One key factor in maintaining a liquid water biosphere on a planet over billions of years is its distance from the Sun, or whatever star it may be orbiting. If it is in too close, then, like Venus, its oceans will boil away. At too great a distance a water world will at best have icy polar caps on the surface.
In a new study, researchers based at NASA's Exoplanet Science Institute at the California Institute of Technology, in Pasadena, Calif., carefully analyzed the location of both a planet called Kepler-69c and its habitable zone. Their analysis shows that this planet, which is 1.7 times the size of Earth, lies just outside the inner edge of the zone, making it more of a Super Venus than a Super Earth, as previous estimates indicated. "On the way to finding Earths, Kepler is telling us a lot about the frequency of Venus-like planets in our galaxy," said Stephen Kane, lead author of the new paper on Kepler-69c appearing in the Astrophysical Journal Letters.
Keep in mind that Venus has an atmosphere that has quickly consumed every spacecraft we’ve dropped in there, although it is actually Earth-like in some profound ways. As extreme and hostile as the environment there seems to us, says David Grinspoon, author of Lonely Planets, it represents a delicate and subtle balance of ongoing geological, meteorological, and climatic activity. Much planetary exploration involves studying dead worlds, surveying places that were once active but have long been still, and trying to reconstruct the events of billions of years ago.
To determine the location of a star’s habitable zone, one must first learn how much total radiation it emits. Stars more massive than our sun are hotter, and blaze with radiation, so their habitable zones are farther out. Similarly, stars that are smaller and cooler sport tighter belts of habitability than our sun. For example, the Super Earth planet called Kepler-62f, discovered by Kepler to orbit in the middle of a habitable zone around a cool star, orbits closer to its star than Earth. The planet takes just 267 days to complete an orbit, as compared to 365 days for Earth.
Knowing precisely how far away a habitable zone needs to be from a star also depends on chemistry. For example, molecules in a planet's atmosphere will absorb a certain amount of energy from starlight and radiate the rest back out. How much of this energy is trapped can mean the difference between a turquoise sea and erupting volcanoes.