If life exists on planets orbiting white dwarf stars, it almost certainly evolved after the stars’ deaths, according to a new study from the University of Warwick, UK. The study, led by Dimitri Veras and published in Monthly Notices of the Royal Astronomical Society, sought to understand how the death of a star like our sun affects the planets around it,and any life that may dwell on them.
According to the authors, as a star like our sun runs out of hydrogen in its core, it contracts and heats up, driving a massive expansion of its outer atmosphere to form a ‘red giant’. As it expands, the star would swallow up any inner planets. Moreover, red giants are incredibly volatile, with raging solar winds that would batter any planets in the vicinity.
The Earth has a strong magnetic field (magnetosphere) because its core is made of iron that rotates, and this protects it from stellar winds. But the authors’ model shows that under red-giant conditions, even this sort of magnetosphere would not be sufficient to protect the planet from the furious solar winds.
“We know that the solar wind in the past eroded the Martian atmosphere, which, unlike Earth, does not have a large-scale magnetosphere,” says Aline Vidotto of Trinity College, Dublin, co-author of the study. “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.”.
The core problem is that the density and speed of stellar wind, combined with an expansion of the planetary orbit as the star’s gravitational pull weakens, would shrink and expand the magnetosphere of a planet over time. The model found that for a planet to maintain its magnetosphere throughout these stages of stellar evolution, it would need to have a magnetic field at least 100 times stronger than that around Jupiter, which is one million times the volume of Earth’s magnetosphere. Compounding the problem, the star’s habitable zone would also expand at a faster rate than the movement of the planet outwards in response to the star’s weakening gravitational pull.
It’s tough to see how life on any planet could survive this assault.
Eventually, a red giant will shed its outer atmosphere, leaving behind a dense, hot, white-dwarf remnant. White dwarfs do not emit stellar winds, so at this stage, life becomes feasible again.
“This study demonstrates the difficulty of a planet maintaining its protective magnetosphere throughout the entirety of the giant branch phases of stellar evolution,” says Veras.
“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.”
White dwarfs, unlike red giants, are potentially supremely hospitable, being stable for long periods of time.
“A planet that’s parked in the white-dwarf habitable zone could remain there for billions of years, allowing time for life to develop, provided that the conditions are suitable,” adds Veras.
The team hopes its modelling will help inform the search for life on planets orbiting white dwarf stars. Future missions, such as the James Webb Space Telescope, due to launch this year, will look for biomarkers that hint at life among planets orbiting in the habitable zone of white dwarfs.