The Search Is On for ‘Pale Blue’ Planets

ShowImage.aspx Since the early 1990s, astronomers have discovered more than 400 planets orbiting stars other than our sun, nearly all of them gas giants like Jupiter. Powerful space telescopes, such as the one that is central to NASA's recently launched Kepler Mission, will make it easier to spot much smaller rocky extrasolar planets, or exoplanets, more similar to Earth.

But seen from dozens of light years away, an Earth-like exoplanet will appear in telescopes as little more than a "pale blue dot," the term coined by the late astronomer Carl Sagan to describe how Earth appeared in a 1990 photograph taken by the Voyager spacecraft from near the edge of the solar system.

Using instruments aboard the Deep Impact spacecraft, a team of astronomers and astrobiologists has devised a technique to tell whether such a planet harbors liquid water, which in turn could tell whether it might be able to support life.

Colors of planets are telltale signs for the astronomers eye: Mars for example is red because its soil contains rusty red iron oxide. And the famous tint of the "pale blue dot” is because Earth's atmosphere scatters blue light rays more strongly than red ones creating an atmosphere looks blue from both above and below.

Astronomers are using color to target potentially habitable exoplanets. In a study led by NASA astronomer Lucy McFadden and UCLA graduate student Carolyn Crow describe a simple way to distinguish between the planets of our solar system based on color information. “Eventually, as telescopes get bigger, there will be the light-gathering power to look at the colors of planets around other stars,” McFadden says. “Their colors will tell us which ones to study in more detail.”

Color data about Earth, the moon, and Mars became available from NASA’s Deep Impact spacecraft. En route to a planned encounter this November with Comet 103P/Hartley 2, Deep Impact observed Earth. The idea was to determine what our home looks like to alien astronomers and eventually use that insight to figure out how to spot earthlike worlds around other stars.

As Deep Impact cruised through space, its High Resolution Instrument (HRI) measured the intensity of Earth’s light, feeding light through seven different color filters mounted on a revolving wheel. Each filter samples the incoming light at a different portion of the visible-light spectrum, from ultraviolet and blue to red and near-infrared. On May 28, 2008, Deep Impact even caught a glimpse of the moon’s light as it crossed in front of Earth. Later, in 2009, HRI scoped Mars.

McFadden sougth to discover what combination of color information from the filters would best distinguish Earth from the other planets and moons of the solar system.

The Deep Impact color data covered Earth, the moon, and Mars. The relative amounts of light passing through the filters vary for each planet or moon, providing a kind of color fingerprint. To this the team added existing color information about Mercury, Venus, Jupiter, Saturn, Uranus, Neptune, and Saturn’s moon Titan.

NASA researchers analyzed the light reflected by the planets and plotted the results on a "color-color" diagram. By plotting the ratios of red to green light as well as blue to green, the planets cluster into "color families." On the diagram, Earth is easily distinguishable from the other major planets.

On a special “color-color” diagram the team created, the planets cluster into groups based on similarities in the wavelengths of sunlight that their surfaces and atmospheres reflect. The gas giants Jupiter and Saturn huddle in one corner, Uranus and Neptune in a different one. The rocky inner planets Mars, Venus, and Mercury cluster off in their own corner of “color space.”

But Earth is unique in color space, which traces to the scattering of blue light by the atmosphere, called Rayleigh scattering, after the English scientist who discovered it. The other reason Earth stands out in color space is because it does not absorb a lot of infrared light due to the fact that our atmosphere is low in infrared-absorbing gases like methane and ammonia, compared to the gas giant planets Jupiter and Saturn.

“It is Earth’s atmosphere that dominates the colors of Earth,” Crow says. “It’s the scattering of light in the ultraviolet and the absence of absorption in the infrared.”

In the search for Earth-like exoplanets, the three-filter approach may provide a rough “first cut” look at exoplanet surfaces and atmospheres. “There are some things we can tell from the colors but there are some things that we can’t quite tell without additional information,” Crow says. For example, if an exoplanet shows a similar color fingerprint to Earth’s, it would not necessarily mean that the planet has the blue skies and vast oceans of our home. But it would tell us to look at that planet more closely.

And that would be an important first step toward making sense of the colorful complexity of the 490 (and counting) exoplanets already discovered, and the scores more on the way from the success of the Kepler mission.

A similar study at the University of Washington concluded: "Liquid water on the surface of a planet is the gold standard that people are looking for," said Nicolas Cowan, a University of Washington doctoral student in astronomy and lead author of a paper explaining the new technique.

As part of NASA's Extrasolar Planet Observation and Characterization mission, the scientists obtained two separate 24-hour observations of light intensity from Earth in seven bands of visible light, from shorter wavelengths near ultraviolet to longer wavelengths near infrared. Earth appears gray at most wavelengths because of cloud cover, but it appears blue at short wavelengths because of the same atmospheric phenomenon that makes the sky look blue to people on the surface.

The researchers studied small deviations from the average color caused by surface features like clouds and oceans rotating in and out of view. They found two dominant colors, one reflective at long, or red, wavelengths and the other at short, or blue, wavelengths. They interpreted the red as land masses and the blue as oceans.

The analysis was undertaken "as if we were aliens looking at Earth with the tools we might have in 10 years" and did not already know Earth's composition, Cowan said. "You sum up the brightness into a single pixel in the telescope's camera, so it truly is a pale blue dot."

Since Earth's colors changed throughout the 24-hour-long observations, the scientists made maps of the planet in the dominant red and blue colors and then compared their interpretations with the actual location of the planet's continents and oceans.

"You could tell that there were liquid oceans on the planet," Cowan said. "The idea is that to have liquid water the planet would have to be in its system's habitable zone, but being in the habitable zone doesn't guarantee having liquid water."

The observations on March 18 and June 4, 2008 were made when the spacecraft was between 17 million and 33 million miles from Earth, and while it was directly above the equator. Observations from above a polar region likely would show up as white, Cowan said.

Casey Kazan via University of Washington News and NASA

Image credit: NASA. Earth appears gray at most wavelengths because of cloud cover, but it appears blue at short wavelengths because of the same atmospheric phenomenon that makes the sky look blue to people on the surface. Using instruments aboard the Deep Impact spacecraft, a team of astronomers and astrobiologists has devised a technique to tell whether Earth-like exoplanets harbor liquid water.


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