A team of astronomers may have found exoplanets that are inhabitable. The exoplanets are not unlike earth. Previously it was believed that exoplanets would be too cold to sustain life. Scientists are rethinking that.

The range of ideal orbital distances from a star that makes the conditions necessary for life are called the stellar habitable zone (SHZ), or Goldilocks zone. Outside that zone, it is difficult or impossible for a planet to sustain life. But a paper that was recently published in Astrobiology magazine theorizes that that some planets outside this zone may be even more habitable for life than on Earth. The authors say tidal heating, orbital and rotational energy released as heat in the crust of the planets, can create conditions in which life could emerge.

An international team of astronomers led by Dr Vincent Van Eylen of Aarhus University, Denmark,  at Stellar Astrophysics Centre in Aarhus, Denmark, discovered a new exoplanet that they have called "Kepler-410A b." The exoplanet planet is roughly the same the size as Neptune. The newly discovered planet has a radius of about 2.8 times that of Earth.

Kepler 410Ab orbits the brightest star in a double star system that is 425 light years from Earth. The planet is much closer to its star than the Earth is to the Sun and unlikely to be suitable for life.

Stanford University earth sciences professor Norman Sleep, who is an editor for Astrobiology said "A great place for hydrothermal microorganisms and a volcanic eruption in the weather forecast every morning and evening, a tidally heated planet would be unpleasant though spectacular to visit."

The exoplanet can be seen with through strong binoculars. It has an orbit period of about 18 days, and is therefore too hot in order to sustain life. Such planets are usually classified as being too frigid for life, a "snowball" state, or too hot for life because a thick layer of atmospheric insulation causses a runaway greenhouse state. They noted that tidal heating can be a heat source that causes a planet, like Venus, to go into a runaway greenhouse state and become uninhabitable. Conversely, tidal heating could make a world orbiting outside the HZ inhabitable by preventing it from entering a snowball state.

Astronomers working out of the Stellar Astrophysics Centre in Denmark were studying the star that the new planet revolves around. They found that star's rotation appeared to be in strong alignment with the planet's movement.

Lead author Vincent Van Eylen, a doctoral student at Aarhus University in Denmark, told the Stellar Astrophysics Centre's Peter F. Gammelby  "Ultimately, to understand anything about exoplanets, we need to understand the stars they revolve around. In this case, asteroseismology has even allowed us to measure the inclination angle of the star. We now know we are looking at the equator of the star, not at the pole. This can be compared with the orbit of the planet to learn about planetary formation. The star is around 2.7 giga-years old and is a little larger than the Sun. We will never be able to go there, as it is located at around 425 light years from Earth."

Exoplanets orbiting stars outside our Solar System are now known to be very common. These extrasolar planets have been found orbiting stars of widely varied ages and chemical compositions and are scattered across the sky. But, up to now, very few planets have been found inside star clusters.

A few months ago a new rogue planet was found by a team of astronomers who were working at the University of Hawaii at Mamoa. It is about 80 light years away from earth. Astronomers compared the newly discovered planet to a young Jupiter.

In a statement, astronomy team leader Michael Liu said "We have never before seen an object free-floating in space that looks like this. It has all the characteristics of young planets found around other stars, but it is drifting out there all alone. I had often wondered if such solitary objects exist, and now we know they do."

The planet without a sun is a free-floating planet. It was given the name "PSO J318.5-22." The new planet is estimated to be about 12 million years old, which is young for a planet. For example, it is believed that the Earth is about 4.5 billion years old.