The Quest To Find Other Earths
Of all the issues that the cosmos poses for humanity, one of the greatest and most resonant focuses on the search for Earth's cousins, planets that offer a world on which life can both originate and flourish. The past half decade has taken us from almost total ignorance of planets orbiting other stars to knowledge of the existence of more than 70 such planets. All of these extrasolar planets have masses comparable to the masses of Jupiter and Saturn, more than 80 times Earth’s. Their large masses allow the planets to produce detectable gravitational pulls on their starry masters. Only by observing these tugs have astronomers been able to deduce the planets' existence and their masses. Finding planets with masses comparable to Earth's, either by indirect methods or by direct observation, will require observational systems far superior to those so far devoted to the search for other worlds.
Most of those efforts will require space-borne interferometer systems. Such systems combine the light and infrared waves collected by an array of individual small telescopes to achieve the resolving power of a single giant telescope whose size equals the size of the array. Radio astronomy has long employed interferometer systems to achieve high resolution. Today astronomers are making rapid progress to create optical interferometers for the infrared and visible-light spectral domains. The Space Interferometry Mission (SIM), now under development for launch in the second half of the decade, will demonstrate the feasibility of space-borne interferometer missions to observe visible-light and infrared radiation. But SIM will still enable discovery of planets not by direct observation but indirectly, through highly accurate measurements of the displacement of stars' positions on the sky.
To make direct observations of Earth-sized planets around other stars will require an interferometer system with more powerful telescopes, capable of maintaining their separations at distances sufficiently large to provide much finer angular resolution than SIM's. By the end of this decade scientists should be poised to develop an interferometer array capable of direct detection of Earth-like planets around nearby stars — the Terrestrial Planet Finder (TPF).
Analysis of the infrared radiation and visible light from any such planets can reveal telltale signs of atmospheric gases. Oxygen or ozone would signal the possible existence of conditions favorable to aerobic life forms. Methane in the presence of oxygen would suggest the actual presence of life on these faraway worlds.
Read More About The Terrestrial Planet Finder (TPF)