White dwarfs are the final stage of many stars. Once stars run out of hydrogen to fuse in their core, they swell up into red giants burning helium. For medium and small stars, this is the end of the line – they’ll eventually run out of fuel again and turn into a white dwarf.
While this process might not be as spectacular as a supernova explosion, it’s likely just as deadly to planets surrounding the star that is turning into a white dwarf. New research suggests that if life is found on a planet around a white dwarf, it probably evolved after the red giant star died. The findings are reported Monthly Notices of the Royal Astronomical Society and presented at the National Astronomy Meeting.
The work showed that as a red giant ages, its stellar winds are capable of stripping nearby planets of their atmospheres. Red giant stars balloon up to a size of tens of millions of kilometers. Their mass doesn’t change, so the gravitational pull on these inflated outer layers is lower.
So, chunks of stars are pushed out into the cosmos. Not in a continuous way, according to the model, but with wild fluctuations that over time will erode a planet’s atmosphere and vastly reduce its magnetosphere. A planet ought to have a magnetic field 100 times stronger than Jupiter (the strongest among the planets) to survive this perilous time unscathed.
If this wasn’t enough, the orbits of planets and habitable zones are shifted during such a stellar phase. Some planets are engulfed by the star expansion, but others are pushed out. Unfortunately, the habitable zone is pushed out more quickly than the planets. In this case, a life-supporting planet might find itself stripped of protection and favorable conditions.
"This study demonstrates the difficulty of a planet maintaining its protective magnetosphere throughout the entirety of the giant branch phases of stellar evolution," lead author Dr Dimitri Veras, from the University of Warwick, said in a statement.
"One conclusion is that life on a planet in the habitable zone around a white dwarf would almost certainly develop during the white dwarf phase unless that life was able to withstand multiple extreme and sudden changes in its environment."
So when the Sun expands into a red giant, Earth – with a strong magnetic field and in a great position to support life – might not be sufficiently shielded against a more powerful solar wind and changes to our star.
"We know that the solar wind in the past eroded the Martian atmosphere, which, unlike Earth, does not have a large-scale magnetosphere. What we were not expecting to find is that the solar wind in the future could be as damaging even to those planets that are protected by a magnetic field", says Dr. Aline Vidotto of Trinity College Dublin, the co-author of the study.