Monday, September 27, 2010; 6:34 PM
Aliens have been in the news this year. In April, cosmic oracle Stephen Hawking, the legendary theoretical physicist, proclaimed that extraterrestrial life is almost certain to exist. He also mentioned, by the way, that we should stay as far away from aliens as possible, since they're probably scavenging the universe for resources after destroying their own homes.
Then, in August, two separate teams of astronomers discovered that distant solar systems had as many as 10 planets, one of which may be capable of hosting life because of its near-Earth size. That brings the total count of confirmed planets outside our solar system to more than 400.
How do astrophysicists find these far-off planets? What makes a planet a good candidate for hosting life? And when can we expect to be Skyping with E.T.? Let's pull on our alien-hunting boots.
The first step is defining our quarry. Hawking could be right - maybe there are fearsome extraterrestrials zipping around the universe at hyper-speed. Or maybe silicon-based life forms are riding across a distant galaxy on an asteroid as you read this. But most astrophysicists aren't betting on that.
Because current technology and resources limit our search to the nearest few hundred stars, our best odds lie in searching for the only kind of life we know to exist: carbon-based cells that consume oxygen and carbon dioxide. That means we're looking for a planet a lot like Earth.
The problem is that it's really hard to find planets more than 20 trillion miles away, because the bright light of their nearby star obscures them from view. It's like trying to find a needle in a haystack, but with someone shining a floodlight in your eyes.
Over the last couple of decades, scientists have gotten around this problem through indirect observation: detecting the planets' effects on other objects.
The first method they use involves gravitational wobble.
Think of a drum major at the head of a marching band spinning his baton. He holds the staff at the center, because the weight is equally distributed. But if one end weighed more than the other, he'd have to grasp the baton closer to the heavy end.
The same thing happens with a planet and its star, which have vastly unequal masses. (Our sun is 333,000 times as massive as Earth.) The greater the difference, the closer the rotational center moves toward the star.
Most stars are so much heavier than their planets that the point of rotation is inside the star itself, just off its geometric center. That makes the star wobble a little bit as it rotates. Scientists used wobble to identify the first extrasolar planets in the 1990s.
Transiting is another technique.
Every once in a while, the light intensity from a distant star dips, as an orbiting planet crosses between the star and our telescopes, causing a tiny eclipse. This is a great method, because the shadow offers clues about the planet's shape and size.
While we've learned a lot about planets through their gravitational and eclipsing effects, studying planets by indirect observation is a little bit like studying an animal based on its tracks: You can only learn so much without seeing the real thing.
In the last few years, however, scientists have snapped the first pictures of extrasolar planets using a technique called coronagraphy.
Ben R. Oppenheimer, an astrophysicist at the American Museum of Natural History in New York and a pioneer in the field, describes the coronagraph as an artificial eclipse machine, because it blocks out the star's light. (Planets emit light, even though our eyes can't detect it. They produce heat, which is infrared light just outside the field of human vision.)
"It's like a performer blocking out the spotlight by holding his hand in front of his face, so he can see the audience," Oppenheimer says.
The coronagraph attaches to a telescope and teases apart a beam of light into constituent wavelengths. Through the psychedelic red, blue and yellow chaos of a coronagraphic image, an astrophysicist looks for a constant light source - an indication that a particular beam of light has nothing to do with imperfections in the device or other electromagnetic noise. They look like little blue spots, a lighthouse in an interstellar storm.
Oppenheimer has used the device to find brown dwarfs, young stars that look like planets. Now he's preparing to deploy it to answer what he calls "the most compelling question in science": whether there is extraterrestrial life. (The excitement of the hunt isn't lost on the enthusiastic Oppenheimer, who mingles phrases such as "micro-arc-seconds" and "pretty cool.")
Why is direct observation so important? It will be a very long time before we get close enough to an extrasolar planet to photograph its bacteria. For now, we have to rely on light from the planet itself, since the bacteria are too small to see from here.
Fortunately, light carries a few secrets on its interstellar voyage.
Molecules in a planet's atmosphere absorb certain wavelengths of light, preventing them from reaching us. If astrophysicists can identify those missing wavelengths in light that has made it to Earth, they can determine what kind of molecules are floating around the distant planet.
"Oxygen wouldn't be on Earth in this abundance if there were no life here," says Oppenheimer. It's only here because carbon-based life forms, such as bacteria and plants, suck in carbon dioxide, break it up, and expel oxygen.
If we can find a planet rich in oxygen, carbon dioxide, methane and water - the molecules crucial to life - we'll know that something is living on that distant rock.
Is it reasonable to expect that a headline like "Alien Life Found" will splash across The Post's front page in your lifetime? Oppenheimer thinks so (although he envisions something more like bacteria than Hawking's marauding alien hordes).
Hawking may be right: We may soon find a race of bullies scouring the galaxy for inferior life forms to exploit. Except maybe the bullies will be us. Chew on that for a while, sci-fi fans.