odd couple

Scientists get a rare glimpse of the first planet to orbit a dying star

The discovery changes astronomers' perception of how exoplanets form and survive.

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After it has exhausted all of its nuclear fuel, a star like our Sun will shed all of its material to become a stellar core.

White dwarf stars are the haunting remains of large stars that dwindle down to about the same size as the Earth. However, a recent discovery proved that these dying stars are capable of much more than astronomers believed.

Scientists recently discovered the first massive planet orbiting around a small white dwarf star, defying previous theories detailing how planets and stars co-exist in the cosmos.

The discovery was detailed in a study published Wednesday in the journal Nature and gives scientists a rare glimpse of what might occur to our Solar System once the Sun reaches its white dwarf phase.

Using NASA's Transiting Exoplanet Survey Satellite (TESS) mission, which has been scouring the universe for exoplanets since 2018, the team of researchers behind the new study spotted a white dwarf star whose brightness dimmed by half about every one-and-a-half days.

When the light of a star dims periodically, it is usually a tell-tale sign that a planet is orbiting around it, momentarily blocking our view of its light as it transits across.

In order to make sure that what they were seeing was in fact a planet, the researchers supplemented the data with observations by two large ground-based telescopes in the Canary Islands.

They were shocked to find that not only was there a planet orbiting this small remanent of a star but that it was, in fact, a massive Jupiter-sized planet speeding its way around the white dwarf at a close distance.

"We've never seen evidence before of a planet coming in so close to a white dwarf and surviving. It's a pleasant surprise," lead author Andrew Vanderburg, an assistant professor at the University of Wisconsin-Madison's astronomy department, explained.

The planet was dubbed WD 1856 b, and it orbits the smaller star remnant every 34 hours, meaning that a year on this planet is equal to 1.4 days on Earth.

WD 1856 b is located about 80 light-years away in the northern constellation Draco.

The researchers believe that the planet started off much further away from its host star, and moved in closer as the star transitioned into a white dwarf. The transitioning process for most stars ends up devouring their orbiting planets, but somehow, WD 1856 b managed to survive the violent process.

In order to figure out how this odd pairing came to be, the team of researchers ran through multiple simulations and discovered that the process may have taken billions of years.

As the star ran out of fuel, it destroyed all of its nearby surrounding planets and destabilized WD 1856 b, which was a little further away from its stellar host. The giant planet then took on an exaggerated, oval orbit that tossed it further out. Over time, the gravitational tug between the star and the planet slowly moved them closer together, putting the planet in a tight, circular orbit billion of years after its host star became a white dwarf.

The white dwarf star has been around for 6 billion years.

The latest discovery in star-planet pairing opens up new doors in the search for exoplanets and habitability in the universe, as scientists can now search for signs of life in these dead star systems.

The white dwarf system also provides a unique glimpse into the future of our Solar System, where our star will transition to a white dwarf in about five billion years.

"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," co-author Ian Crossfield, assistant professor of physics and astronomy at Kansas University, said in a statement.

"Can planets actually survive that — or is that impossible? And until now, there weren’t any known planets around white dwarfs.”

Abstract: Astronomers have discovered thousands of planets outside the Solar System1 , most of which orbit stars that will eventually evolve into red giants and then into white dwarfs. During the red giant phase, any close-orbiting planets will be engulfed by the star2 , but more distant planets can survive this phase and remain in orbit around the white dwarf3,4 . Some white dwarfs show evidence for rocky material foating in their atmospheres5 , in warm debris disks6–9 or orbiting very closely10–12, which has been interpreted as the debris of rocky planets that were scattered inwards and tidally disrupted13. Recently, the discovery of a gaseous debris disk with a composition similar to that of ice giant planets14 demonstrated that massive planets might also fnd their way into tight orbits around white dwarfs, but it is unclear whether these planets can survive the journey. So far, no intact planets have been detected in close orbits around white dwarfs. Here we report the observation of a giant planet candidate transiting the white dwarf WD 1856+534 (TIC 267574918) every 1.4 days. We observed and modelled the periodic dimming of the white dwarf caused by the planet candidate passing in front of the star in its orbit. The planet candidate is roughly the same size as Jupiter and is no more than 14 times as massive (with 95 per cent confdence). Other cases of white dwarfs with close brown dwarf or stellar companions are explained as the consequence of common-envelope evolution, wherein the original orbit is enveloped during the red giant phase and shrinks owing to friction. In this case, however, the long orbital period (compared with other white dwarfs with close brown dwarf or stellar companions) and low mass of the planet candidate make common-envelope evolution less likely. Instead, our fndings for the WD 1856+534 system indicate that giant planets can be scattered into tight orbits without being tidally disrupted, motivating the search for smaller transiting planets around white dwarfs.

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