So far, scientists have said they have identified twelve planets that might be habitable. In fact, there are significant problems with every single planet
identified to date. For a list of the most habitable exoplanets click here
First, many of these exoplanets (Gliese 581d, Gliese 581g, Gliese 667 Cc, Gliese 163 c, HD 40307 g, 55 Cancri f, GJ 1214 b, HD 85512 b) circle small mass stars. Many seriously doubt whether life could evolve in these star systems.
M-dwarfs are cooler and much smaller than our sun. They account for about three-quarters of all stars in the Milky Way. Although Earth size planets have been found around such dwarfs, these planets tend to orbit very close to the sun. Because M dwarfs are small and cool, their temperate zone--also known as the "habitable zone," the region where liquid water might exist is further inward. New research by the Harvard-Smithsonian Center for Astrophysics (CfA) suggests that there are 17 billion earth-like planets in the Milky Way in an orbit closer than Mercury. In other words, one in six stars has a Earth-like planet in a close orbit. The problem is, there planets may all be dead worlds.
Red dwarf planets emit less light so the planet must be closer to it's sun. Flares on M-dwarfs can be a thousand times stronger than compared to our Sun. Such mega-flares can double the brightness of the star in minutes. Life on the surface of GJ667Cc would have to find a solution to this problem.
In addition, many red dwarfs can lose their light. They may become covered by sunspots that could reduce the energy output of the star by as much as 40% for periods that may last months. In other words, random nuclear winters. Also, red dwarf stars emit very little ultraviolet light, these varying light conditions would be a potential problem for life as we know it.
However, a recent study posed even more problems for low mass stars.
1. Habitable planets that are close to their sun are probably tidally locked so one side of the planet is constantly in daylight while the other side would be frozen in constant darkness. The dark side of these planets will always be too cold to support life and the lighted side could become exposed to too much radiation and the temperature differences would cause extreme weather across the planet.
2. Similarly, tides can cause the planet`s rotation to become perpendicular to its orbit eliminating seasonal variation and again creating dark and light sides with similar problems to 1. In addition, the equator of such a planet could become so hot that it would burn off the atmosphere.
3. Tidal heating causes massive vulcanism. Planets where there are constant lava flows are obviously not habitable.
A recent study estimated there may be as many as 100 billion Earth size planets surrounding dwarf stars in the Milky Way. The only problem is that they may all be dead worlds.
Second, many other exoplanets revolve around binary or trinary star systems (Kepler-22 b, Kepler-47c). However, recent research suggests that planets from binary and trinary star systems that produce unstable orbits. Computer simulations suggest that in most cases the binary star system will eventually radically change the orbits of the planets.
Third, Tau Ceti (Tau Ceti e, Tau Ceti f) has an asteroid belt ten times the size of our asteroid belot that would destroy all life in the system. Also, it's a Tau Ceti is metal-deficient, which deficiency is usually few rocky planets. In addition, the best guess is that the surface temperature on these planets are oven-like (155 °F, 122 °F). However, Tau Ceti f may also not have a greenhouse effect so it's average surface temperature would be -31 °F which would leave all the water frozen.
After that quick screening, we're left with no habitable confirmed planets.