Planets could survive the death of their star and be able to sustain life – and now astronomers are hunting for them.
Stars don’t survive forever, the sun included. In about five billion yearsEarth’s star will begin to deplete its supply of hydrogen used to generate energy nuclear fusion at its core. The Sun’s core will then begin to contract, raising its temperature, so that hydrogen in its outer shell can then unleash fusion reactions that will cause the Sun – and other stars like it, when they reach this stage – to expand into a core of the sun. red giant.
The red giant phase is bad news for all nearby planets. In our solar systemthe expanding sun will swallow Mercury, Venus and probably also the Earth.
Planets further away will do better. Worlds that are five to six times further away from their star than The earth belongs to the sun will be warmed by the expanding star, melting their ice and creating surface oceans and possibly life. In our solar system Jupiter‘s icy moons, like Europe And Ganymedewould be in such a top position.
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But it’s a good thing. Too close, and their water will evaporate. If you’re too far away, the worlds remain frozen. Essentially the Goldilocks zone of habitability will move away from an expanding star, and a planet or icy moon will have to inhabit this zone to have any chance of developing liquid water.
The red giant star will continue to evolve. Eventually, all fusion reactions will cease and the star’s puffy outer layers will be expelled, leaving only the star’s compact core, also known as the star’s core. white dwarf.
White dwarfs are born hot and shine brightly, but they are also small, about the size of Earth. Due to their small size, they do not radiate much heat at all. A planet orbiting one of these exotic objects would have to be about 1.5 million kilometers from the white dwarf – about 1% of Earth’s distance from the Sun – to be warm enough to host liquid water.
Therein lies the problem. Any nearby planets would have been fried and swallowed long ago, and the outer planets and moons that have now melted will be too far from the white dwarf to support surface water.
So how does one transfer a planet from hundreds of millions of miles away to the new, nearby Goldilocks zone?
“It’s a dangerous journey,” said Juliette Becker of the University of Wisconsin-Madison rack. She noted that it is “difficult for oceans to survive this process, but it is possible.”
Becker, who discussed how exoplanets could survive this process and then be detected via ‘transits’ – passes over the face of their host star, from our perspective – at the 244th meeting of the American Astronomical Society earlier in June, explained that the mechanism for creating a planet moving closer to a white dwarf is called tidal migration.
‘During tidal migration, a certain dynamical instability between the planets in the system causes one of them to end up in an orbit with a high eccentricity, such as a cometwhere it swings in very close to the central body in the system and then swings far out again.”
The migrating planet does not stay in this comet-like orbit for long. Gravity works to make its path circular, keeping the planet close to the white dwarf. And it is here where astronomers could discover their transits.
One caveat is that white dwarfs do not appear to be a hotbed of exoplanetary action. Earlier this year, the James Webb Space Telescope (JWST) observed two candidate planets around white dwarfs, but overall they were scarce. None of these candidates pass by its white dwarf.
As a planet passes its white dwarf, transit spectroscopy – which looks at whether the planet’s atmosphere absorbs and filters certain wavelengths of starlight during a transit – can reveal the presence of water in that planet’s atmosphere. Such measurements have been made for exoplanets that pass by ordinary stars, but it may actually prove easier to do this with a white dwarf.
“White dwarfs are so small and so featureless that if a terrestrial planet were to pass in front of them, you could characterize its atmosphere much better,” says Becker. “The planet’s atmosphere would have a much larger, clearer signal, because a larger portion of the light you see passes through exactly what you want to study.”
Water is no guarantee of life, of course, but even the possibility that previously frozen worlds can be made habitable by the death of their star, and then pulled into a close orbit around that dead star where they can remain habitable, gives astrobiologists a new arena what they need to delve into extraterrestrial life. Such a world would be the ultimate case of a ‘phoenix’ world and prove that there can be life after death.
Becker has a paper describing her work studying the search for habitable planets transiting white dwarfs that are currently under peer review.