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At first, astronomers detect a giant planet orbiting a dead star



wd-1856

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

NASA Goddard

About 80 light-years from Earth is the white dwarf WD 1856, a dead star that entered the final stages of its life some 6 billion years ago. This slow death is typically quite lonely. While they are dying, some stars are expanding dramatically, becoming a huge “red giant”, like Betelgeuseand engulfing any of the nearby orbiting planets. Eventually, they consume all of their fuel and collapse again into white dwarfs, having destroyed everything in their wake.

Not so for WD 1856. For the first time, astronomers have detected a giant planet, the size of Jupiter, orbiting the dead star. They dubbed it WD 1856 b and it is a surprising discovery: it avoided destruction and proves that dead stars could still host planets with the right living conditions.

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

Astronomers have recently begun to grapple with the idea that these dead stars can still host a number of planets. In December, the researchers found a planet that was slowly being devoured by a white dwarf about 1,500 light years away. However, that detection was based on light emitted from a disk of debris and gas surrounding the star, which the researchers suggest must have been ripped from a Neptune-like planet.

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

Whenever the Jupiter-sized planet transits in front of WD 1856, as seen from Earth, the star’s light decreases by nearly half. The process is incredibly short, however, as the planet completes a full orbit every 1.4 days. The white dwarf itself is only about 40% larger than the Earth. As a result, the drop 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, Australia. “In this case the planet is larger than its host star by a factor of 7!”

Using data collected from ground-based telescopes, the team was also able to get an estimate of how massive the planet is. Infrared data from the dearly extinct Spitzer Space Telescope suggests it is probably 14 times more massive than Jupiter.

But if it’s that close to its star, how did WD 1856 b survive the expansion phase? The team provided 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 sideways. The disordered cosmic dance may have helped launch a planetary body such as WD 1856 b towards the star, where it has continued to spin ever since. Because she’s such an old white dwarf, it also gives the planets plenty of time to sneak up on them. Potentially, it could mean that there are other planets orbiting the white dwarf as well.

“While it’s not impossible, I don’t think we know how likely it is, since things get chaotic when orbits are disturbed,” Campbell said. “This is where a remark like this is important.”

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, our current theories on this process conclude most likely to suggest that it did not form 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 whether it is a failed star known as a “brown dwarf”. They indicate the next, but long-delayed, James Webb Telescope and the Gemini Observatory as keys to better understanding WD 1856 b. And, of course, if there are planets, then they might be able to host life.

“There are people who are now looking for planets transiting around white dwarfs that could potentially be habitable,” Ian Crossfield said in a news release. “It would be a rather strange system, and you should think about how the planets actually survived all that time.”

Of course, if we can wait a few billion years, the fate of our solar system will give us front row seats for the white dwarf afterparty. As our sun begins to die, it will swell to dimensions that extend beyond the orbit of Mars. Sara really massive. All four inner planets of the solar system will be incinerated during expansion until, as in WD 1856, it runs out of fuel and collapses into a cold white dwarf. Will the outer planets, such as Jupiter, Saturn and Neptune, be closer in the carnage? I’m sure we won’t be around to find out.


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