Some day, scientists hope to find the Holy Grail of planetary astronomy -- an Earth-like planet circling another star in the not-too-hot, not-too-cold region known as the "Goldilocks Zone."
But then what? From as far as 100 light years away, how can astronomers ever determine whether that tiny dot of a planet, like its cousin Earth, actually nurtures living things? To solve the problem, they are turning their telescopes in an unlikely direction: to Earth's lifeless moon.
They are studying "earthshine," the dim, ghostly light reflected from the shadowed portions of the moon during its slim crescent phases.
It's called earthshine because, as Leonardo da Vinci first recognized, it's produced by sunlight that's first reflected off the sunlit side of the Earth, and then bounced back onto the moon, faintly illuminating the surface "between the horns of the New Moon."
What has fired scientists' imaginations is that, when sunlight first strikes the Earth, portions of the light are absorbed -- by the land, water and atmosphere. And the light that remains is encoded with Earth's chemical signature.
By learning to read the language of Earth's biochemistry in that code, scientists hope one day to be able to recognize it again in the starlight reflected off a planet circling another star.
Climate scientists also use earthshine to measure the reflectivity of Earth's surface and cloud cover. This affects how much solar energy reaches the surface.
Changes in that reflectivity have been detected over the decades, and they are among the most powerful -- and least understood -- factors influencing global temperature change.
"It [earthshine] is quite beautiful, but also for us, it turns out to be scientifically very valuable," said solar physicist Philip R. Goode, director of the Big Bear Solar Observatory in California and a professor at the New Jersey Institute of Technology.
Goode was one of seven scientists who spoke about earthshine studies last week at the 2006 Joint Assembly of the American Geophysical Union in Baltimore.
So far, astronomers have found evidence of more than 180 "extra-solar" planets. But none of them as yet qualify as "Earth-like."
Most have been inhospitable, Jupiter-like giants, detected because their huge gravitational pull causes their stars to "wobble," or because they repeatedly dim the light of their stars when they orbit between those stars and observers on Earth.
Many are also orbiting their stars in regions that are likely to be too close (and too hot) to support life, or too distant and cold to provide liquid water and breathable air.
Unfortunately, ground-based telescopes, trapped beneath the turbulence of the Earth's atmosphere, will never be good enough to spot Earth-like planets in the habitable zones around other stars.
"It's totally hopeless to even think about it," said Wesley A. Traub, a project scientist with NASA's Terrestrial Planet Finder Coronagraph Mission, or TPF-C.
Not surprisingly, Traub believes the agency's two planned spacecraft, TPF-C and companion TPF-I (for Interferometer) have the best chance to tease the dim reflected light of habitable planets from the blaze of their parent stars. This assumes that NASA gives the project go-ahead funding, which is far from certain.
After launch, the spacecraft would be moved to a point about 1 million miles from Earth, where their large, 8-meter telescopes would begin a search among nearby stars. That means stars within 100 light years of our sun.
At that distance, not even a TPF telescope would be able to see any surface features. Planets would appear as a single point of reflected light comprising all the planet's chemical signatures.
To learn what that might look like, and what information they might extract from it, scientists turned to earthshine -- the sum of all the Earth's reflected light, and the spectral equivalent of our planet as a single dot, seen from billions of miles away in space.
Viewed from observatories in California, Texas, Hawaii and the Canary Islands, and broken into its constituent frequencies, earthshine has revealed sharp evidence of water vapor and even the signature of blue skies on our planet, Traub said.
The analyses of visible light in earthshine also signal the presence of atmospheric oxygen, and the ozone that forms from it in the stratosphere. That's the ozone layer that shields Earth's life forms from deadly solar radiation.
"These are prime indicators of life on a planet. The only way we can think of that you can generate a lot of oxygen on a planet is to have life," Traub said. "You can generate tiny amounts, like you have on Mars or Venus by just photo-dissociating CO2 or water. But to generate large amounts -- 20 percent or so -- you really need plants creating the stuff."
Spectral analysis of earthshine can also reveal the presence of chlorophyll and green vegetation, which reflects very brightly in the near-infrared wavelengths. But it's more difficult than scientists had imagined.
Work by NJIT research professor Pilar Montanes-Rodriguez found that Earth's cloud cover, which averages 60 percent, obscures a surprising amount of the plant life. But when the clouds, the Earth's rotation and solar illumination all align, she said, the light signature of plant life "can be unambiguously detected."
But what if life on the target planet has only just begun to evolve? Using Earth as his model, Traub has also calculated what those light spectra might look like. "Back before any life arose on Earth you would see a lot of CO2 and methane in the spectrum," he said. "And you wouldn't see any oxygen line. You wouldn't see any ozone line because the ozone comes from the oxygen.
"And as the Earth evolves you see the CO2 disappearing, and you see the oxygen increasing, and at some late point, 700 million years ago or so, you see the [infrared signal] of vegetation coming on," he said.
None of this work will happen, however, without the Terrestrial Planet Finder mission. With sufficient funding, it could launch as early as 2015, Traub said, but its future is in doubt.
Although President Bush's 2004 Vision for Space Exploration called for searches for habitable planets around other stars, the demand for cash to support the initiative's manned space flight priorities threatens to cripple or delay the planetary quest. All TPF funding, from fiscal year 2007 forward, has been "indefinitely deferred," Traub said.
"We're hoping that NASA will change their mind and at least put some funding into thinking about how to build these missions, because if we don't think about it, we aren't ever going to be able to do it when serious funding becomes available," Traub said.
There's no price tag yet, but it's "a major strategic mission for NASA," Traub said. "One could presume this would cost several billion dollars."
The good news is that the scientists searching for Earth-like planets are getting closer to success. Recently discovered planets have been as small as 5 1/2 times Earth's mass. That sounds big, but it's small compared with earlier discoveries of planets many times Jupiter's size.
"I do believe there are many Earths out there to look for," Traub said. "That's just a matter of faith, but it's based on watching what people are measuring these days. And the planets people have been measuring are getting smaller and smaller as the technology gets better and better."