A white dwarf's surprise planetary companion

For the first time, an intact, giant exoplanet has been discovered orbiting close to a white dwarf star. This discovery shows that it is possible for Jupiter-sized planets to survive their star's demise and settle into close orbits around the remaining stellar ember, near the habitable zone. This foretells one possible future for our own Solar System when the Sun ages into a white dwarf.

Astronomers have used the international Gemini Observatory, a Program of NSF's NOIRLab, and other telescopes around the globe and in space to find and characterize a giant planet, less than 13.8 times as massive as Jupiter, orbiting a white dwarf star. The research is published in the journal Nature.

This is the first example of an intact giant planet orbiting close to a white dwarf star - in this case a particularly cool and dim stellar ember known as WD 1856+534. "The discovery came as something of a surprise," according to lead author Andrew Vanderburg, assistant professor at the University of Wisconsin-Madison. "A previous example of a similar system, where an object was seen to pass in front of a white dwarf, showed only a debris field from a disintegrating asteroid."

After detecting the planet with the TESS satellite, which observed it transiting its white dwarf star, the team took advantage of the tremendous light-collecting power of Gemini North's 8.1-meter mirror and used the sensitive Gemini Near-Infrared Spectrograph (GNIRS) to make detailed measurements of the white dwarf star in infrared light from Maunakea, Hawai'i. The spectroscopic observations captured the unique fingerprint of the star, but not that of the planet or any debris surrounding this system.

"Because no debris from the planet was detected floating on the star's surface or surrounding it in a disk we could infer that the planet is intact," said Siyi Xu, an assistant astronomer at Gemini Observatory and one of the researchers behind the discovery.

"We were using the TESS satellite to search for transiting debris around white dwarfs, and to try to understand how the process of planetary destruction happens," explains Vanderburg. "We were not necessarily expecting to find a planet that appeared to be intact."

"Additionally, because we didn't detect any light from the planet itself, even in the infrared, it tells us that the planet is extremely cool, among the coolest we've ever found."

Xu adds that the precise upper limit of the planet's temperature was measured by NASA's Spitzer Space Telescope to be 17C (63F), which is similar to the average temperature of Earth.

"We've had indirect evidence that planets exist around white dwarfs and it's amazing to finally find a planet like this," said Xu. White dwarfs are extremely dense and very small, so the exoplanet is much larger than its tiny parent star, making the system extremely unusual.

The surprising discovery of this planet, known as WD 1856b, raises interesting questions about the fate of planets orbiting stars destined to become white dwarfs (like our Sun). Of the thousands of planets outside the Solar System that astronomers have discovered, most orbit stars that will eventually evolve into red giants and then into white dwarfs. During this process, any planets in close orbits will be engulfed by the star, a fate that WD 1856b somehow managed to avoid.

"Our discovery suggests that WD 1856b must have originally orbited far away from the star, and then somehow journeyed inwards after the star became a white dwarf," said Vanderburg. "Now that we know that planets can survive the journey without being broken up by the white dwarf's gravity, we can look for other, smaller planets."

"The study of planets in extreme locations is giving us new perspectives on the history and fate of the billions of worlds around other stars," said Martin Still, NSF Program Director for the international Gemini Observatory partnership. "Gemini's sensitivity was critical in following up the TESS space-based detection of this planet, revealing a more complete story of the exoplanetary system."

This new discovery suggests that planets can end up in or near the white dwarf's habitable zone, and potentially be hospitable to life even after their star has died. "We're planning future work to study this planet's atmosphere with Gemini North," concludes Xu. "The more we can learn about planets like WD 1856b, the more we can find out about the likely fate of our own Solar System in about 5 billion years when the Sun becomes a white dwarf."

KU astronomer helps confirm first-ever planet found orbiting white dwarf
Lawrence KS (SPX) Sep 17 - A University of Kansas astronomer played a key role on the team that today announced the first-ever discovery of a planet orbiting a white dwarf. The finding, published in Nature, shows the likely presence of a Jupiter-sized planet, named WD 1856 b, orbiting the smaller star remnant every 34 hours.

"This planet is roughly the size of Jupiter, but it also has a very short orbital period - a year on this planet is only 1.4 days, so it's quickly whipping around its white dwarf star," said Ian Crossfield, assistant professor of physics and astronomy at KU, who is a co-author on the paper.

A white dwarf is the vestige of a star, like our Sun, that has ballooned into a red giant then collapsed back into a dense, dim core that's often about the size of Earth; so this planet is much larger than what's left of its star! The process usually devours orbiting planets - but not in the case of WD 1856 b, which appears somehow to have avoided destruction.

"This tells us white dwarfs can have planets, which is something we didn't know before," Crossfield said.

"There are people who now are looking for transiting planets around white dwarfs that could be potentially habitable. It'd be a pretty weird system, and you'd have to think about how the planets actually survived all that time. But it's a big universe! Now we at least know some kinds of planets can survive and be found there, so that gives greater support and greater interest in continuing the search for even smaller planets around these white dwarfs."

At first, WD 1856 b captured astronomers' interest when they noticed a possible transiting object with NASA's TESS Space Telescope survey.

"TESS finds a planet by looking at a star, and it measures how bright the star is continuously for weeks," he said. "If a planet is orbiting the star, and if the planet passes between you and the star, some of that star's light is going to be blocked.

Then the star will get brighter again as the planet passes - we call this the 'transit' of the planet. And so, TESS looks for transiting exoplanet satellites. It tells you that something is there - but it doesn't necessarily tell you what it is because it could be another dim star passing in front instead of a planet."

To help the international team of scientists confirm if WD 1856 b indeed was a planet orbiting the white dwarf, Crossfield studied the object's infrared emissions with NASA's now-defunct Spitzer Space Telescope in the months leading up to the satellite telescope's decommission.

"For this white dwarf object, it's tough to measure the mass of it - so we knew how big it was, but not how heavy it was," he said.

"This new object could have been a small star or a big planet. The way we could tell the difference was to look and see - is this thing emitting infrared light as well? If it's a star, stars are generally hotter than planets and it should be glowing in the infrared.

But if it's just a planet, planets are generally colder than stars and so there should be a little or no infrared light. What our Spitzer data showed is there's basically no infrared light at all. And the depths of these transits are identical between the TESS data, and our Spitzer datasets. That really put the final nail in the coffin that this thing is almost certainly a planet, rather than a star."

WD 1856 b is located about 80 light years away in the northern constellation Draco. The team behind the paper believes the gaseous planet was pulled in by the white dwarf's gravity long after the star had dwindled down from its red giant phase - otherwise the planet would have been obliterated in its current orbit.

Asked if the discovery of the first planet orbiting a white dwarf meant the Earth stood a chance of surviving the sun's red-giant phase in the distant future, Crossfield said it remained unlikely.

"In around five billion years our Sun will become a white dwarf. There's a lot of open questions about whether planets can survive the process of a star inflating up to become a red giant, swallowing up some of the inner planets, and then shrinking back down and just being left over as the white dwarf again. Can planets actually survive that - or is that impossible? And until now, there weren't any known planets around white dwarfs."

Research Report: A Giant Planet Candidate Transiting a White Dwarf

Related Links
Association Of Universities For Research In Astronomy (AURA)
University Of Kansas
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