Sniffing Out ET

Research Blogging / by Dave Munger /

The discovery of potentially habitable planets beyond our solar system is imminent. But no one really knows when we might learn whether any of those distant worlds are inhabited.

Credit: L. Calçada/ESO

You might think looking for planets outside of the solar system is a little like looking for a needle in a haystack. Actually, finding a needle in a haystack is easy as long as you have a powerful enough metal detector. Try finding that same needle in a barn full of cannonballs, using only your metal detector. If you’re looking for planets similar to Earth, reduce the size of the needles a hundred-fold. Now you might have a better idea of what searching for planets orbiting stars hundreds of light-years away is really like.

Last week, NASA announced that its team of scientists working on the Kepler mission had found a likely candidate for a planet outside of the solar system that’s roughly the same size as Earth. Unfortunately, even if it is verified, it’s unlikely to harbor life, as surface temperatures for a planet so close to its sun are estimated at over 2,000° C.

So what would it take to actually find a planet similar enough to our own that carbon-based life—the only form of life known here on Earth—would be possible? Last month, astronomy graduate student Alexander Bastides Fry discussed efforts to find extraterrestrial life. The most obvious planet to support this type of life would be roughly Earth-size, Earth-distance from a star similar to the Sun.

The Kepler mission takes a brute-force approach to solving the problem: This space telescope points at a region of the sky near the Milky Way’s galactic plane and filled with lots of Sun-like stars. Kepler was launched in 2009 and is a relatively small satellite, about 5 meters long, but unlike the much larger Hubble Telescope, it orbits the Sun, not the Earth. This gives it an unobstructed view to the same area of the sky all year long. The telescope observes the same 150,000 stars every 30 minutes, in hopes of catching a planetary transit—a planet crossing in front of its sun. This registers as a slightly dimmer star in the image. But since planetary systems like ours are disk-shaped, with all the planets more or less in the same orbital plane, only a few of them are likely to be oriented so that their planets are detectable: If Kepler photographs the disk from the top instead of the side, then it won’t ever see a transit. An Earth-like planet orbiting a Sun-like star would be seen in transit in just an estimated 0.5 percent of all such systems. So if planets similar to Earth are common, then NASA calculates Kepler should spot anywhere from 50 to several hundred of them among the stars it surveys. If they are rare, then many fewer will be found.

NASA doesn’t expect to see these results for a few years: A star must dim at least three times before it is reasonable to conclude that a planet is causing the dimming (think about it—you need to see three transits to confirm a regular pattern indicating a planet’s orbit). For a planet exactly like Earth with a 365-day orbit, this would take two years if you were lucky enough to spot it on the first day you started taking pictures, and it could just as easily take three years (if you missed it the first time around). The planets Kepler has found so far are much closer to their stars, with faster orbits.

But suppose a suitable planet was found. How would we know if life existed there? According to Fry, University of Washington astrobiologist Shawn Domagal-Goldman says there are a number of possible clues, but just as finding the planet in the first place is difficult, so too is seeing what it’s made of once you find it. The classic technique astronomers use to determine what objects in space are made of is to examine the spectral signature of the light they emit, reflect, or absorb. In the case of a star, you just attach something like a prism to your telescope, then examine the “rainbow” created by its light. Under close analysis, different elements and compounds have different spectral signatures. But planets don’t emit much light, especially compared to the bright stars nearby. Fry says detecting the spectral signature of an Earth-size planet orbiting close in to a distant star is “like trying to tell the color of the wings on a gnat hovering around a spotlight on the moon.” It can be done, but it requires a far more powerful telescope than Kepler, and a lot of patience.

In 2008, Virginia graduate student Nicole Gugliucci explained how one planet, Fomalhaut b, was spotted using the Hubble Space Telescope. The telescope used a shade to block out the much brighter light of the central star, but this still left a blinding glare, which again made it difficult to see the planet. To remove the glare from the images, scientists took an image of a similar star’s corona and then mathematically subtracted it out. Comparing images taken two years apart confirmed that the suspected planet had moved the way it should if it was actually orbiting the star. Then scientists could analyze the planet’s spectrum and figure out what its atmosphere was made of.

So if a planet has life like that on Earth, what would be in its atmosphere? Domagal-Goldman says that ozone, an unstable form of oxygen found in Earth’s upper atmosphere, would be a near sure sign of life. In the absence of life forms to replenish it, ozone rapidly breaks down. Water is also a good sign, but on its own doesn’t necessarily indicate life is present. Oxygen, while generated by plants here on Earth, could be a product of runaway greenhouse effects—without ozone, or some other compounds in the right proportions, we wouldn’t know for sure. Unfortunately, at the distances and contrast ratios we’re talking about for most of Kepler’s potential discoveries, we do not currently have the technology to measure those things. While Kepler can help us spot planets that might be able to support life, researchers now need to develop larger, more sophisticated telescopes to find out if life is actually present.

Dave Munger is editor of, where you can find thousands of blog posts on this and myriad other topics. Each week, he writes about recent posts on peer-reviewed research from across the blogosphere. See previous Research Blogging columns »

Originally published September 1, 2010

Tags biology proof space technology

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