The star, WD 1145+017, and planet were found by NASA’s Kepler K2 mission. When astronomers look for planets far away from Earth, focus on the star. Any dip in brightness usually means a planet. In this case, data showed a consistent dip every 4.5 hours.
Using that, astronomers can figure out the planet orbits the white dwarf star at a distance of about 520,000 miles. That’s close, real close. 520,000 miles is about twice the distance from the Earth to the Moon.
The discovery of this system marks the first time a planetary object has been seen transiting a white dwarf. It also helps confirm a theory about why white dwarf stars sometimes show signs of heavier elements in its light spectrum.
First, let’s briefly talk white dwarf stars. As a sun-like star approaches the end of its life, it swells and turns into a red giant. Then, it sheds its outer layers – leaving a hot, Earth-sized core behind. The star is now a white-dwarf star. Now, white dwarf stars are usually made up of carbon and oxygen with a thin hydrogen or helium shell.
But occasionally, astronomers will see signs of heavier elements like silicon and iron in its light spectrum. What’s the big deal? Well, the gravity of a white dwarf is so strong that these heavier elements should sink in the star’s interior.
“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.
Astronomers didn’t have a concrete answer for why metals would be on the surface. But they did have a theory. Heavy element ‘pollution’ could be explained when white dwarf stars consumed rocky debris (planets). There was just one problem. Astronomers didn’t have the evidence to back it up. A small number of polluted white dwarfs showed evidence of surrounding debris disks, but the origin of the disks was always unclear.
This system has it all. A polluted white dwarf star, a 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.
Here’s Vanderburg writing about what they found. “We saw two beautiful, 40% deep, asymmetric transits separated by the 4.5 hour period we saw in K2. At this point, we knew that what we had been seeing was real, and we realized that the asymmetric shape of the transits indicated strongly that we weren’t watching the transits of a normal planet – we were in fact witnessing a small rocky object disintegrating.”
More than likely, the planet’s orbit was kicked inward by the extreme gravity of the white dwarf star. And the planet’s days are numbered. The planet is orbiting perilously close to the white dwarf star. Observations indicate the white dwarf’s pull is already ripping the planet apart. Over the next million years, small pieces of asteroid will be all that remains of the planet.
Research will continue on WD 1145+017. Astronomers want to know more about the debris field orbiting the star. Is it from multiple fragments of the planet? The data hints at it, but follow-up observations are needed.