Giant Planet Detected Around White Dwarf Star for First Time

Astronomers have found the first evidence of a giant planet orbiting a white dwarf star.

White dwarfs are the small, dense remnants of stars that have shed their outer layers and spent most of their hydrogen and helium fuel. Ninety-seven percent of stars in our galaxy—including our sun—will meet their end in this way.

According to a study published in the journal Nature, the planet orbits the star—known as WDJ0914+1914—roughly once every 10 days. Intriguingly, the Neptune-like planet is about four times bigger than the star itself.

The finding is significant because, until now, astronomers had not found direct evidence of a planet that had survived the transition of a star into a white dwarf. This study puts an end to two decades of speculation that planets exist around white dwarfs, indicating that there could be many more that have yet to be identified, the researchers said.

The white dwarf in question was first discovered by an astronomical project known as the Sloan Digital Sky Survey. But the discovery of the planet orbiting the star happened, partially, by chance, according to Boris Gaensicke, lead author of the study from the University of Warwick in the U.K.

"One of our co-authors, Nicola Gentile Fusillo, was sifting through thousands of observations of white dwarfs, obtained by the Sloan Digital Sky Survey, for a large catalog he was working on," Gaensicke told Newsweek. "He flagged a few hundred that he wanted my opinion on, and I noticed a very weak signature of oxygen in one of them—barely noticeable—that I'd never seen before."

"We therefore decided to use the Very Large Telescope of the European Southern Observatory to get much better quality data, which almost immediately showed that this white dwarf is surrounded by a large disk of hydrogen, oxygen and sulfur. Talking to [co-author] Matthias Schreiber, he came with the idea that we may be looking at atmospheric material from a giant planet," he said.

At first, the researchers thought they were looking at a binary, or double, star system with a disk of material surrounding it.

"However, our observations show that it is a single white dwarf with a disc around it roughly ten times the size of our sun, made solely of hydrogen, oxygen and sulfur. Such a system has never been seen before, and it was immediately clear to me that this was a unique star," Gaensicke said in a statement.

WDJ0914+1914
The giant planet is orbiting the white dwarf just outside the gas disc. The blast of high-energy photons that it receives from the white dwarfs evaporates its atmosphere, which is mainly composed of hydrogen, oxygen and sulfur. University of Warwick/Mark Garlick

While the astronomers could not directly observe the planet, the extreme heat of the 28,000 C star is causing it to slowly evaporate. During this process, high-energy photons—or particles of light—being emitted from the white dwarf blow away hydrogen from the planet, creating a comet-like tail. Meanwhile, oxygen and sulfur—in addition to smaller amounts of hydrogen—from the planet fall towards the white dwarf, creating the disk of gas that was detected.

From this disk, the scientists were able to infer the existence of the planet because its composition closely matches those of the ice giants in our own solar system—Uranus and Neptune.

"This star has a planet that we can't see directly, but because the star is so hot it is evaporating the planet, and we detect the atmosphere it is losing. There could be many cooler white dwarfs that have planets but lacking the high-energy photons necessary to drive evaporation, so we wouldn't be able to find them with the same method," Gaensicke said.

"This discovery is major progress because over the past two decades we had growing evidence that planetary systems survive into the white dwarf stage," he said. "We've seen a lot of asteroids, comets and other small planetary objects hitting white dwarfs, and explaining these events requires larger, planet-mass bodies further out. Having evidence for an actual planet that itself was scattered in is an important step."

Giant Planet Detected Around White Dwarf Star for First Time | Tech & Science