While it's not strictly impossible that life in the universe exists in an as-yet-unknown form (say, silicon-based), we don't really know how to look for it. We do, however, know how to look for the conditions that have given birth to the life forms we know -- that is, the conditions on our home planet, Earth.
This is the Kepler mission: to locate planets enough like Earth to be considered habitable. There are several key factors to this. First, the planets must be in the "Goldilocks" zone -- that is, not too hot, not too cold, but juuuust right. This refers to the planet's orbit position around its star: a distance where it's not so close that it's too hot for liquid water, but not so far that it's so cold all water freezes.
In addition, the planet needs to be rocky -- like Earth.
Among the recent crop of eight new Kepler planets discovered in the Goldilocks zone around their stars -- announced at a meeting of the American Astronomical Society on January 6 -- two are the most similar to Earth of any of the 1,004 Kepler planets identified to date.
Identified as Kepler-438b and Kepler-442b, both orbit red dwarf stars, smaller and cooler than the yellow dwarf that is our own sun.
"For our calculations we chose to adopt the broadest possible limits that can plausibly lead to suitable conditions for life," said Guillermo Torres of the Harvard-Smithsonian Center for Astrophysics and lead author of a paper describing the findings.
Kepler-438b is 12 percent bigger than Earth, and completes an orbit of its star every 35 days. It receives about 40 percent more light than Earth, and has a 70 percent chance of being rocky. The team calculates that it has a 70 percent chance of being in the habitable zone.
Kepler-442b is about 33 percent larger than Earth, and completes an orbit every 112 days. It receives about two-thirds as much light as Earth, and has a 60 percent chance of being rocky. The team said it has a 97 percent chance of being in the habitable zone.
Prior to this analysis, the two most Earth-like planets identified were Kepler-186f, 1.1 times the size of Earth and receiving 32 percent as much light, and Kepler-62f, 1.4 times the size of Earth and receiving 41 percent as much light.
The two planets are very far away -- 470 light-years from Earth for Kepler-438b and 1,100 light-years for Kepler-442b -- which makes analysis challenging. The team used a variety of technologies, such as high-resolution spectroscopy, adaptive optics imaging and speckle interferometry, to draw its conclusions.
To gauge whether the planets do indeed host life, further research will be required, involving more instrumentation, such as the radio telescope array employed by the SETI Institute, which searches for artificially generated radio signals.
"We don't know for sure whether any of the planets in our sample are truly habitable," said co-author David Kipping of the CfA. "All we can say is that they're promising candidates."
The full study, which has been accepted for publication in The Astrophysical Journal, can be found online (PDF).