July 23, 2015
The discovery of a super-Earth-sized planet orbiting a sun-like star brings us closer than ever to finding a twin of our own watery world. But NASA’s Kepler space telescope has captured evidence of other potentially habitable planets amid the sea of stars in the Milky Way galaxy.
To take a brief tour of the more prominent contenders, it helps to zero in on the “habitable zone” around their stars. This is the band of congenial temperatures for planetary orbits — not too close and not too far. Too close and the planet is fried (we’re looking at you, Venus). Too far and it’s in deep freeze. But settle comfortably into the habitable zone, and your planet could have liquid water on its surface — just right. Goldilocks has never been more relevant. Scientists have, in fact, taken to calling this water-friendly region the “Goldilocks zone.”
The zone can be a wide band or a narrow one, and nearer the star or farther, depending on the star’s size and energy output. For small, red-dwarf stars, habitable zone planets might gather close, like marshmallow-roasting campers around the fire. For gigantic, hot stars, the band must retreat to a safer distance.
About a dozen habitable zone planets in the Earth-size ballpark have been discovered so far — that is, 10 to 15 planets between one-half and twice the diameter of Earth, depending on how the habitable zone is defined and allowing for uncertainties about some of the planetary sizes.