Around 80 light-years from Earth lies the white dwarf WD 1856, a dead star that entered the final stages of its life around 6 billion years ago. This slow death is typically quite lonely. In the process of dying, some stars will drastically expand, becoming a huge “red giant,” like Betelgeuse, and engulfing any of the planets orbiting close by. Eventually, they use up all their fuel and collapse back into white dwarfs, having destroyed everything in their wake.
Not so for WD 1856. For the first time, astronomers have detected a giant planet, about the size of Jupiter, orbiting the dead star. They’ve dubbed it WD 1856 b and it’s a surprising find — it avoided destruction and demonstrates dead stars could still host planets with the right conditions for life.
The study, published in the journal Nature on Wednesday, used data obtained by NASA’s planet-hunting TESS satellite and a suite of ground-based telescopes to examine WD 1856 for potential exoplanets. TESS, which examines stars for small dips in brightness signifying potential planets, first looked at the star across July and August 2019. A huge reduction in brightness was seen when the team looked at WD 1856.
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Astronomers have recently begun to grapple with the idea these dead stars may still host a number of planets. In December, researchers detected a planet that was slowly being devoured by a white dwarf about 1,500 light-years away. However, that detection was based on light being emitted by a disk of debris and gas surrounding the star, which the researchers suggest must have been stripped from a Neptune-like planet.
The discovery published in Nature today is different because it records a direct detection of the planet orbiting in front of its host star, which has not previously been achieved for a white dwarf.
Every time the Jupiter-size planet transits in front of WD 1856, as seen from Earth, the light from the star drops away by almost half. The process is incredibly brief, however, because the planet completes one full orbit every 1.4 days. The white dwarf itself is only around 40% larger than Earth. As a result, the dip in brightness lasts for just eight minutes and the planet is about 20 times closer to its star than Mercury is to our sun.
“This system is quite odd,” said Simon Campbell, an astrophysicist at Monash University, Australia. “In this case the planet is bigger than it’s host star by a factor of 7!”
By using data collected by ground-based telescopes, the team was also able to get an estimate for how massive the planet is. Infrared data from the dearly departed Spitzer Space Telescope suggests it’s probably 14 times more massive as Jupiter.
But if it’s so close to its star, how did WD 1856 b survive the expansion phase? The team gave two possible explanations.
When its host star became a red giant, it may have disturbed the planets in its system, causing their orbits to go askew. The disordered cosmic dance may have helped fling a planetary body like WD 1856 b in toward the star, where it has been circling ever since. Because it’s such an elderly white dwarf, that also gives planets plenty of time to sidle up close. Potentially, it could mean there are other planets orbiting the white dwarf, too.
“While not impossible, I don’t think we know just how probable this is, since things get chaotic when you disturb orbits,” said Campbell. “This is where an observation such as this is important.”
Less likely, the researchers say, is the idea the star was able to strip away some outer layers and survive during the expansion phase. However, they conclude our current theories on this process most likely suggest it was not formed in such a manner.
Future observations, the team writes, should be able to confirm whether or not WD 1856 b really is a planet or if it’s a failed star known as a “brown dwarf.” They point to the upcoming, but long-delayed, James Webb Telescope and the Gemini Observatory as keys to understanding WD 1856 b better. And, of course, if there are planets, then they may be able to host life.
“There are people who now are looking for transiting planets around white dwarfs that could be potentially habitable,” said Ian Crossfield, in a press release. “It’d be a pretty weird system, and you’d have to think about how the planets actually survived all that time.”
Of course, if we can wait a few billion years, our own solar system’s fate will give us front-row seats to the white dwarf afterparty. When our sun begins to die, it will swell to a size that extends beyond the orbit of Mars. It will be truly massive. All four of the solar system’s inner planets will be incinerated in the expansion until, like WD 1856, it runs out of fuel and collapses back to a cool, white dwarf. Will the outer planets, like Jupiter, Saturn and Neptune be flung closer in the carnage? I’m certain we won’t be around to find out.