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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. 

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Kepler's preliminary results suggest that Kepler 22b could be on the less-dense side of that spectrum – a water world or a mini-Neptune.

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"Once a planet gets above say, two Earth radii, we could imagine scenarios where it's basically a water world, but most likely what you have is an envelop of hydrogen and helium," says David Charbonneau, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and a member of the Kepler team. "There might be a solid surface, but the surface would exist at very high pressure. It would be very difficult to imagine how life would survive there."

Even if Kepler could find an Earth-mass planet at Earth-like distances from its star, however, scientists couldn't declare victory – at least not immediately. First, they would need to confirm the shape of the planet's orbit. 

When it comes to orbits, shape matters, Fichser says. Earth's orbit is nearly circular – the slightly oval shape never leaves the habitable zone. But a highly elliptical orbit would place a planet within a star's habitable zone for only part of its year. The rapid freeze-thaw cycles wouldn't preclude life, but it might make it tough for life to gain a foothold.

Planets in a multiplanet system tend to assume increasingly circular orbits with time, Dr. Charbonneau adds. But if a sun-like star has a single planet, that allows for a range of eccentric orbits that might bring the planet into the habitable zone only briefly during its year.

Even then, Kepler and its ground-based counterparts can't detect other factors that could render seemingly habitable planets uninhabitable. 

For instance, a magnetic field is a decided plus. Even if a planet has enough mass to retain an atmosphere, it could still lose its gassy envelope to collisions from cosmic rays unless it has a sufficient magnetic field, Charbonneau says.

The presence or absence of an appreciable magnetic field also signals the level of tectonic activity within a planet. Earth, with a highly radioactive iron core for heat, as well as a crust constantly being recycled, has a protective magnetic sheath. Mars, with little or no tectonic activity, does not.

That recycling serves as a planetary thermostat, partitioning the Earth's inventory of heat-trapping carbon dioxide between the atmosphere and the interior. Venus, with about the same total inventory, has no tectonics. Volcanoes over the eons have deposited the planet's CO2 into the atmosphere, leading to a torrid, cloud-covered environment where surface temperatures can melt lead.

Tectonics, magnetic fields, and other traits are features Kepler's data, even backed by ground-based observations, can't characterize.

In the end, Charbonneau says, Kepler is not about finding other livable Earths, per se. Instead, it is conducting a cosmic nose-count of Earth-size planets in habitable zones in order to allow the team to confidently project the number of similar planets much closer to home.

Any such planets in the solar system's general neighborhood would fall within range of planned ground- and space-based telescopes capable of characterizing their atmospheres, establishing their masses, orbits, and densities to very high precision, and even image some of them.

Moreover, sun-like stars are only one set of potential homes for habitable planets. Smaller, cooler red-dwarf stars, which are more numerous, also have grown in favor with planet-hunters.

Much of what scientists have figured out so far regarding the criteria for potential habitability has been based on a survey with a sample size of one: our own solar system.

"The field of exoplanets has been mostly a field of surprises rather than confirmations," Charbonneau says, referring to the range of planetary systems quite unlike our own or defying theoretical predictions.

"If we were not able to predict how planets form" in these other systems, he says, "we probably should be very careful" about using our own solar system as a model for what makes for a habitable planet.

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