In 1992, scientists made one of the most significant discoveries in the history of astronomy; the first confirmed detection of an extrasolar planet. For many decades, it had been widely believed that planets existed around stars other than our own, but it was not until the discovery of two planets orbiting a distant star some 1000 light years away (1 light year = about 6 trillion miles) that their existence was proven beyond doubt. As detection methods advance, particularly thanks to the highly successful Kepler Space Telescope launched in 2009, new alien worlds are being discovered almost every week. As of August, 2013, well over 900 alien planets have been confirmed with another 18,000 still awaiting confirmation. Taking recent discoveries into account, scientists estimate that there are at least 100 billion planets in the Milky Way galaxy alone (est. 60 billion that are habitable) and countless trillions in the entire Universe. However, it is not just these statistics that make the world of exoplanetology fascinating; it is the remarkable diversity of alien worlds already discovered. With the sheer number of possibilities, it is likely just a matter of time before we find another planet much like Earth in the unending vastness of space. All known planets fall into two main categories; terrestrial planets and gas giants. In our own solar system, Mercury, Venus, Earth and Mars comprise the terrestrial planets while Jupiter, Saturn, Uranus and Neptune are gas giants. The Planetary Habitability Laboratory of the University of Puerto Rico further divides these groups by size and average surface temperature. MERCURIAN PLANETS Credit: NASA; Johns Hopkins University; Carnegie Institution of Washington Mercurian planets are those which are similar to Mercury, the closest planet to the Sun in our own solar system. As of August, 2013, only two have so far been discovered, although they are likely to be much more numerous than they appear to be. Due to their small size, they are among the hardest planets to detect. With masses under ten percent that of Earth, such planets are too small to have a high enough gravitational field to retain an atmosphere, and are therefore unable to support life as we know it. SUBTERRAN PLANETS A Mars-size icy exoplanet. Mars is a perfect example of a subterran planet, and around six have extrasolar planets of this kind have been discovered. These are terrestrial planets with a mass between ten and fifty percent that of Earth. If located in a star's habitable zone, such a planet could support life provided that it meets certain other essential requirements. Due to lower masses, and therefore lower gravity, however, it is unlikely that such a planet could retain a large enough atmosphere to meet the requirements of sophisticated Earth-like life. All subterran exoplanets discovered to date orbit near their host stars as ones further away are much harder to detect. TERRAN PLANETS A habitable exoplanet around a red dwarf. When it comes to the search for alien life beyond our solar system, it is terran planets which are potentially the most promising. These are planets with a mass between half and double that of Earth. Earth and Venus are both terran planets. As with other smaller planets, no such worlds have yet been discovered outside of our solar system due to them being so difficult to detect with current technology. Provided that such a planet comfortably resides in the 'sweet spot' of its host star, then such a planet could, just like Earth, be able to retain an atmosphere and surface water. SUPERTERRAN PLANETS Terrestrial (rocky) planets which are more than twice the size of Earth are classified as superterran planets. Since more than 74 such planets have been confirmed orbiting other stars, they present some of the most promising places to look for life and other Earth-like conditions. Due to their larger masses, however, they would have a much stronger gravitational field making surface conditions quite different though not necessarily inhospitable to life. Seven superterran planets have been discovered orbiting in the habitable zones of their host stars. Most of these orbit red dwarf stars and are thought to be tidally locked to them. This means that, on one side of the planet, it would always be daytime while the other would be in permanent darkness. In the middle, however, there could be a sweet spot where life may thrive. Other superterran planets are thought to be water worlds, completely covered by an ocean which is likely to be hundreds of miles deep. Yet another type of superterran planet is described as "Cthonian planets." These are former gas giants which have lost their enormous atmospheres leaving only a rocky core behind. No superterran planets exist in our solar system. NEPTUNIAN PLANETS Hot Neptunian: A Neptune-size hot exoplanet. The smaller of the gas giants are referred to as Neptunian planets. In our own solar system, Neptune and Uranus fall into this category. Neptunians are between ten and fifty times more massive than Earth. Over a hundred have so far been discovered, though the vast majority of them are in the hot zone close to their host stars unlike Neptune and Uranus which are far out in the cold zone of the solar system. Neptunian planets themselves cannot host Earth-like life due to the fact that they have no solid surface save for possible rocky cores under extreme pressures. Just like the gas giants in our solar system, Neptunians are likely to have extensive systems of moons. Given the right conditions, such moons may support life. JOVIAN PLANETS A Jupiter-size hot exoplanet. Due to their size and the fact that they are by far the easiest to detect, the vast majority of extrasolar planets discovered to date are Jovian planets. These are planets with masses over fifty times that of Earth. Jupiter and Saturn are both Jovian planets but, unlike many extrasolar Jovian planets discovered so far, they reside in the cold zone in the outer reaches of the solar system. To date, almost one-hundred Jovian planets have been discovered in the habitable zone of their host stars. While such planets, like Neptunians, cannot host Earth-like life themselves, they could still possess moons with Earth-like conditions. Image Credits: Planetary Habitability Laboratory, University of Puerto Rico at Arecibo unless otherwise noted.
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