Corpse stars could nurture life on alien planets

WHITE dwarfs may be dying, but their light could be just right to sustain life as we know it. That could make habitable planets even more common than we think.

Many planet-hunting missions have focused on finding rocky exoplanets around sun-like stars, based on the notion that an exact Earth twin would be a prime breeding ground for alien life.

White dwarfs, by contrast, would seem unlikely hosts. These smouldering cores form when stars around the same mass as our sun reach the end of their lives. First the stars balloon to red giants, then they shed their outer gas layers and leave behind dim, ultradense orbs not much larger than Earth itself.

Still, previous work suggested that the stellar corpses could maintain habitable zones, regions where liquid water can exist on a planet's surface, for more than 8 billion years. As our own solar system is 4.5 billion years old, a habitable world around a white dwarf should have plenty of time to give rise to some form of life.

Now a new study shows that an Earth-like planet in a white dwarf's habitable zone would get light at the right wavelengths to sustain photosynthesis. Crucially, such a world would not get too much damaging ultraviolet radiation, which can stop life in its tracks.

Luca Fossati at the Open University in the UK and his colleagues started by assuming that this hypothetical planet has an atmosphere similar to Earth's.

By simulating the conditions created by a white dwarf, the team calculated the amount of starlight that would reach the planet's surface. They then compared the results with the wavelengths of light DNA absorbs, particularly UV waves known to damage DNA.

The researchers found that the planet would get just 1.65 times as much UV light as Earth does (arxiv.org/abs/1207.6210). "The dose is remarkably benign from an astrobiological perspective," says Fossati.

For the optical wavelengths that play roles in photosynthesis, the team found conditions almost identical to those on Earth.

Planets surrounding red dwarf stars have also been proposed as alternative sites for life, says Fossati, in part because these small, cool stars are the most common in our galaxy. But they can experience intense stellar activity, including flares of radiation bigger than the ones that affect Earth. White dwarfs are less temperamental, and would provide life with a more stable home, says Fossati.

"The team's evaluation of habitable planets at white dwarfs is an excellent way to smash preconceptions about these systems," says Jay Farihi at the University of Leicester in the UK.

One lingering question is how an Earth-sized planet would come to be in the right orbit around a white dwarf star. In our solar system, the sun will expand into a red giant in about 5 billion years' time, and astronomers think it will pulverise everything well past Earth's orbit before it shrinks back down into a white dwarf.

While what look like planetary remnants have been seen around white dwarfs, no intact planets have been found so far. "That's the most tricky bit," says Fossati. "We don't yet know if a full planet could survive."

Finding a 'Blue Marble'

Starlight could tell us even more about an exoplanet once the world has been spotted. A new technique shows how analysing light reflected off an alien world can give us a glimpse of its surface conditions, with clear distinctions between clouds, continents and oceans, just like the "Blue Marble" image of Earth seen from space.

Hajime Kawahara at Tokyo Metropolitan University and Yuka Fujii at the University of Tokyo in Japan used data from the Earth-orbiting Terra satellite to model the annual variation in light reflected from our planet. The pair then used this to create 2D maps of the light from hypothetical Earth-like planets with varying surface features. These maps can be compared with the light variations of real exoplanets to figure out the kinds of habitats they might hold (The Astrophysical Journal, DOI: 10.1088/0004-637X/755/2/101).

At first a nearby rocky planet might just look like a pale blue dot, says Kawahara. "By looking at that dot over time, our method can retrieve an image more like the Blue Marble," he says.

The method could also reveal whether plants are growing on the distant worlds by revealing something called "red edge", says Fujii. This abrupt jump in the amount of light near the red end of the spectrum is a distinctive feature of vegetation on Earth.

Although current telescopes can't distinguish the light of small, rocky worlds from the glare of the host stars, proposed missions such as the Occulting Ozone Observatory might soon be able to use this technique.

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