by Planets could survive the death of their star and be able to support life – and now astronomers are going to hunt for them.
The stars don’t last forever, the sun included. In about five billion yearsEarth’s star will begin to exhaust its supply of hydrogen which is used to generate energy through nuclear fusion in its core. The sun’s core will then begin to contract, increasing its temperature so that hydrogen in its outer envelope can ignite fusion reactions that will cause the sun — and other stars like it, when they reach this stage — to expand to this step. red giant.
It is the red giant phase sad story of any nearby planets. In our Solar systemthe rising sun will swallow up Mercury, Venus and probably the Earth, too.
Planets further out will do better. A world that is five to six times further from its star than The earth is from the sun heated by the expanding star, melting their ice and forming surface oceans and, possibly, life. In our solar system, Jupiterand icy moons, like Europe and Ganymedebe in excellent condition.
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But it’s a fine thing. Too close, and their water will evaporate. Too far out, and life will remain frozen. Basically, the Goldilocks zone the habit will move away from an accreting star, and an icy planet or moon will have to live in 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 behind only the star’s dense core, known as a white ox.
White dwarfs are hot and glow brightly, but they are also tiny, about the size of Earth. Their small size means they don’t radiate much heat overall. A planet orbiting one of these exotic objects must 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 hot enough to host liquid water.
Therein lies the problem. All nearby planets would have been fried and swallowed deep long ago, and the now molten outer planets and moons will be too far away from the white dwarf to sustain surface water.
So how does one move a planet from hundreds of millions of kilometers out to the new zone, close to Goldilocks?
“It’s a dangerous journey,” said Juliette Becker of the University of Wisconsin-Madison in a statement. She noted that it is “difficult for the oceans to survive this process, but it is possible.”
Becker, to discuss how exoplanet the survival of this process and its subsequent detection through “transits” – passages across the face of its host star, from our perspective – at the 244th meeting of the American Astronomical Society earlier in June, explained that the mechanism to planet moving closer to a white dwarf is called tidal migration.
“During tidal migration, some dynamical instability between planets in the system forces one of them into a highly confined orbit, like cometwhere it swings in really close to the central body in the system and then far out again.”
The migrating planet does not remain in this orbit like a comet for long. Gravity acts to circle its path, keeping the planet close to the white dwarf. And it is here that astronomers could see their behavior.
One caveat is that white dwarfs do not appear to be hotbeds of extraplanetary activity. Earlier this year, the James Webb Space Telescope (JWST) two planet candidates were observed around white dwarfs, but overall they were scarce. Neither of these candidates transforms into a white dwarf.
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If a planet transits its white dwarf, then transit spectroscopy – watching the planet’s atmosphere absorb and filter out certain wavelengths of starlight during transit – could reveal the presence of water in that planet’s atmosphere. Such measurements have been made for exoplanets transiting normal stars, but it may be easier to do so with a white dwarf.
“White dwarfs are so small and featureless that if you moved a terrestrial planet in front of them, you could do a much better job of characterizing the atmosphere,” Becker said. “The planet’s atmosphere would have a much bigger and clearer signal because a larger fraction of the light you’re seeing is going through exactly what you want to study.”
Water is not a guarantee of life, of course, but even the possibility that previously frozen life could be made habitable by the death of their star, and then drawn into a close orbit around that dead star where they can remain habitable . astrobiologists a new field in which to meditate alien life. Such a world would be the ultimate “Phoenix” world, and would prove that life after stellar death is possible.
Becker has a paper describing her work studying the search for habitable planets that orbit white dwarfs currently under peer review.
