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As a first step, astronomers discover a huge planet orbiting a dead star


A tiny white dwarf, WD 1856, is closely orbited by a huge planet.

NASA Goddard

About 80 light years from Earth lies the white dwarf WD 1856, a dead star that entered the final phase of its life about 6 billion years ago. This slow death is usually pretty lonely. As they die, some stars will expand dramatically and become one giant “red giant”. like Betelgeuseand devour one of the planets that orbit nearby. Eventually they use up all their fuel and fall back into white dwarfs after destroying everything they left behind.

Not so with WD 1856. For the first time, astronomers have discovered a giant planet about the size of Jupiter and orbiting the dead star. They named it WD 1856b and it’s a surprising find – it avoided destruction and shows that dead stars can still host planets with the right living conditions.

The study, published Wednesday in the journal Nature, used data from NASA’s TESS planet-hunting satellite and a series of ground-based telescopes to survey WD 1856 for potential exoplanets. TESS, which examines stars for small dips in brightness that indicate potential planets, first examined the star in July and August 2019. A huge decrease in brightness was observed when the team looked at WD 1856.

Astronomers recently began to grapple with the idea that these dead stars could still be home to a number of planets. In December, researchers discovered A planet that was slowly being devoured by a white dwarf about 1,500 light years away. However, this detection was based on the fact that light was emitted by a disk of debris and gas surrounding the star, which the researchers believe must have been removed from a Neptune-like planet.

The discovery published today in Nature is different in that it orbits a direct detection of the planet orbiting in front of its host star, something that has not previously been achieved for a white dwarf.

Every time the Jupiter-sized planet passes through before WD 1856 as viewed from Earth, the star’s light falls off by almost half. The process is incredibly short, however, as the planet completes a full orbit every 1.4 days. The white dwarf itself is only around 40% larger than Earth. As a result, the decrease in brightness only lasts eight minutes and the planet is about 20 times closer to its star than Mercury is to our sun.

“This system is pretty strange,” said Simon Campbell, an astrophysicist at Monash University in Australia. “In this case the planet is seven times larger than its host star!”

Using data collected from ground-based telescopes, the team was also able to get an estimate of the planet’s mass. Infrared data from the late Spitzer space telescope suggests that it is probably 14 times more massive than Jupiter.

But when 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 disrupted the planets in its system and made their orbits crooked. The disordered cosmic dance may have helped to hurl a planetary body like WD 1856b towards the star it has been orbiting in since then. Because it’s such an older white dwarf, it also gives the planets plenty of time to move up close. Possibly this could mean that other planets orbit the white dwarf as well.

“While this is not impossible, I don’t think we know how likely this is, as things get messy when you perturb the orbits,” Campbell said. “Such an observation is important here.”

Less likely, the researchers say, is the idea that the star was able to remove some of the outer layers and survive during the expansion phase. However, they close our current theories about this process, which most likely suggests that it was not formed that way.

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Future observations, the team writes, should be able to confirm whether WD 1856 b is really a planet or whether it is a failed star known as the “brown dwarf”. You point up the upcoming but long-belated James Webb Telescope and the Gemini Observatory as the key to better understanding WD 1856 b. And of course, if there are planets they can host life.

“There are people now looking for transit planets around white dwarfs that could potentially be habitable,” Ian Crossfield said in a press release. “It would be a pretty strange system, and you’d have to think about how the planets actually survived all along.”

Of course, if we can wait a few billion years, the fate of our own solar system will give us front row seats for the White Dwarf’s after party. When our sun begins to die, it swells to a size beyond the orbit of Mars. It will be really firmly. All four inner planets of the solar system are burned during the expansion until, like WD 1856, it runs out of fuel and falls back into a cool, white dwarf. Are the outer planets like Jupiter, Saturn and Neptune hurled closer in the slaughter? I am sure we won’t be there to find out.

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