Exoplanets: Worlds beyond our Solar System
The existence of planets outside of our solar system was, until relatively recently, a matter of conjecture. Astronomers speculated that the universe was populated by planets, but technological limitations meant that they were unable to test that hypothesis. That changed in the latter decades of the twentieth century, when telescopes advanced to such an extent that they were able to map these unchartered areas of the universe. There were several unconfirmed sightings of exoplanets, or extrasolar planets, in the late 1980s but the first definite sightings were of the two planets now known as Draugr and Poltergeist in 1992, both of which are around 2,300 light-years away and were spotted by radio astronomers Aleksander Wolszczan and Dale Frail. Since then over 3,600 exoplanets have been discovered; an incredible rate of discovery which is set to increase as more powerful telescopes are built on Earth or are launched into space. The study of exoplanets is now an important part of astronomy, since these planets can tell us a lot about the past and possible future of the Earth, whilst also offering the tantalising prospect of being home to extraterrestrial life.
Although the discovery of exoplanets now happens so frequently it is almost common place, it took a long time for their existence to be widely accepted. The scientific revolution in Europe during the fifteenth and sixteenth centuries inaugurated a new understanding of the cosmos based on Copernicus’s heliocentric conception of the Earth orbiting the Sun. The friar and philosopher Giordano Bruno proposed that, following Copernicus’s model, the stars were distant suns with planets on which life could exist. Incredibly, he was burned at the stake for his scientific beliefs during the Roman Inquisition. Nevertheless, by the advent of the twentieth century the idea of the universe being populated by planets was widely accepted and the search for sightings of these exoplanets started in earnest, mainly as part of the larger search for extraterrestrial life.
In the 1990s, advances in high-resolution spectroscopy allowed for more precise, and far more rapid, detection of exoplanets. This involved detecting the existence of these planets indirectly, through measuring how their gravitational pull affected the motion of the stars they orbited. Another technique involved using the changes in a star’s luminosity to detect when a planet was orbiting in front of it. These new techniques allowed for more accurate mapping of other solar systems and intensified the search for exoplanets which resembled Earth, and which therefore could support life. Astronomers were particularly interested in finding exoplanets which were a particular distance from the sun. This would determine whether they were in the ‘habitable zone’; a range of orbits within which the surface of a planet can support liquid water because it would receive the optimum amount of warmth from its sun. This zone is colloquially known as the ‘Goldilocks zone’, a reference to the story of Goldilocks, who has to choose between, among other things, three bowls of porridge, picking the one which is not too hot or too cold but is ‘just right’. It is estimated that around one in five stars have a planet of a similar size to Earth within this zone, and therefore it can be extrapolated that there are 11 billion planets which could be habitable in the Milky Way.
NASA’s Spitzer Space Telescope recently identified several of these possibly habitable planets orbiting a single star around 40 light years away from Earth. The telescope identified seven exoplanets orbiting a dwarf star known as TRAPPIST-1. Three of the seven were firmly in the habitable zone of the sun’s orbit, but all seven could possibly have liquid water -- a prerequisite for life on Earth. This discovery set a record for the highest number of habitable planets orbiting a single star . The TRAPPIST-1 planets are also notable for, most probably, being tidally locked to their star, in the same way the Moon is to the Earth. This means they do not rotate in the manner of our planet and therefore have a ‘dark’ side which is always facing away from their sun, which would cause extreme temperature shifts and chaotic climatic patterns . Astronomers have also subsequently tracked an unusually high number of solar flares emerging from the TRAPPIST-1 star. Although each of these are not necessarily large enough to wipe out any trace of life on these planets, the fact of their high frequency suggests that the atmosphere of the TRAPPIST-1 planets would be highly unstable, and life would have to be extremely hardy to withstand such conditions, if it could develop at all.
The search for exoplanets which could support life nevertheless goes on, and the almost infinite extent of the universe suggests that there are likely to be planets out there which do have the required conditions. This search also regularly picks up fascinating planets which reveal the incredible variety of the universe. Kepler-16b, for example, whilst not habitable, was the first planet to be sighted orbiting not one but two stars. Another noteworthy planet is Kepler-186f, which was the first planet to be found with a radius similar to that of Earth. Also in the habitable zone, this planet is 560 light-years away and is thus too far away to detect whether it has an atmosphere. Meanwhile, Kepler-22b is a ‘water world’ which is almost entirely covered in liquid, but which has also been designated a candidate for extraterrestrial life. All these exoplanets have notable features which make them worthy of study, and whilst humankind may not be able to find life on any of them, mapping more of this incredible variety of planets will nevertheless help us to further our understanding of the universe.
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