Imagine for a moment that astronomers have finally discovered an Earth-like planet orbiting another star. After the papers are published, the headlines run, the tweets tweeted, one question will still be burning: Is anyone home?
Discovering a habitable planet is just the first tiny step toward discovering an inhabited planet. So how would astronomers begin to determine whether a newly-discovered Earth-like planet actually harbors life?
First, explains Soren Meibom, we must find out whether the planet has been around long enough for life to take hold. "On Earth it took roughly a billion years for the most primitive microbial forms of life to evolve and another three to three and a half billion years for animals and humans," says Meibom. "Therefore, as we look for life beyond Earth, and in particular beyond our own solar system, the question of time becomes highly relevant."
Because stars and their planets form together, to get at the age of a planet, all you need to know is the age of its star. Sounds easy enough. But in practice, determining a star's age is not so simple. After they are born, stars enter a middle age that continues until shortly before their death; over the course of that sustained middle age, which may last for billions of years, the star's appearance is essentially unchanged. Meibom compares a star to a person who emerges from infanthood as a fully-grown adult, then keeps her smooth skin and brunette hair for eight decades until, finally, she goes grey, gains a dowager's hump, and slips on some orthotics in the time it takes her to finish the early-bird special. (See Meibom's visual interpretation below.)
If people aged as stars age. Image courtesty Soren Meibom.
To pin down stars' ages, Meibom and his colleagues are turning to one property of that does change over time: the rate at which a star spins. "A star's rotation slows down steadily with time, like a top spinning on a table, and can be used as a clock to determine its age," says Meibom. "We can measure a star's rotation period by looking for changes in its brightness caused by dark spots on its surface--the stellar equivalent of sunspots. Any time a spot crosses the star's face, it dims slightly. Once the spot rotates out of view, the star's light brightens again. By watching how long it takes for a spot to rotate into view, across the star and out of view again, we learn how fast the star is spinning." Meibom is now using the Kepler space telescope to do just that.
So if a star and its planets are "of age," then what? It might be possible to look for "biosignatures" in a planet's atmosphere. An alien astronomer looking toward Earth, for instance, would see oxygen produced by plants and bacteria, ozone from the interaction of ultraviolet radiation and oxygen molecules, and nitrous oxide from microbial reactions. If we could pick out similar signatures in the atmosphere of an alien world, we would have strong reason to suspect that life might be present there. As astronomer Lisa Kaltenegger told NOVA back in 2009, "If you have a look at our own Earth, you actually have CO2, you have water, methane, and you have ozone or oxygen...That's the golden fingerprint you're looking for" in the spectrum of a habitable exoplanet.
The most conspicuous biosignatures in Earth's atmosphere are products of tiny microbes, not the plants and animals that loom so large in our notion of "life," points out Harvard astronomer Dimitar Sasselov. Our own planet was transformed by microbial life over the course of more than two billion years; so an exoplanet with lots of oxygen in its atmosphere may not be carpeted in forests, but it may have oceans full of green algae. That algae could be paving the way for more complex, energy-hungry forms of life, says Sasselov.
Technology is only now catching up to this ambitious project, though. Measuring an exoplanet atmosphere directly is only practical if the planet is very large and sits far from its parent star--not a great recipe for a habitable world. If an exoplanet happens to pass in front of its star from the vantage point of our telescopes, astronomers can probe its atmosphere indirectly, by comparing starlight that has passed through the planet's atmosphere to starlight that meets our telescopes directly, when the planet is "behind" the star.
Habitable planets will also make juicy targets for SETI--that is, the search for signals from extraterrestrial intelligence. SETI usually means "listening" for radio waves produced, either incidentally or as a beacon, by an alien civilization. (Think Contact.) Some astronomers have also proposed looking for optical signals produced by high-powered lasers.
The discovery of a bona fide habitable world will bring us closer to finding life beyond Earth, but there is still plenty of work to be done before we know whether we have company in the cosmos--or whether we are truly alone among the stars.
For more information on Soren Meibom's work, check out his recent public lecture at the Harvard-Smithsonian Center for Astrophysics, or watch him explain his results at a press conference from last year's meeting of the American Astronomical Society.
NOVA's Finding Life Beyond Earth will premiere Wednesday, October 19 on most PBS stations. Beginning on Thursday, can also watch it online. Check your local listings to confirm when it will be airing near you.