A news release last week reported what may seem self-evident to most planet hunters: white dwarf stars are lousy places to go looking for inhabited worlds. However, we’ve learned that exoplanets are so eclectic, that we should never say never.
A white dwarf is the burned-out dense core of a sunlike star that has collapsed under gravity. It’s conceivable that planets could wind up orbiting close enough to the dwarf to be warmed by its feeble radiation.
But the odds of them being habitable are very unlikely, concludes Rory Barnes of the University of Washington and René Heller of Germany’s Leibniz Institute for Astrophysics Potsdam in a report in the November edition of Astrobiology. The researchers looked at two planets know to orbit a white dwarf in or near its habitable zone. They predict that tidal heating from their close passage to the white dwarf will desiccate the planets and leave them sterile.
But I’d say not so fast. If there’s one thing we’ve learned from exoplanet discoveries is that we live in a compulsive universe where anything is possible so long as it doesn’t violate the laws of physics and chemistry.
Though white dwarfs are dim, they are very hot and continue radiating heat for billions of years (a white dwarf can be seen in this Hubble Space Telescope image orbiting the bright winter star Sirius). That’s long enough for life to evolve on a post-apocalypse world that survived the star’s burnout. Where there’s a thermal gradient from very hot to to very cold, there is always a chance for life to take advantage of it.
What’s more, a long-lived super-civilization will eventually face the prospect of living around a white dwarf, as an alternative to abandoning the system and sending a small fraction of their population to colonize nearby stars. Coping with the dwarf may be more practical than assembling an interstellar wagon train.
After swelling to a red giant and convulsing off half of it mass as hot gas, the sun will burn out as a white dwarf in about 6 billion years. The remnant core of dense carbon “ash” will be no bigger than Earth. Glowing at tens of thousands of degrees Fahrenheit, the sun’s surface will pour out ultraviolet light. But the sun will only be 1/1,000th its present brightness as an icy blue pinpoint star in the sky, even as seen from surviving planets.
The habitable zone will shrink to a radius of only one million miles, four times the distance between Earth and the moon. Therein lies the problem. A planet in a white dwarf’s habitable zone will be so close to the dwarf that powerful tidal forces will heat the planet. This will evaporate away water the planet might have had, say the authors.
An analog can be found by looking at Jupiter’s innermost Galilean satellite, Io (seen below). Though no bigger that our moon, Io has several active volcanoes spewing sulfur into space. Io’s interior is kept warm enough to power the volcanoes largely by “tidal pumping” from Jupiter’s gravitational grasp.
One reason not to write off white dwarfs is that, as slowing cooling embers, they release energy for billions of years. Over that vast time-span almost anything can happen.
In the first billion years of its existence the white dwarf cools rapidly and the habitable zone shrinks like a tightening noose. But the cooling rate slows between 4 and 7 billion years, leaving plenty of time for interesting things to happen on a planet.
To get that close to the dwarf in the first place a planet would have to migrate from farther out in the surviving system (the red giant phase of the star would have obliterated worlds that originally formed close to the star). Any number of chaotic planetary pinball interactions in the outer system could hurtle a planet inward.
The star’s tidal pull would transform the planet’s orbit from elliptical to circular.
But the price is tidal heating that would bake away any water, turning the planet into a Venus-like world. The dwarf’s intense ultraviolet radiation would efficiently break apart water vapor into hydrogen and oxygen.
If the migrating planet were an ice giant, like Uranus or Neptune, it might not be totally dried out. Or imagine a Titan-sized moon that may have enough of a water reservoir, and geologic mechanism for continually irrigating the surface, to survive being made bone dry. “Planets are complex objects and we hesitate to rule our habitability,” say the authors.
However, they do write off a pair of candidate white dwarf planets discovered by NASA’s Kepler space observatory. The planets, designated KOI 55.01 and KOI 55.02 are rocky worlds that are 75 percent and 87 percent Earth’s diameter, respectively. But the radiation from the star and tidal forces are roasting these planets. What’s more, the planets’ gravitational tug on each other may inhibit them from setting into circular orbits, and therefore tidal stresses with the dwarf would persist. They cannot be habitable now and will unlikely become livable for life as we know it in the future, the researchers conclude. “These planets may be typical of those that orbit white dwarfs.”
A super-civilization that has reach a level of social and technological maturity that makes them virtually immortal, could have a long-term plan for surviving through the star’s late stages of evolution. They would have to shuffle planet orbits to set up worlds that are livable as the star’s habitable zone shifts outward and then inward as stellar brightness rises and fall.
The final stage might be to target asteroid flybys to exchange momentum with a large icy moon and cause it to fall toward the white dwarf. Alternatively, aliens might fashion a compact Dyson Sphere to encase the dwarf. Such astroengineering would buy the civilization billion of years more to remain in their home star system.
Assuming a habitability niche could be created, living conditions around a simmering white dwarf would be more placid than around a petulant star with its seething electromagnetic storms.
Image credits: NASA, S Charbinet