May 10, 2013
Imaging Atmospheres Of Alien Worlds
John P. Millis, Ph.D. for redOrbit.com — Your Universe Online
In the search for life beyond Earth scientists face significant challenges. Given the great distances and faint signatures of the alien worlds that we have found, discovering which ones are even potentially habitable is difficult.
While the first step is attempting to determine which worlds could maintain liquid water on their surfaces, an equally important factor is characterizing the atmosphere maintained above the surface. The ultimate goal is to identify planets with molecular oxygen, a sign of life.
In a few years, new instruments like the James Webb Space Telescope will be able to study planetary atmospheres by studying exoplanets as they pass in front of their host stars. As the star's light passes through the atmosphere, some of the wavelengths will be absorbed, allowing for spectral analysis to determine the molecular composition.
In the meantime, researchers are finding ways to analyze some systems using current technology. Ground based telescopes, armed with spectrometers, have been able to visually resolve planets orbiting nearby stars, and resolve the molecular components of their atmospheres.
"In just one hour, we were able to get precise composition information about four planets around one overwhelmingly bright star," said Gautam Vasisht of JPL, co-author of the new study. "The star is a hundred thousand times as bright as the planets, so we've developed ways to remove that starlight and isolate the extremely faint light of the planets."
A coronograph was used, effectively blacking-out the light from the star, allowing the researchers to isolate the light being reflected from the planets.
"It's like taking a single picture of the Empire State Building from an airplane that reveals a bump on the sidewalk next to it that is as high as an ant," said Ben R. Oppenheimer, lead author of the new study.
In their initial research the team studied HR 8799, a large star orbited by at least four giant planets. Interestingly, it seems that the planets have different compositions, despite being of similar size and orbiting the same star. Furthermore, some of the chemical signatures, such as the presence of ammonia, were unexpected.
Further analysis is needed to complete the study, and the team is already looking for additional planets to study. But a challenge lies ahead; finding candidates is difficult with the current state of technology. And as we try to find smaller and smaller planets, the task becomes even greater.
"Rocky Earth-like planets are too small and close to their stars for the current technology, or even for James Webb to detect. The feat of cracking the chemical compositions of true Earth analogs will come from a future space mission such as the proposed Terrestrial Planet Finder," said Charles Beichman, executive director of NASA's Exoplanet Science Institute at Caltech.