By David Morris
In March last year, following three years of data accumulation and analysis, NASA disclosed the discovery of an exoplanet (a planet from outside our solar system) that shares enough characteristics with the Earth that it could potentially harbour life, Kepler-186f. Discovered by the Kepler space observatory, the body has a volume of approximately 1.48 x 1021 m3, just about 10% larger than Earth. Interestingly, the exoplanet is believed to show very little obliquity, the degree to which the axis of rotation of a mass tilts away from its plane of orbit. This means that it experiences minimal differences in seasonal weather. It is of comparable age to the Earth, is located roughly 490 light years away and can be found in the constellation Cygnus.
Kepler-186f is the first planet outside of our solar system discovered to have both a comparable size to the Earth and to be within the ‘habitable zone’ of its parent star. The habitable zone of a star is the range of distances from it where, given sufficient atmospheric pressure, any present water will be a liquid: a characteristic that is absolutely vital for the existence of life. Many exoplanets have this characteristic, but they are all at least 40 % larger than Earth, making them too dense to sustain life. This is the case for the other four exoplanets that Kepler-186f shares its solar system with. Their large masses draw them closer to their parent star due to gravity, making them too hot and therefore uninhabitable.
Kepler-186f orbits Kepler-186, a star half the size of our Sun. This star is known as a red dwarf, a star cooler and smaller than that of the Sun. Kepler-186f receives roughly a third of the heat energy that Earth receives, making it much less able to support life. The masses of red dwarves are generally so small that orbiting planets are likely to go into tidal locking. This process is caused by the gravitational pull of a mass on an orbiting body, compelling it to rotate at roughly the same rate as it orbits. This means that the same hemisphere is always facing the mass, as is the case for Pluto and one of its moons, Charon. This effect is also employed in getting artificial satellites to orbit the Earth correctly. Tidal locking would cause one side of the planet to be in constant ‘night-mode’ and the other in constant ‘daylight’, resulting in enormous temperature variations and therefore varying phases of atmospheric substances like water. Without a mechanism for swift planetary heat distribution, such as oceans, the planet would be uninhabitable. There is an approximately 50 % chance that Kepler-186f is orbiting its parent star with partial tidal locking. As a result of this, a single rotation of the exoplanet about its axis could take up to weeks or months, contributing to the prevalence of the aforementioned planetary temperature differences.
There are a wide range of atmospheric densities and temperatures that the exoplanet could support depending on its chemical composition. The greenhouse gases that keep a planet hot could be present in any number of concentrations, maintaining a specific surface temperature. The age of the red dwarf is also important as it will emit differing amounts of various types of radiation depending on its age. For example, if the red dwarf emits high levels of high-energy, ultraviolet radiation, as is prevalent in young red dwarves, the energy supplied could cause lighter elements like hydrogen and helium to escape the gravitational pull of the exoplanet, removing mass from its surface.
Given the information NASA has gathered, it is possible that Kepler-186f could support life. However, in order to fully determine its capabilities, the surface composition of the exoplanet needs to be examined further. Exoplanetary surface composition is very difficult information to obtain, especially for a body so far away. This is one of the many tasks that NASA is constructing the James Webb Space Telescope for, due to launch in October 2018. The telescope has been under construction since the beginning of 2011 and is planned to be sent to the L2 Lagrange point, 1.5 million kilometres from Earth, the furthest from the Earth that any man-made device has ever been sent. Consequently, the question as to whether other life outside our solar system exists may be answered within the coming years.