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The first giant planet discovered in orbit around the remains of a dead star



Picture of the Hubble Space Telescope of dozens of stars, including white dwarfs.

Hidden in this image of the Hubble Space Telescope are the burnt scales of dead stars known as white dwarfs (they are not shining)

NASA / X. Judge

About 80 light-years from Earth is the white dwarf WD 1856, a dead star that entered the final stages of its life about 6 billion years ago. This slow death is usually quite lonely. In the process of dying, some stars will expand dramatically, becoming a huge “red giant” like Betelgeuse, and swallowing any of the planets orbiting nearby. Eventually, they run out of fuel and crash back into the white dwarves after destroying everything after them.

Not so for WD 1856. For the first time, astronomers have discovered a giant planet the size of Jupiter orbiting a dead star. They called it WD 1856 b and this is a surprising find – it avoids destruction and shows that dead stars can still accept planets with the right living conditions.

The study, published in the journal Nature on Wednesday, used data from NASA’s TESS satellite to hunt planets and a set of ground-based telescopes to study WD 1856 for potential exoplanets. TESS, which studies stars for small declines in brightness, meaning potential planets, first looked at the star in July and August 2019. A huge decrease in brightness was seen when the team examined WD 1856.

Recently, astronomers began to struggle with the idea that these dead stars could still host a number of planets. In December, researchers found a planet slowly being swallowed up by a white dwarf at about 1500 light years. However, this discovery is based on light emitted by a disk of debris and gas surrounding the star, which researchers believe was supposed to be deprived of a planet similar to Neptune.

The discovery, published today in Nature, is different because it records a direct discovery of the planet orbiting its host star, which was not previously achieved for a white dwarf.

Every time a planet the size of Jupiter passes in front of WD 1856, as seen from Earth, the light from the star is almost halved. However, the process is incredibly short, as the planet completes a full orbit every 1.4 days. The sinking in brightness lasts only eight minutes and the planet is about 20 times closer to its star than Mercury is to our sun.

Using data collected from ground-based telescopes, the team was able to obtain an estimate of how massive the planet is. Infrared data from the expensive Spitzer Space Telescope suggests that it is probably 14 times more massive than 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 turns into a red giant, it may have disrupted the planets in its system, causing their orbits to distort. The disordered cosmic dance may have helped throw a planetary body like WD 1856 b to the star, where it has been orbiting ever since. Because it is such an adult white dwarf that it also gives the planets enough time to get close. Potentially, this could mean that there are other planets orbiting the white dwarf.

It is less likely, according to researchers, that the idea that the star was able to take some outer layers and survive during the expansion phase. However, they conclude our current theories about this process, most likely suggesting that it was not formed in this way.

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Future observations, the team writes, should be able to confirm whether WD 1856 b is indeed a planet or a failed star known as a “brown dwarf.” They point to James Webb’s forthcoming but long-delayed telescope and the Gemini Observatory as keys to a better understanding of WD 1856 b. And, of course, if there are planets, then they can host life.

“There are people now looking for transit planets around white dwarfs that could be habitable,” Ian Crossfield said in a statement. “It would be a rather strange system, and you’ll have to think about how the planets actually survived all this time.”

Of course, if we can wait a few billion years, the fate of our own solar system will give us seats in the front row of the white dwarf afterparty. When our sun begins to die, it will swell to a size that extends beyond the orbit of Mars. Will be really massive. All four inner planets of the solar system will be incinerated in expansion until, like WD 1856, they run out of fuel and collapse back into a cool, white dwarf. Will the outer planets, such as Jupiter, Saturn and Neptune, be thrown closer into the carnage? I’m sure we won’t be around to find out.


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