And if any planets similar to these orbit in their parents stars' habitable zone, substantially farther from the home star where liquid water might more likely exist, their atmospheres will lose even smaller amounts of hydrogen-bearing compounds over time, the researchers note.
Kepler-34b and Kepler-35b are particularly notable because they lay near their stars' habitable zone.
They've also discovered a number of worlds that appear to orbit in their stars' habitable zone — that just-right range of distances that could support the existence of liquid water and thus, perhaps, life as we know it.
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Unlike the others, which skirt the edges of their stars' Goldilocks zones, Kepler 22b orbits comfortably within its own.
A survey of planets, using a different technique to Kepler's, concluded that 90% of exoplanets have less circular orbits than do those in the solar system, suggesting that many may wander in and out of their stars' habitable zones.
Though a moon may become tidally locked to its parent planet, it can never be locked to a star, so it will always have some sort of day-night cycle even if it is an odd one from a terrestrial point of view.The warmth provided by tidal heating might also help make some moons which are outside their stars' habitable zones inhabitable nevertheless.
In other words, some of the Super Earths identified by Kepler may in fact be gas giants, more akin to Neptune and Uranus.Bigger stars' "habitable zones", in which temperatures are low enough for liquid water, and life, to exist, are also farther from a star system's centre.
Kasting says the studies may be most useful for refining estimates of sunlike stars' habitable zones: the range of distances at which orbiting rocky planets can host abundant liquid water.
