As astronomers become more adept at hunting for, and finding, exoplanets orbiting stars beyond the Solar System, an international team of astronomers has figured out just what alien eyes might see using the increasingly sophisticated technologies being developed on Earth.
"Maybe somebody's looking at us right now, finding out what our rotation rate is -- that is, the length of our day," says Sara Seager, associate professor of physics and the Ellen Swallow Richards Associate Professor of Planetary Sciences at MIT.
Future telescopes such as NASA's Kepler, set for launch in 2009, would be able
to discover dozens or hundreds of Earth-like worlds. The Space Interferometry Mission (SIM), to be launched early in the next decade, consists of multiple telescopes placed along a 30 foot structure. With an unprecedented resolution approaching the physical limits of optics, the SIM is so sensitive that it almost defies belief: orbiting the earth, it can detect the motion of a lantern being waved by an astronaut on Mars
The SIM, in turn, will pave the way for the Terrestrial Planet Finder, to be launched late in the next decade, which should identify even more earth-like planets. It will scan the brightest 1,000 stars within 50 light years of the earth and will focus on the 50 to 100 brightest planetary systems. The TPF will allow the follow-up studies to learn about these planets' rotation and weather, and the composition of their atmospheres, Seager says.
All this, in turn, will stimulate an active effort to determine if any of them harbor life, perhaps some with civilizations more advanced than ours.
According to the team's analysis, among other things alien astronomers could probably tell that our planet's surface is divided between oceans and continents, and learn a little bit about the dynamics of our weather systems. The team is headed by Seager, along with Enric Palle and colleagues at the Instituto de Astrofísica de Canarias, in Spain, and Eric Ford of the University of Florida,
Exoplanets have rarely been seen; rather, they have been indirectly observed by looking at the influence they exert on stars they orbit. But even with the most advanced telescopes planned by Earth's astronomers for use over the next several years, a planet orbiting another star would only appear as a single pixel. By comparison, a simple cellphone camera typically takes pictures with about a million pixels, or one megapixel. However, a great deal of information about a planet can be gleaned from that single pixel and the way it changes over time.
The way of analyzing the data that Seager and her co-authors studied would work for any world that has continents and bodies of liquid on its surface plus clouds in its atmosphere, even if those were made of very different materials on an alien world. For example, icy worlds with seas of liquid methane, like Saturn's moon Titan, or very hot worlds with oceans of molten silicate (which is solid rock on Earth), would show up similarly across the vastness of space.
However, the method depends on clouds covering only part of a planet's surface, regardless of what each world is made of. Saturn's Titan, for example, covered by perpetual global smog, would not give up the mysteries of its weather or rotation, nor would the boiling hot Venus, with its complete shroud of clouds.
The key, the astronomers learned after studying data from Earth's weather satellites, is that while clouds vary from day to day, there are overall patterns that stay relatively constant, associated with where arid or rainy landmasses are. Detecting those repeating patterns would allow distant astronomers to figure out the planet's rotation period because a brightening associated with clouds above a particular continent would show up regularly once each "day," whatever the length of that day might be. Once the day's length is determined, then any variations in that period would reveal the changing weather--that is, clouds in a different place than the average.
No telescope now in operation is capable of making the measurements that Seager and her team analyzed. But planned telescopes such as NASA's Kepler, set for launch in 2009, would be able to discover dozens or hundreds of Earth-like worlds. Then even more advanced space observatories being considered, such as NASA's Terrestrial Planet Finder, would allow the follow-up studies to learn about these planets' rotation and weather, and the composition of their atmospheres, Seager says.
Posted by Casey Kazan.
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