In a follow up to our recent post "MIT Asks: Could Extraterrestrial Astronomers Detect Life on Earth," Eric Ford, an assistant professor of astronomy at the University of Florida asks: what if aliens were hunting life outside their own planet? Armed with telescopes only a bit bigger and more powerful than our own, could they peer through the vastness of space and zero in on Earth as a likely home to life?
The answer, Ford says, is a qualified âyes.â With a space telescope larger than the Hubble Space Telescope pointed directly at our sun, âhypothetical observersâ could measure Earthâs 24-hour rotation period, leading to observations of oceans, clouds, and the chance of life.
âThey would only be able to see Earth as a single pixel, rather than resolving it to take a picture,â Ford says. âBut that could be enough for them to identify our planet as one that likely contains clouds and oceans of liquid water.â
Astronomers are beginning to plan how future space telescopes could directly detect planets much closer to Earthâs size and proximity to the sun. One challenge: To figure out how to use a planetâs light to recognize if its surface and atmosphere are Earth-like.
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 planetâs 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 Earthâs rotation period, then they would know when a given part of the planet was in view. But astronomers were unsure whether Earthâs 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, Ford and his colleagues created a computer model for the brightness of the Earth, revealing that on the global scale Earthâs cloud cover is remarkably consistent â 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 â a bit like watching the spots on a spinning ball come into view and then disappear. From those repeating patterns, they could then deduce Earthâs 24-hour rotation period, Ford said.
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.
âVenus is always covered in clouds. The brightness never changes,â Ford said noting that observers could likely also infer the presence of continents and oceans from Earthâs changing light pattern.. âhas virtually no clouds. Earth, on the other hand, has a lot of variation.â
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.
Posted by Casey Kazan. Adapted from a University of Florida release.
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