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Earth-like planets: How will we know if they can sustain life? (VIDEO)

The Kepler spacecraft has made two landmark discoveries of Earth-like planets this month. But determining whether such planets can sustain life would require years of additional study. 

By Staff writer / December 22, 2011

An artist's rendering of Kepler 22b.

JPL-CaltechAmes/NASA/AP

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What makes for a potentially livable planet? That question moved center stage this month as NASA's Kepler mission passed two milestones.

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On Tuesday, the Kepler team announced the discovery of two truly Earth-size planets orbiting another star – but too close to the star for life to emerge. This followed an announcement on Dec. 5 that the Kepler team had found a planet in the host star's habitable zone, but 2.4 times larger that Earth.

The findings move the Kepler team closer to its goal of finding other planets like ours. The spacecraft is searching 150,000 stars to see how many are like the sun and have planets roughly the same size and distance away as Earth.

But researchers caution that even when Kepler eventually scores a direct hit, that will not be the end of the story. Astronomers will have to answer many more questions about such planets before they can suggest that any of them may be Earth-like, let alone livable for some form of life.

According to Yale University astronomer Debra Fischer, three important pieces of this habitability puzzle begin with: a planet's distance from its sun, its mass, and the shape of its orbit.

Watch video aboot the newly discovered habitable planet Kepler-22b here:

Other traits come into play, but "if we can find 100 planets that meet the three conditions, we will have have gone a long ways in our search for life," she writes in an e-mail exchange.

Distance is most straightforward for Kepler to gauge. The distance from the sun to the Earth is about 93 million miles, or 1 Astronomical Unit (AU). By some estimates, the habitable zone around a sun-like star – where with a little help from an atmosphere, water can exist on the surface as solid, liquid, and gas – is between 0.95 and 1.37 AU.

The planet announced Dec. 5, Kepler 22b, is almost exactly 1 AU from its star. But its mass has yet to be confidently established.

Mass is important, because if a planet is a lightweight, with less than about half Earth's mass, it won't have enough gravity to retain much of an atmosphere. Mars, at 10 percent of Earth's mass, has had much of its atmosphere stripped away.

"Too big is harder to quantify," Dr. Fischer adds.

If a planet has only a few times Earth's mass, it might still be potentially habitable. But if a planet becomes too massive, its gravity could be too strong, meaning that it builds a thick, deep atmosphere, resulting in crushing atmospheric pressures on the surface. 

For Kepler 22b, the best the Kepler team can do at the moment is give an upper limit to the mass – 124 times Earth's mass. The reason: Kepler's technique for pinpointing planets. It does this by gauging how they briefly dim the light of their host star when they pass in front of it.

While the team can make some rough estimates about a planet's mass from this technique, the best information on mass, as well as the shape of the orbit, comes from a different technique used by ground-based astronomers. This approach measures the wobble the planet imposes on host star's spectrum as it orbits. 

Knowing the planet's mass and its volume, researchers can also estimate its density, and so glean something about the planet's general composition by comparing its density with that of water. If the density is relatively small, it could be more gaseous, like a mini-Neptune. If the density is larger, it could suggest a denser, rocky planet. 

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