravitational pull is tearing the rock apart, creating an enormous cloud of dust and debris that follows the planet on its orbit.

This dance of the dead was observed by NASA's Kepler spacecraft last year and is described today in the journal Nature. The discovery of the planetary system is the first of its kind. It helps to confirm what many scientists have suspected for years: that planets can actually orbit white dwarfs. Experts estimate that at least 15 percent of white dwarfs have planets or debris around them, but no one has ever observed a planet passing in front of this type of star before. "This is the first time we’ve caught that process in action," said lead author Andrew Vanderburg, an astronomer at the Harvard Smithsonian Center for Astrophysics.

This dance of the dead was observed by NASA's Kepler spacecraft

Finding this system could help strengthen the search for intact planets orbiting white dwarfs, which could drastically help the search for extraterrestrial life. Some astronomers think that Earth-like planets could be circling white dwarfs at just the right distance, where the temperature is ideal for water to pool and life to exist. Planets like this would be roughly the same size as the white dwarf they orbit, making them easy to see; they'd block out a ton of light as they passed in front of the star, allowing researchers to easily find these planets and study what's on their surface.

Planets around white dwarfs

White dwarf Sirius B. (NASA)

A planet that orbits a white dwarf in its habitable zone has likely survived through a lot of chaos. White dwarfs are the remnants of dead stars. After a regular star uses up all its nuclear fuel, it turns into a red giant, expanding outward to 100 times its original size. Any planets that might have been orbiting closely around the star are engulfed by the giant and spiral down into the star's core. This will likely happen to Earth when our Sun dies sometime in the next 5 billion years.

Eventually, a red giant will start to contract and shed its outer layers, growing much smaller and more dense. The end result is a white dwarf — a compact leftover of the original star's inner core. White dwarfs are much fainter than the stars they once were but still radiate a great deal of heat. They begin to cool after a few billion years, though, and at some point reach the same surface temperature as our Sun.

White dwarfs are the remnants of dead stars

It's possible that any outer planets that were originally orbiting the star may still remain around the white dwarf. However, such a major shift in the star's mass may throw these planets off their original paths a bit. "The mass of the star is what determines the orbits of the planets," said Vanderburg. "When the star's mass changes, the orbits change." Vanderburg said it's possible that this may cause some of the outer planets to bump into each other, and one could "kick" the other into an orbit much closer around the dwarf. This is how a planet could theoretically wind up in the star's habitable area.

It's a very particular process, with a lot of caveats — but it's possibly how the white dwarf star system discovered by Vanderburg came to be.

Decaying world

This white dwarf — given the eloquent name of WD 1145+017 — was spotted by NASA's Kepler space telescope as part of the spacecraft's K2 mission. For this mission, Kepler stares at a patch of sky continuously for 80 days at a time. Between July and early September of last year, Kepler captured an interesting signal from WD 1145+017 that sparked Vanderburg's attention. The star dimmed a bit, making it seem like something might be passing in front of it.

Vanderburg and his team started following the star from ground-based telescopes to see if they could confirm something was transiting in front of the star. Sure enough, the star dimmed periodically, indicating that something was moving in front of it — but the lengths of these dimming periods weren't completely uniform; sometimes they lasted longer than expected and they didn't always happen at the exact same times. Normally, if a planet is passing in front of a star, it dims the star in a very predictable pattern, because the planet's orbit is symmetric and stable.

Instead, the researchers believe that the dimming they saw was caused by the cometary dust tail coming off a smaller disintegrating planet. They think this planet got "kicked" a little too close to WD 1145+017, and the white dwarf's gravity is pulling the object apart. The debris created by the destruction has turned into a giant dust cloud, which periodically blocks out the light coming from the white dwarf. "The dust becomes a much, much larger cloud than the smaller core of the planet, and it's easier to see big things than smaller things," said Vanderburg.

Alien life?

An artist conception of a polluted planet orbiting around a white dwarf. (Christine Pulliam/Harvard)

If alien live did exist around a white dwarf, it'd be much easier to find than planets elsewhere. Since WD 1145+017 is tearing apart this particular planet, chances are no life exists on it. But it's possible if the planet orbited a little farther outward, life could survive in such a zone. "This is a planet that is a little too close," said Avi Loeb, an astronomy professor at Harvard, who was not involved in this study. "If you just go two times closer than the habitable zone, then you’re in danger of being tidally disrupted by the gravity of the white dwarf."

If such a planet were to exist in the habitable zone around a white dwarf, Loeb says it'd be very easy to study and characterize. Astronomers could study the light that shines through the planet's atmosphere as it passes in front of the white dwarf, to see what gases the air contains. This is nearly impossible to do for exoplanets orbiting typical stars, because they get almost completely drowned out by the star's brightness.

Alien life would be much easier to find on planets around white dwarfs

Studying a planet's atmosphere could tell us a lot about what the world contains. Loeb wrote a couple papers about how certain gases in a planet's atmosphere may indicate the presence of alien life. "The amount of oxygen in the atmosphere of Earth, if there was no life, it would be diminished in a million years or so," said Loeb. "The main reason we have oxygen in the atmosphere is because we have life." Loeb suggests that gases like methane or oxygen — which typically arise from organic matter — may indicate that primitive life may have existed on an exoplanet at some point.

Loeb also thinks it might be possible to find intelligent life on an exoplanet by searching for chemical pollutants in the atmosphere. It's possible that an alien civilization's technology would pump a lot of artificial gases into the air. But in order to detect this type of gas, Loeb says the pollution would have to be 10 to 100 times more massive than the amount of air pollution we have on Earth. "You might say if they pollute so much, they’re not so intelligent," he said. "But maybe the planet is a little farther out from the star, so they may actually do it to artificially warm up the planet. It's possible they produce a thick layer of pollution so the planet is warmed up and the conditions for life are better."

These are just theories for now, since no one has had the chance to study an exoplanet around a white dwarf. But Loeb and Vanderburg hope the discovery of this decaying world will inspire others to study white dwarfs even further. "White dwarfs are not exotic; they're not something that is rare," said Loeb. "Hopefully this will to motivate people to search for planets around white dwarfs."