science

The planet that survived the death of its star



When our sun enters its death throes in about five billion years, it will incinerate our planet and then dramatically collapse into a dead ember known as a white dwarf – but the fate of more distant planets, such as Jupiter or Saturn, is less clear.

In the journal Nature, astronomers report a tantalising preview of our solar system’s afterlife in a Jupiter-sized planet orbiting a white dwarf 6,500 light years from here.

Known as MOA-2010-BLG-477Lb, the planet occupies a comparable orbit with Jupiter. The discovery not only offers a glimpse into our cosmic future, it raises the possibility that any life on surviving worlds might endure the deaths of their stars.

Joshua Blackman, a postdoctoral researcher at the University of Tasmania and lead author of the study, says: “While there is quite a lot of evidence of rocky planetary debris orbiting around white dwarfs, we have very few data points of intact planets.

“The fate of our solar system is likely to be similar to MOA-2010-BLG-477Lb. The sun will become a white dwarf, the inner planets will be engulfed and the wider-orbit planets like Jupiter and Saturn will survive.”

The planet was spotted because of the light-warping effects of its gravitational field, a phenomenon known as micro-lensing. After searching for years for its host star with the Keck II telescope in Hawaii, Blackman and his colleagues concluded it was orbiting a white dwarf too faint to directly observe.

Astronomers using a different method last year reported spotting another intact Jupiter-like planet, known as WD 1856 b, closely orbiting a white dwarf. But MOA-2010-BLG-477Lb circles its hidden stellar husk at nearly three times the distance between Earth and the sun, making it the first known planet to occupy a Jupiter-like orbit around a white dwarf. WD 1856 b, by contrast, orbits its white dwarf every 34 hours, suggesting that it migrated into its current position after the death of its star, though the exact mechanics of that journey are still being hashed out.



Dying stars spew out harmful radiation as they grow into a phase called red giants and introduce turbulence in their systems that could obliterate life

Andrew Vanderburg, an assistant professor of physics at the US Massachusetts Institute of Technology who led the team that discovered WD 1856 b, says the conclusions of the new study appear solid. He also notes that planets with wide orbits around white dwarfs are probably more abundant than those in tight orbits but that the latter group is simpler to detect.

“If I had to guess, I would say that theirs is a much more common population because it just has to stay there and have nothing happen to it,” Vanderburg says. “That feels to me like the most likely outcome, at least at this point in the universe’s history.”

The discoveries can yield insights about the search for extraterrestrial life and the potential habitability of white dwarf systems. Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University in the US has suggested that some life-bearing star systems might even experience what she calls a “second genesis” in which new organisms emerge in the reconfigured fallout of a white dwarf system.

“I find the study exciting because it adds to the growing evidence that planets can survive the death of their star, which leads to intriguing questions about the future of the cosmos,” says Kaltenegger, who was part of the team that discovered WD 1856 b. “If planets can survive the death of their stars, can life as well?”

Dying stars spew out harmful radiation as they grow into a phase called red giants and introduce turbulence in their systems that could obliterate life. But there are some speculative scenarios that might preserve the habitability of white dwarf systems.

“There are a lot of things that have to go right,” Vanderburg says. He imagines a planet distant from a red giant star that then moves inward after the star becomes a white dwarf and retains, “enough water to potentially be a nice place to live”, when the star turns into a white dwarf.

Because white dwarfs are small and dim, such a planet would have to be in a very close orbit for liquid water to exist. However, if life were to emerge on a world like Jupiter’s moon Europa, which might contain a subsurface ocean warmed by Jupiter’s tidal forces, it could potentially survive at a greater distance from the star.

“If humanity is somehow still around in five billion years, we would probably have a better chance of surviving the sun’s red giant phase on a moon of Jupiter than on Earth,” Blackman says.

Though the existence of life around white dwarfs remains a matter of speculation, next-generation observatories could help to provide concrete answers to some of these evocative questions. As more intact planets are spotted orbiting white dwarfs, scientists will gain a clearer picture of the life, and afterlife, of these mysterious systems.

Blackman concludes: “This is the first detection of a planet orbiting a white dwarf made using the micro-lensing technique but almost certainly not the last.”

This article appeared in The New York Times.



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