Billions of years after humanitys last call and the Suns final gasp as a hydrogen-burning star Or so say astronomers searching for super Earths around thousands of relatively nearby dying White Dwarfs burned-out stellar remnants of stars that used to be very similar to our own.
Like a hot charcoal briquette, a White Dwarf has no internal heat source, but unlike the hot cubes on your backyard grill, these remnants take billions of years to cool. Thus, there would still be plenty of time for life to evolve on newly-formed, very close-in planets around a White Dwarf, says John Tonry, an astronomer at the University of Hawaii at Manoa. That is, even after a solar systems original planets had been torn asunder by the stars expansion during its phase as a Red Giant; when it has exhausted its supply of hydrogen and is reduced to burning helium in its core.
Habitable zones around such White Dwarfs would last between three to eight billion years, says B.J. Fulton, an NSF graduate research fellow at the University of Hawaii at Manoa.
A very cold white dwarf star. Although scientists believe them to be mostly the remnants of dead stars, it is possible that some may have habitable zones, or even planets remaining in orbit around them. (Credit: Wikipedia)
In a paper to appear in The Astrophysical Journal, lead author Fulton and colleagues detail Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) observations of some 1700 bona fide White Dwarfs that all lie within 325 light years of our Sun. Their five years of observations were sensitive to planetary transits (or eclipses) of super Earths five Earth masses and larger across the face of the White Dwarfs.
We found no planetary transits at all, said Tonry, the papers second author. If we had, then we could have immediately taken spectra to measure whether the planet had water or oxygen in its atmosphere.
In contrast to NASAs Kepler Mission, which could spot the tiny transits of terrestrial-mass planets around normal stars, Tonry says their groups survey was a much lower-sensitivity Hail Mary attempt at planet detection.
Even so, as Tonry explains, Hawaiis Pan-STARRS can survey a tenth of the sky in the visible spectrum during a given night and build up deep images by digitally stacking observations. As a result, the papers authors were able to conclude that super Earths in a White Dwarfs circumstellar habitable zone the orbital region warm enough to support liquid planetary surface water are extremely rare.
Most planets would have been destroyed as the star expanded into a Red Giant in the final few million years of its life, said Fulton. In our solar system, Mercury, Venus, and perhaps even Earth will be swallowed and destroyed during this phase of the Suns life.
Our own solar future is mirrored in the thousands of White Dwarfs that lie within a few hundred light years of Earth. But once our Sun expands into a Red Giant some five to six billion years from now, will new planets form out of the detritus of our old inner planetary system?
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