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Earth-size, habitable zone planets may be closer than you think

Harvard-Smithsonian researchers say there are more planets closer to Earth that previously thought

By Layer 8 on Wed, 02/06/13 - 2:01pm.

nasaDistances in space are usually impossibly large and hard to get a handle on but this week astronomers said as many as six percent of stars known as Red Dwarfs have planets around them in the habitable zone and may only be 13 light years away from Earth.

Now 13 light years is still an impressive distance but habitable planets in such proximity arouses more than its share of astrobiology interest.  Astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) who made the discovery said also worth noting is that Red dwarf stars are smaller, cooler, and fainter than the sun. An average red dwarf is only one-third as large and one-thousandth as bright as the sun. Consequently, the not too hot or not too cold habitable zone would be much closer to a cooler star than it is to the sun, the researchers said.

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"This close-in habitable zone around cooler stars makes planets more vulnerable to the effects of stellar flares and gravitational interactions, complicating our understanding of their likely habitability," said Victoria Meadows, professor at the University of Washington and principal investigator with the NASA Astrobiology Institute. "But, if the planets predicted by this study are indeed found very nearby, then it will make it easier for us to make the challenging observations needed to learn more about them, including whether or not they can or do support life."

Using publicly available data from NASA's Kepler space telescope, astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) estimate that six percent of red dwarf stars in the galaxy have Earth-size planets in the "habitable zone," the range of distances from a star where the surface temperature of an orbiting planet might be suitable for liquid water.

Specifically the CfA team looked at 95 planet candidates orbiting 64 red dwarf stars using publically available data obtained by NASA's star-gazing telescope Kepler.  Most of these candidates aren't the right size or temperature to be considered Earth-like, as defined by the size relative to Earth and the distance from the host star. However, three candidates are both temperate and smaller than twice the size of Earth the researchers said.

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The three planetary candidates highlighted in this study are Kepler Object of Interest (KOI) 1422.02, which is 90 percent the size of Earth in a 20-day orbit; KOI-2626.01, 1.4 times the size of Earth in a 38-day orbit; and KOI-854.01, 1.7 times the size of Earth in a 56-day orbit.  The three candidates orbit stars with temperatures ranging from 3,400 to 3,500 degrees Kelvin. By comparison, the temperature of the sun is nearly 5,800 degrees Kelvin.

The research team went on to say that locating nearby, Earth-like worlds may require a dedicated small space telescope, or a large network of ground-based telescopes. Follow-up studies with instruments like the Giant Magellan Telescope and James Webb Space Telescope could tell us whether any warm, transiting planets have an atmosphere and further probe its chemistry. 

Such a world would be different from our own. Orbiting so close to its star, the planet would probably be tidally locked. However, that doesn't prohibit life since a reasonably thick atmosphere or deep ocean could transport heat around the planet. And while young red dwarf stars emit strong flares of ultraviolet light, an atmosphere could protect life on the planet's surface. In fact, such stresses could help life to evolve, the researchers said.

"You don't need an Earth clone to have life," said Harvard astronomer and lead author Courtney Dressing during a press conference on the study.

In related planet news also utilizing NASA's Kepler data, another CfA research group said about 17% of stars have an Earth-sized planet in an orbit closer than Mercury. Since the Milky Way has about 100 billion stars, there are at least 17 billion Earth-sized worlds out there, according to Francois Fressin, of the CfA.

The research team found that 50% of all stars have a planet of Earth-size or larger in a close orbit. By adding larger planets detected in wider orbits up to the orbital distance of the Earth, this number increases to 70%.

The researchers said Kepler's currently ongoing observations and results from other detection techniques, they have determined that nearly all sun-like stars have planets.   Planets closer to their stars are easier to find because they transit more frequently. As more data are gathered, planets in larger orbits will be detected. In particular, Kepler's extended mission will enable the detection of Earth-sized planets at greater distances, including Earth-like orbits in the habitable zone.

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