White dwarfs are the remnants of Sun-like stars that once underwent the stage of becoming a red giant and shedding the outer shells of the star. A recent study has been done on whether the planets that still orbit around them could have oceans.
White dwarfs and habitable planets
Recently, the arXiv preprint server has published a study whose authors wondered whether Earth-like planets with oceans of liquid water could orbit around white dwarfs. The answer to it may affect our assessments of how livable the surrounding Universe is in general.
In general, potentially Earth-like planets outside the Solar System have been discovered so many that they are no longer particularly surprising to anyone. Worlds orbiting white dwarfs have also been known for a long time. But here we should take into account how unusual these objects are.
White dwarfs are the remnants of stars that have exhausted their hydrogen fuel, survived the red giant stage, and are now just hot cores. Nuclear fusion hasn’t occurred in them for a long time, but they can still shine and support life on planets orbiting close enough to them for a very long time.
Red giant stage
But in order to bask in the white dwarf’s rays, the Earth-like planet has to somehow preserve its atmosphere and hydrosphere during the red giant stage. Even when the evolution of a star is just getting close, the star’s luminosity increases dramatically and stays that way for hundreds of millions of years.
At some point, the Earth-like world may be beneath the outer surface of the incredibly inflated luminary altogether. And even if that doesn’t happen, the star should still shed its outer shells at the moment of turning into a white dwarf, and they will pass through all the planets, gradually turning into a planetary nebula.
Of course, the possibility of preserving oceans for planets at distances that are comparable to Earth and Mars is unrealistic. However, the researchers have suggested that in the case of icy worlds ten times farther away from their star, this is quite likely.
Is it possible to conserve the oceans?
It remains to be seen how these worlds should then end up where a white dwarf can warm them and melt the ice. After all, once the shells are dropped, their initial orbits will provide them with even less light than they had before.
Fortunately, scientists have a scenario that explains it. They believe that gravitational chaos forms in the system during the shedding of the gas shells. The newborn nebula will begin to slow the planets down a lot and they will move into orbits closer to the star.
However, all this will take place in a rather narrow “corridor of opportunity”. The planet must retain its oceans until the shells drop, and then very abruptly change orbit so that it is in a close orbit to the star.
Actually, the scientists’ study was about determining whether any of this was even likely. They found that the vast majority of worlds must either lose their oceans while still in the red giant stage, or remain icy. Still, a small fraction of them must move to orbits where liquid water on the surface is possible.
It should be noted that there should be so few Earth-like worlds near white dwarfs that we can search for them for quite a long time. But it’s still worth doing, because the spectra of these stars are so clean that these planets will be clearly visible when they pass in front of them.
According to phys.org
