As more planets emerge, astronomers are confident they'll find one like Earth
Tuesday, January 12, 2010
It seems increasingly likely that, as they stare at the heavens, astronomers are going to find an Earth out there, or at least something that they can plausibly claim is a rocky planet where water could splash at the surface and -- who knows? -- harbor some kind of life. But it's also clear that, when they make their big discovery, the astronomers might want to hire movie director James Cameron to help with the special effects.
The roughly 400 planets that astronomers have found outside our solar system have not been Earthlike by any stretch of the imagination. Most are hot Jupiters, which is to say they're gas giants in scorching orbits.
They've also been pretty much invisible, their presence inferred from fluctuations in starlight. The planet emerges from the data. Astronomers will announce a new planet find with a graph, typically with a nice curving line that represents the periodic changes in starlight associated with the orbiting body. There are no pictures. Which is fine for scientists.
"To me, a spectrum is more beautiful than a picture," said David Latham, a Harvard astronomer who came to Washington last week for the annual meeting of the American Astronomical Society. "A spectrum can tell you about the physics."
Yeah, but what about seeing some clouds, continents, jungles? How about some flying reptilians or thundering ungulates? Why can't astronomers produce a planet that looks more like the "exomoon" Pandora from "Avatar"?
"Sorry, they have a bigger budget than we do," Latham quipped.
Finding an Earthlike planet is really hard. Even detecting one of those hot Jupiters requires prolonged observations that pick up extremely subtle shifts in starlight. Keep in mind that it's been only a decade and a half that we've been able to detect any of these planets outside our solar system.
But the hunt has been fruitful. Astronomers say it's increasingly obvious that the galaxy is lousy with planets.
Planets seem to be a natural byproduct of star formation. But from our vantage point, the feeble, reflected light of a planet is utterly lost in the glare of the star right next to it. Astronomers are developing techniques for blocking that starlight so that only "planetlight" remains. Such occultation requires exquisite engineering. It's just a bit like trying to write the Book of Genesis on a grain of rice.
In the meantime, most planets will be discovered through indirect techniques. The most common is the radial velocity method, in which astronomers study the slight Doppler shift in starlight as the star is tugged to and fro by an orbiting planet. Another technique watches for the dimming of a star as a planet passes in front of it.
An "Earth" is especially hard to detect because it is by definition small and fairly far from the parent star, and thus leaves only the faintest fingerprint on the starlight. Improvements in detection will be incremental; new discoveries are likely to carry some level of uncertainty. There won't be a single eureka moment, like sighting land from the crow's-nest of a ship and having immediate confirmation when the sailors wade onto the beach.
There will surely be debate among astronomers about whether the next Earth is sufficiently Earthlike. Astronomers will ask: Are we sure about the size and density? Is the orbit truly in the "habitable zone" of the star, where water could be liquid? Is this a stable, long-lived star like our sun or some hothead star that's been around only a few million years? Does the planet have an atmosphere? Does the atmosphere have water, oxygen, ozone, methane or some other signatures of life?
There was much discussion at the Washington astronomy meeting about a planet named COROT-7-b, discovered by European astronomers. In a NASA press release last week, it was referred to as the most Earthlike planet found so far. But in fact, it's only Earthlike in its size and density. It's about 60 times closer to the star than Earth is to the sun, and in those heat-lamp conditions may have a surface temperature of 4,000 degrees.
Several astronomers acknowledged in conversations last week that the planet-hunting field is vulnerable to hype. The news media are all too eager to get attention with sensational claims of alien-world discoveries.
Scott Gaudi, an astronomer at Ohio State University, says of the scientific process, "We make mistakes. We have uncertainties in our conclusions which we are aware of. But it's a net assimilation of knowledge."
"I think it's important that people understand that it is incremental," says Charles Beichman, executive director of the NASA Exoplanet Science Institute at Caltech. "We're going to have to have 10 incremental steps to get there. But we'll get there." The scientists behind NASA's Kepler space telescope, launched last March to search for Earthlike planets, are being careful with their announcements precisely because they don't want to proclaim a discovery that turns out to be a false positive. Kepler has already found five planets and is likely to find many more.
Astronomers are getting so bold as to say they will be able to examine the atmosphere of such planets to see if they have the kind of components that we associate with life. They might even be able to tell if a planet has weather, continents -- and jungles.
Green plants on Earth, for example, reflect a great deal of light in the infrared part of the spectrum. Scientists refer to this as the "red edge."
Says Sara Seager, an astrophysicist at MIT, "If our eyes could see a little bit farther to the infrared, the world would be a completely different place. Plants would be as bright as fresh snow. And red. Very red."
So that's one of the dreams: Find a planet and look for the red edge.
Which might not work, of course, if the alien plants are blue.