Could our Milky Way鈥檚 many red and white dwarf stars be home to alien life? These tiny, dim stars seem inhospitable with intense flares and聽destructive tidal forces. But with just the right circumstances, life forms on nearby聽planets could survive.
Our galaxy is full of small, cool stars. Red dwarfs are common but rambunctious stars that lash out in fierce flares. White dwarfs are calmer smoldering remnants of dying stars too small to explode in a nova, but only form after a star swells into a planet-destroying red giant.
These dwarf stars have narrow habitable zones 鈥� the region around each star that could have liquid water 鈥� yet their prevalence makes them tempting targets in the search for life.
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Any planet inside a dwarf star鈥檚 habitable zone is close enough to be tidally-locked into a perpetual blazing dayside and frigid eternal night, or, worse, shredded by tidal forces. 鈥淭here is a relatively limited range of distances where a planet can be habitable and not ripped apart by the strong tides,鈥� says 聽at Harvard University.
White dwarf dilemma
鈥淚f you park a planet in a habitable zone around a white dwarf, it will stay there for a very, very long time 鈥� potentially enough time for life to manifest. The problem is getting them there,鈥� says 聽at Rochester Institute of Technology.
When a star like our sun dies, it puffs into a red giant that engulfs and shreds nearby planets before collapsing into a white dwarf. Any planet in a white dwarf鈥檚 habitable zone must either form from rubble of freshly destroyed planets or migrate in from system outskirts after the star鈥檚 collapse.
Planets can migrate if a large object yanks its orbit, says Nordhaus. But migrating planets need to shed enormous energy to snuggle close to its star, burning into an ember and leaving a desiccated husk unable to support life unless comets restore its water.
While a white dwarf鈥檚 stability is hospitable to life, the turbulent conditions necessary to place a rocky world in the proper orbit are so rare that we鈥檝e yet to discover any candidates.
Up to 75 per cent of the stars in the Milky Way are red dwarfs, many hosting Earth-sized exoplanets like those around our nearest neighbor Proxima Centauri or the tantalizing TRAPPIST-1.
Red dwarfs are incredibly long-lived, creating stable solar systems where life might evolve. But they also hurl violent flares that swamp planets in radiation while powerful solar winds strip protective atmospheres.
Shielding life
Strong magnetic shields could protect a planet from solar winds. Alternately, a thick haze like the one that engulfed primordial Earth and currently swathes Saturn鈥檚 moon Titan might survive the onslaught of solar flares, suggests a team led by at the University of Washington in Seattle.
鈥淔loating droplets of hydrocarbon vapour clouds are really efficient at absorbing ultraviolet radiation,鈥� says at the University of British Columbia. 鈥淚t鈥檚 not an ozone layer; it鈥檚 a smog shield.鈥�
Loeb and his team suggest searching slightly outside the habitable zone where planetary surface temperatures drop below freezing. Even without an atmosphere oceans could survive protected by ice. If they鈥檙e relatively deep, says Matthews, those oceans could shield life from radiation.
In the last thirty years, we鈥檝e found thousands of worlds in more variety than we ever imagined. 鈥淓arth is not an extremely rare gem in the jewel box of the sky,鈥� says Matthews. As this explosion of planetary discovery continues, we鈥檙e gaining a more colourful palette of places where life could evolve.
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