Astronomers Discover First Known Planet to Orbit a White Dwarf Star

Artist's depiction of a Jupiter-sized planet in orbit around a white dwarf. Incredibly, the two objects in the image are actually close to scale, as white dwarfs are roughly the size of Earth.  (Image: NASA/JPL)
Artist's depiction of a Jupiter-sized planet in orbit around a white dwarf. Incredibly, the two objects in the image are actually close to scale, as white dwarfs are roughly the size of Earth. (Image: NASA/JPL)

The unprecedented discovery of a Jupiter-sized planet in orbit around a white dwarf star suggests it’s possible for planets to survive the tumultuous death throes of their parent stars.

Imagine a dim, late-stage star roughly the size of Earth with a gigantic Jupiter-sized planet orbiting around it every 34 hours. It would be a bizarre sight, with the smaller but denser object seemingly in control of the visually mismatched celestial configuration. A white dwarf could fit inside Jupiter’s Great Red Spot, to give you a sense of how odd this star system would look.

Such a system actually exists, according to new research published today. As Ian Crossfield, an assistant professor of physics and astronomy at the University of Kansas and a co-author of the study, explained in an email, it’s “the first clear discovery of a planet orbiting a white dwarf.” The new study, published in Nature, was led by astronomer Andrew Vanderburg from the University of Wisconsin-Madison.

In 2015, a Nature study led by Vanderburg chronicled the discovery of a small planet, or possibly an asteroid, that was in the process of being shredded by its white dwarf host. The new finding is unique in that the planet is large, cohesive, and in a fairly stable orbit.

School children are often horrified to learn about the fate of our solar system. A few billion years from now, our Sun will start to run out of fuel, causing it to swell into an enormous red giant. In the process, all the inner planets — Earth included — will be devoured by our bloated dying star. Once this phase is complete, the red giant will shrink into a white dwarf — an orb roughly the size of Earth but containing half the mass of our current Sun. Technically still a star, this white dwarf will still give off some heat and light as it continues to cool, though it will no longer be capable of conducting nuclear fusion.

Such has been, and will be, the fate of similar stars throughout the cosmos. Astronomers weren’t sure if planets, especially the outer planets, are able to survive this highly disruptive process, but the new research suggests they can. This finding should now inspire astronomers to search for similar objects around other white dwarfs.

To detect this white dwarf planet, named WD 1856b, the astronomers used the tried-and-true transit method, along with some infrared scans of the system. Using NASA’s Tess Space Telescope, the team recorded the dimming of the white dwarf once every 1.4 days. This dimming is a potential sign of an orbiting planet transiting in front of a star from our perspective on Earth. The authors confirmed this with infrared data gathered by NASA’s Spitzer’s Space Telescope prior to the satellite’s retirement this past January. By analysing this system in infrared wavelengths, the researchers saw that it was a planet, and not another star, in orbit around the white dwarf.

Artist's conception of the white dwarf (foreground) and its planet. (Image: NASA/JPL-Caltech/NASA’s Goddard Space Flight Centre)

“Hotter stars should emit lots of infrared light, while cooler planets should emit less,” Crossfield said. “We saw no extra infrared light, which helped confirm this new discovery as a real planet.”

Several ground-based telescopes were also recruited to confirm these observations, which was a necessity, given that the glare from a nearby star had clouded the TESS data.

The authors speculate that WD 1856b survived its star’s transformation into a red giant owing to a large distance between the two objects.

“The planet could not have survived if it started out where we see it now: it must have been orbiting much farther from the star,” explained Crossfield. “Later, after the star became a white dwarf, the planet must have moved closer to the star.”

Simulations run by the researchers suggests this is a likely scenario: As the red giant devoured the inner planets, WD 1856b’s orbit became destabilized, causing it to enter into a highly elliptical orbit, taking the planet both very close to and very far from its dying parent star. This orbit shrunk over the course of cosmological timescales, placing the planet into its current tight, circular orbit.

“We think this star died and became a white dwarf roughly 6 billion years ago — so long ago that the Sun, Earth, and solar system hadn’t even been formed yet,” said Crossfield.

We asked Crossfield if it’s possible that the dying star captured a rogue planet, either during its red giant or white dwarf phase. A recent study suggests trillions of rogue planets could be careening through the Milky Way, untethered from the stars they once circled.

“It’s not impossible, but not especially likely either,” he replied. “We might be able to tell for sure by measuring the planet’s atmospheric composition with the James Webb Space Telescope, once it launches late next year.”

Indeed, WD 1856b is in a fairly stable orbit, and it’s reasonably close, at 80 light-years from Earth, so it should be possible for scientists to study it in the years to come. The only things known about this planet are its size (roughly the size of Jupiter, but it could be larger), its orbital period (34 hours), and approximate temperature (well below freezing). Future observations could “could tell us more about what it’s made of, how it formed, and perhaps how it got to be where we see it today,” said Crossfield.

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As an interesting aside, white dwarfs go through yet other transformations, becoming even dimmer brown dwarfs and eventually black dwarfs, which no longer emit any heat or light. As recent research suggests, the largest of these black dwarfs will eventually explode as supernovae, but that won’t happen until trillions upon trillions of years from now.