Searching for Signs of Life: If an Alien Civilization Pointed its Telescopes at Earth, Will they Determine whether Earth has life?

Searching for Signs of Life: If an Alien Civilization Pointed its Telescopes at Earth

, Will they Determine whether Earth has life?

Somewhere not too far away in the Milky Way Galaxy, an alien civilization may have advanced space telescopes trained on Earth. Does it recognize Earth has conditions to support life? Can it determine whether Earth has life? Astronomers and engineers are asking these questions as they design an ambitious space telescope with the goal of detecting and characterizing earthlike planets around nearby stars. NASA plans to launch a visiblewavelength telescope called Terrestrial Planet Finder (TPF) in 2014, followed by a complementary, midinfrared-wavelength version in 2019. To design TPF, scientists must consider what an alien astronomer would be able to learn about Earth from a distance of tens of light-years. C? Even looking for a terrestrial planet - a small, rocky world - poses technological challenges, but those are few compared with the problems in identifying a terrestrial planet possessing liquid-water oceans, an "Earth twin." Perhaps surprisingly, such a planet's dimness is not a major concern. TPF will search for Earth twins out to a distance of about 50 light-years. At this distance, an earthlike world would not be too faint for today's telescopes. The Hubble Space Telescope often views galaxies fainter than such a world would be. C The real problem stems from the brightness of the adjacent parent star. Such a star would shine 10 million to 1 billion times brighter than a planet like Earth, depending on the wavelength of light observed. This huge contrast between faint target planet and bright host star creates a daunting technological challenge, and is the most serious issue facing Earth-hunting astronomers. To image an earthlike planet around another star, astronomers must first get rid of the starlight. Visit here for a creative solution to the challenge and for a comprehensive scientific account of the 'life beyond' without the prescriptive claptrap.

LIVING VEGETATION strongly reflects near-infrared wavelengths. The photo above was taken with color slide film, the one at right with infrared-sensitive film. If our eyes saw the near-infrared, vegetation would appear a bright red. Notice the dead grass, which appears yellow in the color photograph, does not have a reddish hue in the infrared photo.

Because the Moon is not a good mirror, earthshine provides astronomers with only a spatially unresolved image of Earth. That image is contaminated, however, because earthshine is sunlight reflected off both Earth and the Moon. To study Earth alone, scientists have to remove the Sun and Moons spectra. Astronomers subtract light from the illuminated part of the lunar crescent - which also contains the Sun and Moon's spectra - to make this correction.

Earthshine contains mostly wavelengths of visible light with a little near-infrared included. The reason: Solar radiation first must penetrate Earth's atmosphere, then reflect off the ground and back into space. The atmosphere absorbs most mid-infrared radiation and prevents it from ever reaching the ground. Also, the Moon's surface radiates heat and appears bright at these wavelengths, swamping any Earth signal.

You might wonder why astronomers don't use observations from satellites that image Earth directly. Although these satellites provide excellent images of specific parts of Earth in narrow-wavelength windows, they do not provide the distant overview astronomers need. Earthshine provides hemispherical data that astronomers can measure over a broad wavelength range - just the kind of quantities an extraterrestrial astronomer would see.

Although earthshine won't allow astronomers to observe Earth like an extrasolar planet at mid-infrared wavelengths, an earthshine spectrum tells us a great deal about our own planet that we can use as a test case. Oxygen, ozone, and water vapor in our atmosphere create several strong absorption features as the molecules absorb photons from incoming sunlight.

None of these molecules could exist on a rocky, terrestrial planet with an atmosphere in thermal and chemical equilibrium. An alien civilization with its space telescopes trained on Earth should recognize our atmosphere as being in a state of extreme disequilibrium, and the molecules present at visible wavelengths suggest activity of a biological nature.

Beyond earthshine

Interplanetary spacecraft that can look back at Earth - without simultaneously looking at the Sun - can give us more information about how Earth would appear as an extrasolar planet. While on its way to Mars in 1996, the Mars Global Surveyor spacecraft did just this. Astronomers saw the strong signature of carbon dioxide (CO^sub 2^) at midinfrared wavelengths. Venus, Earth, and Mars all have strong signals from carbon dioxide, while none of the solar systems gas-giant planets do. That makes CO2 a good, although not perfect, indicator of a terrestrial planet. Ozone, a by-product of molecular oxygen, also showed a strong mid-infrared absorption feature in the Mars Global Surveyor spectrum.

Understanding what Earth would look like from light-years away requires observations that smear all of the details together. Astronomers must try to extract those details from data and compare them with Earth's "ground truth." This exercise has already produced surprising insights. We've recognized Earth as a pale blue dot from its spectrum. And we've detected strong oxygen and water-absorption features along with color changes induced by vegetation.

Although astronomers have yet to find a single earthlike planet beyond the solar system, they expect that to change radically in the next 2 decades. TPF and other advances may reveal a huge variety of terrestrial planets. Thoroughly understanding Earth as an extrasolar planet is but the first step in making sense of what these planets can tell us. And maybe, just maybe, it can give us the first clues that life exists elsewhere in the galaxy.

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