What Would ET See?

Most extrasolar planets known today cannot be directly observed. Instead, astronomers look at the influence the planets have on the stars they orbit.
Credit: Spaceref

As astronomers become more adept at searching for, and finding, planets orbiting other stars, it’s natural to wonder if anybody is looking back. Now, a team of astronomers that includes a professor from MIT has figured out just what those alien eyes might see using technologies being developed by Earth’s astronomers. The team is finding that a great deal can be learned about a planet, even if the amount of data we collect is small. The study could provide a better understanding of what astronomers should look for when scanning the skies for habitable, Earth-like planets.

According to their analysis, among other things E.T. 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.

"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.


Using current technology, like data from the Hubble Telescope and the Spitzer Space Telescope, astronomers are only able to collect limited amounts of data on extrasolar planets. These telescopes are not powerful enough to view most extrasolar planets directly. Future missions, such as Kepler and the Terrestrial Planet Finder (TPF), will provide vast improvements in our ability to study these distant planets.
Credit: NASA/Ball Aerospace and Technologies Corp., 2003

Seager, along with Enric Palle and colleagues at the Instituto Astrofisica del Canarias, in Spain, and Eric Ford (MIT class of 1999) of the University of Florida, have done a detailed analysis of what astronomers here and on other worlds could learn about a planet from very distant observations, using telescopes much more powerful than those currently available to Earth’s astronomers. Their study, which has just been published online in the Astrophysical Journal, will appear in the journal’s print edition in April.

Most of the planets astronomers have discovered beyond the Solar System have not actually 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-that is, a single point of light, with no detail except its brightness and color. By comparison, a simple cellphone camera typically takes pictures with about a million pixels, or one megapixel.

"The goal of [our] project was to see how much information you can extract" from very limited data, Seager says. The team’s conclusion: 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. So Titan, covered by perpetual global smog, would not give up the mysteries of its weather or rotation, nor would the hellishly hot Venus, with its complete shroud of clouds.

NASA’s Kepler Mission will survey our region of the Milky Way galaxy to detect hundreds of Earth-sized and smaller planets near the habitable zone of their parent stars. Currently, the mission is set for launch in 2009.
Credit: NASA

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.

The research was funded in part by a Ramon y Cajal fellowship for Palle, and a Hubble Fellowship grant for Ford and by NASA.


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