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The hunt for human-habitable worlds continues with a new theoretical study suggesting we could more readily detect oxygen in the atmosphere of planets orbiting a white dwarf star, rather than a star more like our own sun.
The best method for finding planets is a transit search — looking for a star that dims as an orbiting planet crosses in front of it. Although astronomers have determined that the closest habitable planet probably orbits a red dwarf star (a cool, low-mass star undergoing nuclear fusion) the glare from a red dwarf can still overwhelm the faint signs from a an orbiting planet’s atmosphere. White dwarfs, smaller and dimmer, are usually about the same size as Earth, so an Earth-sized planet in transit would block a most of its light and create an obvious signal to observers.
So what is a white dwarf, and how is it different from other start?When a star dies, it first swells into a “red giant”; its outer layers burst away, leaving behind a hot core called a white dwarf. This white dwarf can retain sufficient heat to warm nearby planets for eons, although they must orbit closely in order to have liquid water on their surfaces. The preceding red giant would have engulfed and destroyed any nearby planets, so any worlds in this habitable zone must arrive after the white dwarf forms. They could be “second-generation” worlds formed from leftover gas and dust, or planets migrating in from a greater distance. The abundance of heavy elements on the surface of white dwarfs suggests that a many of them host rocky planets.
When the white dwarf’s light shines through the atmosphere surrounding a planet, the atmosphere absorbs some starlight and leaves chemical “fingerprints” showing whether that air contains water vapor or oxygen. NASA’s James Webb Space Telescope (JWST), scheduled for launch by the end of this decade, might be able to detect oxygen and water vapor on such a planet within only a few hours of observation time. Some researchers estimate that surveying the 500 white dwarfs nearest to Earth could identify one or more habitable worlds.
When it comes to evaluating planets, the term “habitable” is terribly relative. We tend to assume that any form of life must be organically similar to ourselves: carbon-based, dependent on oxygen and water to survive. This is both terribly arrogant and terribly unimaginative. Given the size of the universe, it’s likely there are thousands of planets perfectly suitable for life-forms with different requirements than those on Earth. With this in mind, it seems more appropriate to say that the technique suggested in this study will help identify planets suitable for our own colonization.
Imagine that a decade or so from now, the JWST spots a planet that seems ideally suited for us homo sapiens. A rocky world with temperate climes, liquid water, and oxygen in its atmosphere, orbiting one of our nearest white dwarf. What would we do? Would the discovery exist only within academic papers, or would it inspire us to finally step out into the stars? If we’d already gotten our collective feet wet with a lunar base or colony on Mars, we might have the confidence to consider such a journey.
But here we encounter another relative term: “nearby”. Worlds that are next-door neighbors on a galactic scale could still take decades or longer to travel between with our current technology. It wouldn’t be practical to send out multiple sallies (robotic probes, then manned scouting missions, etc) if it took a hundred years for each one to reach its destination. If the state of our homeworld were truly dire, would we take the leap of faith and travel to an alien world that looks good in the telescope? Pack up the kids and the 3D printer and set out for a destination that could either give our race a new start or, should we arrive and find it less than we hoped, leave us to die in space? How brave or desperate (or both) would be have to be to buy that one-way ticket?