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.
"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.
The most obvious, visible sign of something actively disturbing the atmosphere of Venus, according to NASA astrobiologist David Grinspoon,"is the global clouds themselves. In an extreme form of acid rain, gases spewing from Venus’s volcanoes are actively maintaining the sulfuric clouds and supporting the intense greenhouse climate. Without a continuous source of fresh sulfur gases from active volcanoes, the clouds of Venus would disappear in a mere 30 million years, as sulfur was consumed by reactions with surface rocks. The bright clouds of Venus are the smoking gun of active volcanoes on the surface in the geologically recent past."
Researchers led by Ravi kumar Kopparapu of Penn State University, University Park, Pa., used chemical information to nudge the habitable zone out a bit farther than previously thought. The team's 2013 Astrophysical Journal study is the current gold standard in determining how a star's total radiation output relates to the location of its habitable zone. Kane and his colleagues used this information to fine-tune the boundaries of Kepler-69c's habitable zone, in addition to careful measurements of the star's total energy output and the orbit of the planet.
"Understanding the properties of the star is critical to determining planetary properties and calculating the extent of the habitable zone in that system," said Kane.
"There are a lot of unanswered questions about habitability," said Lucianne Walkowicz, a Kepler science team member based at Princeton University, N.J., who studies flaring stars. "If the planet gets zapped with radiation all the time by flares from its parent star, the surface might not be a very pleasant place to live. But on the other hand, if there's liquid water around, that makes a really good shield from high-energy radiation, so maybe life could thrive in the oceans."
Stellar flares can also scrape off the atmospheres of planets, complicating the picture further. This is particularly true for the smaller, cooler stars, which tend to be more hyperactive than stars like our sun.
Ideally, astronomers would like to know more about the atmosphere of potentially habitable planets. That way they could look at the planet's molecular makeup for signs of runaway greenhouse gases that could indicate an inhospitable Venus-like planet. Or, future space telescopes might even be able to pick up signatures of oxygen, water, carbon dioxide and methane -- indicators that the planet might be somebody's home.
NASA's upcoming James Webb Space Telescope will bring us closer to this goal, by probing the atmospheres of planets, some of which may lie in habitable zones. The mission won't be able to examine the atmospheres of planets as small as Earth, so we'll have to wait for another future telescope to separate out the Venuses from the Earths.
The Daily Galaxy via http://www.nasa.gov/kepler