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

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

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. 

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.

"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.