"Long before it's in the papers"
August 03, 2010

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Technique would detect watery worlds

May 27, 2009
Courtesy University of Washington
and World Science staff

Since the early 1990s as­tro­no­mers have dis­cov­ered more than 300 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, are de­signed 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 would ap­pear in tele­scopes as lit­tle more than a “pale blue dot.” That was the term coined by the late as­tron­o­mer Carl Sagan to de­scribe how Earth looked in a 1990 pho­to tak­en by the Voy­ag­er space­craft from near the edge of the so­lar sys­tem.

Us­ing in­stru­ments aboard NASA’s 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.

“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 that has been ac­cept­ed for pub­lica­t­ion in As­t­ro­phys­i­cal Jour­nal.

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 dif­fer­ent col­ors. Earth ap­pears blue from space for the same rea­son the sky looks blue down he­re: air mo­le­cules scat­ter blue light from the sun more than they scat­ter oth­er col­ors.

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 de­via­t­ions at two dom­i­nant col­ors, or wave­lengths, of light: red and blue. 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 did­n’t 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 of last year 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.

It will be some years be­fore the launch of space tele­scopes capa­ble of mak­ing si­m­i­lar ob­serva­t­ions for Earth-sized exoplan­ets, but de­vising this tech­nique now could guide the con­struc­tion of those in­stru­ments, he said. And while those plan­ets will be much far­ther away, the tech­nique still will be applica­ble, he added.

Cowan notes that some non-habita­ble plan­ets, such as Nep­tune, al­so can look blue, but the col­or is con­stant and, in the case of Nep­tune, likely caused by meth­ane in the atmosphe­re. “It looks blue from eve­ry an­gle, the same blue all the way around. If you had an ocean plan­et it might look like that, but you can do oth­er tests to de­ter­mine that,” he said. “For Earth, the blue varies from one place to anoth­er, which in­di­cates that it’s not some­thing in the atmosphe­re.”

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Since the early 1990s astronomers have discovered more than 300 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, are designed 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.” That was the term coined by the late astronomer Carl Sagan to describe how Earth looked in a 1990 photo taken by the Voyager spacecraft from near the edge of the solar system. Using instruments aboard the agency’s 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. “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 that has been accepted for publication in Astrophysical Journal. 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 different colors. Earth appears blue from space for the same reason the sky looks blue down here: air molecules scatter blue light from the sun more than they scatter other colors. 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 deviations at two dominant colors, or wavelengths, of light: red and blue. 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 didn’t 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 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. It will be some years before the launch of space telescopes capable of making similar observations for Earth-sized exoplanets, but devising this technique now could guide the construction of those instruments, he said. And while those planets will be much farther away, the technique still will be applicable. Cowan notes that some non-habitable planets, such as Neptune, also can look blue, but the color is constant and, in the case of Neptune, likely caused by methane in the atmosphere. “It looks blue from every angle, the same blue all the way around. If you had an ocean planet it might look like that, but you can do other tests to determine that,” he said. “For Earth, the blue varies from one place to another, which indicates that it’s not something in the atmosphere.”