Until the early 1990s, it was not known for certain whether planets existed outside of our own Solar System. While it stood to reason that other stars in other parts of the galaxy would have planetary systems like the one we are part of, they had never been observed. In 1992, a planet was discovered outside of our Solar System for the first time. The planet is PSR B1257+12B, it is four-times the size of Earth and orbits a pulsar approximately 1,000 light years away. Further discoveries followed, 51 Pegasi b was found in 1995, the first exoplanet found orbiting a main sequence star. As of late-July 2013, there are 926 confirmed exoplanets, with thousands of candidates awaiting confirmation.
As the number of confirmed exoplanets increases, certain questions begin to be answered, while others can be investigated with increasing vigour. It's becoming clear that it is common for stars to have at least one planet in orbit, with the trends suggesting that continued research will confirm that where there is one planet, there'll be a whole system of planets of various sizes and types.
The hunt is being led by the Kepler Spacecraft, which launched in 2009 and since then has had its photometer fixed on a section of our galaxy in which 145,000 main sequence stars are in view. The photometer tracks when the view of a particular star is momentarily dimmed by an exoplanet passing in front of it. This data can then be used to determine the size and orbit of the exoplanet.
As far as we know, life can only live in conditions like those we find on Earth. Our first port of call in the search for life elsewhere in the universe should be in planets with similar conditions to those at home. The Habitable Zone is a region around a star in which an orbiting planet with an atmosphere can support liquid water. On our own planet, we have only found life where liquid water persists, so it stands to reason that liquid water persisting elsewhere in the galaxy could provide the opportunity for life to take hold.
There are other aspects of our planet and Solar System that have helped create an environment stable enough for live to thrive. The Sun is in a stable phase within its life that has lasted for billions of years and will continue to last for billions more. The Earth is surrounded by a magnetic field that protects the atmosphere from cosmic rays that may otherwise strip the atmosphere away. Jupiter may have acted and may continue to act as a shield for the Earth against large asteroid impacts, ensuring stable surface conditions. The Moon, as a single large satellite helps to create stable tides, which create circumstances favourable to life in the oceans.
Some exoplanets have been found orbiting their star within the Habitable Zone. So far, these have all been Super-Earths (exoplanets with a mass larger than the Earth). The first was Kepler-22b in 2011, found to be 2.4 times the size of Earth and orbiting a Sun-like star. Better candidates for life-supporting exoplanets have since been found, Kepler-62 has two planets orbiting in its Habitable Zone. One is 1.6 times the size of Earth, the other is 1.4 times the size, with both having a reasonable likelihood of supporting liquid water.
The search for more exoplanets continues. The Kepler Spacecraft is currently our of commission, due to to a fault, but has already provided plenty of data on candidate exoplanets that must be checked and verified in order to become confirmed. As the hunt goes on, there is an increased chance of finding a planet with similar conditions to those on Earth, where the star is in an appropriately stable phase, the planet itself is within the Habitable Zone and is potentially protected by gas giants in the outer system. Who knows what the discovery of such a system may do for science on this planet. Would the discovery of a truly earth-like planet within a few light years spark a new space race? How far might our desire to seek out further life in the universe take us once an X is marked on a spot so tantalisingly?
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