With powerful instruments scouring the heavens, astronomers have found more than 240 planets in the past two decades, none likely to support Earth-like life, but as astronomers become more adept at finding planets orbiting other stars it’s natural to wonder if anybody is looking back. A team of astronomers has speculated at just what those alien eyes might see using technologies similar to those available to Earth’s astronomers.

“They would only be able to see Earth as a single pixel, rather than resolving it to take a picture,” said Eric Ford, a UF assistant professor of astronomy and one of five authors of the paper. “But that could be enough for them to identify our planet as one that likely contains clouds and oceans of liquid water.”

This research may sound whimsical, but it has a serious goal: to provide a road map for Earth-bound astronomers trying to study Earth-like planets — a task expected to become possible in coming decades as more powerful telescopes come on line, said Enric Palle, the lead author of the paper and an astronomer with the Instituto de Astrofisica de Canarias.

“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.

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.

Astronomers have long recognized that even a large telescope would need to observe Earth for several weeks to collect enough light to identify chemicals in the planet’s atmosphere. During these observations, the brightness of the Earth would change, primarily because of clouds rotating into and out of view. If astronomers could measure Earth’s rotation period, then they would know when a given part of the planet was in view. The hitch was that astronomers were unsure whether Earth’s seemingly chaotically changing cloud patterns would make it impossible for alien observers to determine this rotation rate.

Based on data retrieved from satellite observations of Earth, they created a computer model for the brightness of the Earth, revealing that on the global scale Earth’s cloud cover is remarkably consistent — with rain forests usually turning up cloudy, arid regions clear, and so on. As a result, extraterrestrial astronomers who watched Earth for a period of several months would notice repeating patterns – a bit like watching the spots on a spinning ball come into view and then disappear. From those repeating patterns, they could then deduce Earth’s 24-hour rotation period, Ford said.

That done, the “E.T.” astronomers could infer that anomalies in the pattern were caused by changing weather patterns, most prominently, clouds, he said. Although some uninhabitable planets are extremely cloudy, the repeated presence and absence of clouds indicates active weather. On Earth, this variability results in water turning from gas to a vapor and back again, so finding similar variability on another planet would be a reasonable indication of liquid water.

“Venus is always covered in clouds. The brightness never changes,” Ford said. “Mars has virtually no clouds. Earth, on the other hand, has a lot of variation.”

Not only that, but observers could likely also infer the presence of continents and oceans from Earth’s changing light pattern.

The research will be useful to astronomers designing the next generation of space telescopes because it provides an outline of the capabilities required for studying the surfaces of Earth-like planets, Ford said. He said it appears that zeroing in on Earth-like planets orbiting the nearest stars would require a telescope at least twice the size of the Hubble Space Telescope. Ford said he hopes that his research will help to motivate an ever larger space telescope that could search for Earth-like planets around many stars.

Planned telescopes such as NASA’s Kepler, set for launch in 2009, will 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.