In death, it has been said, there is often life. And while it may sound counterintuitive, this old saw may hold especially true in the search for life of the extraterrestrial variety. NASA's Kepler mission may be just years away from discovering Earth 2.0, but the first signs of life probably won't come from planets orbiting a vivacious, active, Sun-like star. On the contrary — exciting new research indicates that if dying stars can host planets with life (and evidence suggests they very well could), that life could be detected quite easily — and well within the decade, at that.
Above: "White Dwarf" by I3a12C1, via deviantART
It goes without saying that the search for life on other worlds is a complicated endeavor. I've said it anyway, because what usually goes unsaid is that the reasons for its complexity have changed dramatically in recent years.
The search for life on planets beyond our solar system can be divided into two broad tasks. The first is finding bodies worthy of investigation (i.e. potentially habitable, Earth-like planets). The second is investigating said planets for evidence of their habitability, including traces of pre-existing life.
Task one, it so happens, has become rather straightforward. For almost four years now, NASA's planet-hunting Kepler observatory has been searching for Earth-like planets coursing around Sun-like stars; and it is damn good at its job. At the rate it's finding exoplanets, most astronomers agree the Kepler mission is a few short years from the first-ever discovery of an Earth twin — assuming it hasn't found it already.
According to Harvard astrophysicist Avi Loeb, it's the second task — the search for signs of life on a distant exoplanet — that now complicates things. Since we can't visit these planets in person, Loeb tells io9 one of our best options is to search their atmospheres for biomarkers — chemical fingerprints, indicative of life, left by chemicals like oxygen. But dusting for these fingerprints can be very difficult.
The best way to search for them is by observing a planet as it crosses the face of its parent star. When it does, light shines through the planet's atmosphere, which absorbs some of the starlight on its way to whoever/whatever is observing it. Different atmospheric chemicals give rise to different absorption patterns, which our telescopes can, in theory, read and then decode.
The problem, says Loeb, is that for most planets in orbit around Sun-like stars, the area of the planet is thousands of times smaller than the ball of gas it's orbiting. (Consider the composite image above, which shows Venus transiting the face of the Sun last June.) The vast majority of the light that we see fails to pass through the planet's atmosphere. "This makes the detection of oxygen absorption very challenging since it amounts to a tiny feature in the spectrum of the background star," Loeb explains.
Not so, he says, for a planet orbiting a white dwarf (a small, cool, dense stellar remnant, known colloquially as a "dead" or "dying" star). Together with astrophysicist Dan Maoz, Loeb has conducted a theoretical study of Earth-like planets orbiting white dwarf stars and concluded that they, of all planets, would be the most likely to yield detectable signs of life. Their findings demonstrate that the chemical fingerprints of atmospheric oxygen could be detected within the next decade, thanks to the unprecedented power of the James Webb Space Telescope, successor to Hubble.
"The size of the white dwarf is comparable to that of the Earth," Loeb tells io9, "so a transit [of a planet across its face, as depicted above] could produce an substantial dimming in the background light of the star." This, in turn, would make bio-markers like atmospheric oxygen far easier to detect.
Loeb says that by current estimations, there are likely billions of white dwarfs in the Milky Way alone, and evidence suggests that as many as a third of them may harbor planets. "We can now design an observing program that could detect bio-markers in the atmosphere of an Earth twin orbiting in the habitable zone around a white dwarf," says he tells us, "if such a planet exists."
The first step of this program, says Loeb would be to identify a thousand nearby white dwarfs, and check whether any of them shows evidence for a transitting Earth twin at the appropriate distance from its parent star. After that, it's up to the massive imaging abilities of the James Webb Space Telescope to scrutinize the planet's atmosphere. (Pictured here is the technology comprising the heart and soul of the JWST: an ultra-sophisticated beryllium mirror system. Click to enlarge.)
Would Loeb prefer to investigate an Earth twin around a Sun-like star? "Of course," he says, but this simply isn't practical in the near future. The steps in his outline could be easily achieved within the decade. ]
"It's exciting, he says "to live at a time when we could answer the question Are We Alone? scientifically, using state-of-the-art technology."
"Indeed, until now, the prospects seemed grim that, within the foreseeable future, we would have the technological capability to detect signs of ET life, whether it is there or not," Moaz tells io9, echoing his colleague's sentiment. "Things look much more encouraging now."
The researchers' findings have been accepted for publication in the Monthly Notices of the Royal Astronomical Society. They're also available, free of charge, on arXiv.