Scientists have spotted heaps of white dwarf stars encircled by debris disks, the remnants of planets obliterated during the star’s evolution. However, they’ve stumbled upon one fully intact planet, about the size of Jupiter, orbiting a white dwarf. Are there more of these white dwarf planets out there? And could there be Earth-like worlds nestled around these celestial bodies?
A white dwarf (WD) is what’s left of a once-bigger star, like our Sun. When a star of similar size to the Sun finishes its main life phase, it swells and turns into a red giant. As the red giant gets older and runs out of fuel, it sheds its outer layers, creating a planetary nebula, which is that beautiful gas cloud you see in Hubble pictures. After about 10,000 years, the planetary nebula fades away, leaving behind just a white dwarf, hanging out at the center of where all that dazzling show used to be.
White dwarfs may be tiny compared to their mass, akin to the size of Earth, and they’ve retired from their fusion days, radiating only leftover warmth. Despite their proximity, some white dwarfs could boast habitable zones.
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White dwarfs and the intact Jupiter-mass planet discovery
Astronomers are pretty convinced that most stars play host to planets. However, these planets face a precarious fate when their star evolves from the main sequence into a red giant, leading to potential devastation. Such transitions can engulf certain planets and tear others asunder due to tidal disruption. Some white dwarfs sport debris disks, likely the remnants of their past planets torn apart during the star’s red dwarf phase.
Yet in 2020, scientists revealed they had found a whole planet in the debris disk, chilling in the habitable zone near the white dwarf WD1054-226. If there’s one, there’ve gotta be others hanging around somewhere. So why haven’t we stumbled upon them yet? And does the fact that the first one we spotted is as big as Jupiter suggest that most of the WD exoplanet crew are of the same hefty caliber?
A fresh study delves into the topic of exoplanets around white dwarfs and wonders why we don’t see many rocky ones. Titled “The giant nature of WD 1856 b implies that transiting rocky planets are rare around white dwarfs,” the paper has made its way into the Monthly Notices of the Royal Astronomical Society. Its author, David Kipping, is an Assistant Professor in the Department of Astronomy at Columbia University in New York.
White dwarfs stick around for a good while and are pretty steady. So, even though their habitable zones are much tinier compared to a star like ours, they’re still there. In theory, planets in those zones could be home to life. The lone intact planet we’re sure about around a white dwarf was found by NASA’s TESS spacecraft, and it’s a hefty one: 13.8 times the size of Jupiter.
“Given the relative paucity of giant planets compared to terrestrials indicated by both exoplanet demographics and theoretical simulations (a “bottom-heavy” radius distribution,) this is perhaps somewhat surprising,” Kipping explains.
Continuing the search for WD terrestrial planets
Just taking a peek at NASA’s Exoplanet Catalogue, we can see there are 5,535 confirmed exoplanets. Out of those, 1898 are like Neptune, and 1756 are gas giants. Only 1675 are Super-Earths, and there’s just 199 that are rocky. Considering Kipping’s talk about the exoplanet lineup being ‘bottom-heavy,’ which implies there are more small-sized planets than big ones, seems a bit baffling when you look at it this way.
We’ve got a good amount of proof suggesting there are small rocky planets hanging around white dwarfs. However, this proof mainly comes from the rocky debris disks left over after some planets got wrecked. This makes us wonder: are there any whole ones chilling in those habitable zones? And does the spotting of WD 1856 b tell us anything about the likelihood of finding these rocky planets around white dwarfs?
You can make sense of the proof for small planets and the discovery of WD 1856 b in two ways. First off, there’s no solid reason why either small rocky planets or big ol’ Jupiter+ mass planets should be the main players in the WD exoplanet crew. Kipping did the math and figured the chances of the initial WD planet being a huge one were only at 0.37%. That’s seriously rare, but it doesn’t really help us draw any solid conclusions.
“For these reasons, we don’t consider our hypothesis in any way established with conviction,” Kipping writes. Perhaps it’s just one of those things that, even though it’s intriguing, might only lead to off-base conclusions. As is often the deal, we’re gonna need more info to figure this one out. “It would certainly be premature to abort on-going and future efforts to look for terrestrial planets around WDs.”
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Potential advantages with white dwarfs
The study of exoplanets around white dwarfs is still just starting out. But there’s some promise here because white dwarfs are known for being pretty steady and hanging around for a long time. And the same goes for their habitable zones.
White dwarfs are different from other stars in that they’re about the same size as Earth. They’re tinier than most stars, which could make it easier to spot planets that are the size of Earth.
It could also make it easier to study atmospheres, including spotting potential signs of life that might be trickier to detect around much bigger stars. Kipping’s theory that there aren’t many rocky planets around WDs can be put to the test pretty easily. If we really concentrate our search, we’ll surely start to uncover the real count of planets hanging around white dwarfs.
If we stumble upon more Earth-like planets around white dwarfs, that gives us another route for habitability and more chances for life to keep going in the Universe.
