A planet is habitable if it is capable of sustaining life that either originates there or has been carried to it by a meteorite or other space satellite. The habitable environment would normally consist of large regions of liquid water. Ground and atmospheric conditions should favour the creation of complex organic molecules. The molecules should be combinations of the four elements necessary for life: carbon, hydrogen, oxygen and nitrogen. Up to 96% of Earth's biomass is formed from these elements. In addition, there has to be an internal or external energy source that sustains such a metabolism.
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Astronomers assume that habitable planets are terrestrial. This means that they are between half and double the size of Earth and are composed mostly of silicate rocks. They do not have the hydrogen and helium atmospheres of gas giants such as Jupiter, Saturn and Uranus. The gas giants are thought to be incapable of supporting life. A habitable planet’s atmosphere has to be thin enough to transfer heat from the star, but also thin enough to prevent an inferno on its surface.
The habitable zone is that region around a star where an orbiting planet may have liquid water on its surface and sustain life from energy radiated by the star. The challenge has been to find terrestrial planets that orbit within the habitable zone of stars.
NASA’s Kepler Mission was launched in March 2009 with the aim of finding these Earth-sized planets in the habitable zone. NASA has extended the duration of this mission until the end of fiscal year 2016. The Kepler spacecraft is an observatory in a heliocentric orbit. Its main instrument is a photometer that monitors the brightness of stars. Its orbit avoids the Earth’s gravitational forces and stray light. The spacecraft is operated for NASA by Colorado University-Boulder Laboratory for Atmospheric and Space Physics (LASP). It has identified over 2,300 candidates for habitable planets among the Milky Way’s stars, of which 74 have been confirmed. Kepler also identified planets of a similar size to Earth or Mars, as well as double-star planetary systems.
Six of these habitable planets are smaller than half the size of Earth. In December 2011, NASA confirmed that a planet just 2.4 times the size of Earth had been discovered in the habitable zone of a star much like the Sun. Situated towards the Lyra and Cygnus constellations some 600 light years from Earth, the planet has been called Kepler-22b. NASA believes that at least 500 million of the Milky Way’s 50 billion planets orbit in the habitable zone.