Planets could survive the death of their star and become capable of supporting life – and now astronomers are on the hunt for them.
The stars don’t last forever sun included. In about five billion yearsthe terrestrial star begins to deplete its supply of hydrogen used to generate energy through nuclear fusion at its core. The Sun’s core then begins to contract, increasing its temperature so that the hydrogen in its outer shell can then ignite the fusion reactions that cause the Sun – and other stars like it when they reach this stage – to expand into red giant.
The red giant phase is bad news for all surrounding planets. In ours Solar Systemthe expanding sun will engulf Mercury, Venus, and possibly Earth.
Farther planets will do better. Worlds that are five to six times farther from their star than The earth comes from the sun they will be heated by the expanding star, melting their ice and creating surface oceans and potentially life. In our solar system, Jupiter‘s ice moons, such as Europe and Ganymedewould be in such a prominent position.
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But it’s a good thing. Too close and their water evaporates. Too far and the worlds remain frozen. In fact, Goldilocks zone habitability will move away from the expanding star and a planet or icy moon will have to inhabit this zone to have any chance of liquid water forming.
The red giant star will continue to evolve. Eventually, all fusion reactions cease and the bloated outer layers of the star are expelled, leaving only the star’s compact core, known as white dwarf.
White dwarfs are born hot and shine brightly, but they are also small, about the size of Earth. Their small size means they don’t generate much heat overall. A planet orbiting one of these exotic objects would have to be about 930,000 miles (1.5 million kilometers) from the white dwarf—about 1% of the distance from Earth to 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 sustain surface water.
So how does one transfer a planet from hundreds of millions of kilometers away to a new, nearby Goldilocks zone?
“It’s a dangerous road,” said Juliette Becker of the University of Wisconsin-Madison. declaration. She noted that it is “difficult for the oceans to survive this process, but it is possible.”
Becker, who discussed how exoplanets could survive this process and subsequently be detected through “transits” — passages across the face of their host star, from our point of view — at the 244th meeting of the American Astronomical Society earlier in June, he explained that the mechanism for bringing a planet closer to a white dwarf is called tidal migration.
“In tidal migration, some dynamical instability between the planets in the system will put one of them into a high-eccentricity orbit, e.g. Cometwhere it tilts really close to the central body in the system and then goes far out again.”
A migrating planet will not stay in this comet-like orbit for long. Gravity acts to circle its orbit and keep the planet close to the white dwarf. And it was here that astronomers could record their transitions.
One caveat is that white dwarfs do not appear to be hotbeds of exoplanetary action. Earlier this year, the James Webb Space Telescope (JWST) observed two planet candidates around white dwarfs, but overall there were few of them. None of these candidates pass through their white dwarf.
If the planet transits its white dwarf, then transit spectroscopy—watching whether the planet’s atmosphere absorbs and filters out certain wavelengths of starlight during transit—could reveal the presence of water in the planet’s atmosphere. Such measurements have been made for exoplanets transiting regular stars, but in reality it might prove easier to do so with a white dwarf.
“White dwarfs are so small and so inconspicuous that if a terrestrial planet were transiting in front of them, you could actually do a much better job of characterizing its atmosphere,” Becker said. “The planet’s atmosphere would have a much bigger and brighter signal because more of the light you see is going through exactly what you want to study.”
Of course, water is not a guarantee of life, but so is the possibility that previously frozen worlds could be made habitable by the death of their star and then be pulled into close orbit around that dead star where they can remain habitable. astrobiologists a new arena to consider alien life. Such a world would be the ultimate case of a “phoenix” world, proving that life can exist after stellar death.
Becker has a paper describing her work studying the search for habitable planets transiting white dwarfs, which is currently under peer review.