Observation of ozone, CO2, and H2O in the atmosphere of an unknown planet strongly suggests habitable conditions and the presence of life. Water vapor at moderate levels can indicate that a planet is essentially in the habitable zone of a star. Water vapor abundance depends on the temperature of an atmosphere as well as on the availability of surface water. CO2 abundance also provides a clue to the "habitability" of a terrestrial planet. At least on Earth, the ability to keep this dangerous greenhouse gas locked in sediments requires moderate surface temperatures and an active land-ocean weathering cycle, whereby "excess" CO2 can be sequestered in carbonates and thus kept out of the atmosphere. The strengths of the CO2 and H2O absorptions provide strong clues for identifying planets that are truly Earth-like, with land, oceans, and moderate surface temperatures. The detection of ozone signals the presence of active, photosynthetic life. Ozone (O3) is a molecule composed of three oxygen atoms. It is a highly chemically active molecule, so it is not very stable. It is produced in the atmosphere by the interaction of ultraviolet light from the sun and normal oxygen (O2). Light splits O2 into individual atoms, and they in turn react with O2 to form ozone. Only a minuscule fraction of the atmosphere's oxygen is in the form of ozone, but it provides strong infrared absorption. Ozone implies the presence of oxygen, which, in sufficient concentration, implies the existence of life. Maintaining moderate oxygen levels requires continuous production to balance the many processes that lead to its removal from the atmosphere.
Several space-borne projects have been proposed to detect infrared spectral signatures that would suggest Earth-like planets, oceans, moderate surface temperatures, and the presence of biological activity. A project in the planning stage at NASA is the Terrestrial Planet Finder, and a similar project being studied by the European Space Agency (ESA) is appropriately called Darwin. Both of these missions would use very large telescopes to image terrestrial planets and take infrared spectra. The basic challenge is to measure the spectral signature of terrestrial planets close enough to their parent stars to be in the habitable zones. This is a formidable task, in part because of the faint-ness of planets but also because of the proximity of these planets to the star. Viewed from a great distance, Earth itself would look very close to the sun and comparatively very faint. From the distance of the nearest stars, the angular separation of Earth and the sun is comparable to the diameter of a quarter as viewed from a distance of 4 miles. This is near the limit of what can be resolved with conventional ground-based telescopes, but it is easily within the reach of space telescopes and of ground-based telescopes with adaptive optics to reduce the blurring effects of the atmosphere.
The major problem with the study of extrasolar planets, even for nearby stars, is that planets are much fainter than stars. At visual wavelengths, Earth is seen only by the sunlight it reflects. The sun is a billion times brighter than the reflected "Earth-light," and in any telescope system, the glare from such an intensely bright object overwhelms the image of a faint nearby planet. In the infrared, at wavelengths of 10 micrometers, the situation is better. The sun is much fainter, and this is the peak of the region where the warm Earth radiates energy back to space. At this wavelength the sun is only ten thousand times brighter than Earth, and separation of the two images is feasible via special techniques involving interferometry.
The grand plan for detecting life in extrasolar planets involves the construction of very large telescopes. It is not efficient—or perhaps even possible—to build a single-mirror telescope large enough to detect individual extrasolar planets, and this limitation suggests using groups of smaller telescopes combined to work in concert. The ability to resolve ultrafine angular detail depends on the diameter of the telescope. Telescopes together in an array have resolving power equivalent to a single mirror as large as the entire array. Present plans for the Terrestrial Planet Finder call for a cluster of four telescopes, each with an individual mirror 4 to 8 meters in diameter. These could be mounted on a truss or could be free flyers with a total separation of about 100 meters. In either case, the separation of the individual telescopes would have to be controlled to incredible precision: small fractions of the wavelength of light. The telescopes would combine light beams and would function as an interferometer with a very special property. Their sensitivity would be a minimum in the center of the imaged field and a maximum at a slight offset equal to the expected angular spacing of a planet and the star. When the telescopes were then pointed directly at the star, the image of the star would effectively be attenuated by a factor of a million, whereas the light from a nearby planet would not be attenuated.
This special design produces a "null" on the star, minimizing the great difference in brightness between planet and star. The technique uses interference, the same process the produces the iridescence of soap bubbles and the brilliant color of some butterfly wings. A laser pointer projected onto a distant wall shows an analogous effect in reverse. It has a bright spot in the middle that is surrounded by faint dark and bright rings. The planet finders will use interference to produce the opposite effect: a null in the center and sensitivity in rings around it. Doing precision interferometry with such huge telescopes in space will require extraordinary effort and billions of dollars. The technique is just at the edge of available technology but appears practicable. The system could search for Earth-like planets around several hundred of the nearest stars, and in it lies our best hope for detecting life outside the solar system any time soon.
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