Note: The study of exoplanets is a rapidly developing field, and new discoveries are constantly being made. The information given here is correct at the time of posting, but it is possible - indeed, very likely - that new findings will render some of the comments here obsolete over time.
An extrasolar planet - commonly shortened to 'exoplanet' - is any planet outside the Solar System, either a planet orbiting another star, or a so-called 'rogue planet' floating independently through space. Before the late 1980s, extrasolar planets belonged entirely to the realm of speculation. Until that time, it remained a theoretical possibility that planetary systems were a rare or perhaps even a unique phenomenon. A number of discoveries then led up to the first confirmed extrasolar planet in 1995, and since then a number approaching four thousand have been identified.
The first potential exoplanet candidate was located in 1988: a body in orbit around the primary star of the binary Errai system in Cepheus. There were some initial questions over the data (and so this is not generally formally credited as the first extrasolar planet to be discovered) but it is now recognised as a massive planet (at least 1.6 times the mass of Jupiter) and has been given the name Tadmor.
Further discoveries followed in 1992, with a potential planet around the star HD 114762 in Coma Berenices, now confirmed as an immense gas giant more than eleven times Jupiter's mass. In the same year, two much smaller bodies were discovered orbiting, not a star as such, but the pulsar designated PSR B1257+12 in Virgo. These were the first first confirmed extrasolar planets of any kind, and are now known to belong to a planetary system with at least three members. The pulsar has been given the name Lich, and the original two planets Poltergeist and Phobetor (with a tiny third planet Draugr being located two years later).
In 1995 came the announcement of the first confirmed exoplanet in orbit around a main sequence star. That star was 51 Pegasi, a yellow dwarf like the Sun lying off the western edge of the Square of Pegasus. Its planet - the only known planet in the 51 Pegasi system - is a gas giant with a mass rather lower than that of Jupiter in a very close orbit around its star. The planet was originally designated 51 Pegasi b, but later named Dimidium (and the star 51 Pegasi was given the name Helvetios).
Since these early pioneering discoveries, developing technologies and techniques have raised the count of known extrasolar planets to a number approaching 4,000, a count that is increasing all the time. Based on the sample of exoplanets so far found, it has become clear that the formation of planets is a relatively common phenomenon, and the total number of planets within the Milky Way can be estimated to lie in the hundreds of billions (of which a significant proportion are rogue planets, orbiting the Galaxy independently of any star).
Naming Extrasolar Planets
The standard method of naming extrasolar planets is to add a lower-case letter to the name or catalogue number of the planet's parent star, starting with 'b'. These letters are added in order of discovery, and have no necessary relation to its relative position in its stellar system (so in some systems planet 'b' might be the outermost planet, in others it might be the innermost). There is a single exception to this rule, that of the early discovery of planets around the Lich pulsar, which were named before the standard came into effect. These were originally designated PSR 1257+12 A, B and C, equivalent to b, c and d on the standard system.
From 2015, the IAU have assigned actual names, selected from public submissions, to a number of the more significant exoplanets and the stars they orbit. Typically these form a theme for each system, so for example the star Mu Arae is now named for the Spanish writer Cervantes, with its four planets Mu Arae b, c, d and e being given the names Quijote, Dulcinea, Rocinante and Sancho after characters from Cervantes' Don Quixote.
These named exoplanets are the exception rather than the rule, but their numbers are growing. A handful more orbit stars that already have common names, like Pollux or Kochab, but the majority of extrasolar planets are known by the catalogue numbers of their parent stars. So, for example, HD 20781 b, a gaseous planet orbiting a star in Fornax, is identified using that star's pre-existing number in the Henry Draper ('HD') star catalogue.
In many cases, a star is catalogued according to the mission or project that discovered the planet. Many planet designations are therefore derived from the highly successful Kepler planet-hunting mission, giving rise to terms like 'Kepler-99 b' (the first planet to be discovered around Kepler star number 99). The Kepler mission focussed its attention on a small region of the sky mainly within the constellations Cygnus and Lyra, with a small patch in Draco, so all exoplanets with the 'Kepler-' prefix lie within one of these three constellations. The common exoplanetary prefixes 'KIC' and 'KOI' are also related to this mission, indicating 'Kepler Input Catalog' and 'Kepler Object of Interest' respectively.
Like Kepler, other exoplanet-hunting projects give prefixes of their own to new discoveries. Some common examples include HAT ('Hungarian Automated Telescope'), OGLE ('Optical Gravitational Lensing Experiment') or WASP ('Wide Angle Search for Planets'). Because stars fall within several catalogues, it is unavoidable that their planets can, and often do, have multiple overlapping names.
Finding Extrasolar Planets
In general, it is not possible to see exoplanet directly, though there are some exceptional cases where images of an extrasolar planet have been achieved. Planets identified by this rare technique tend to be large, hot bodies relatively distant from their parent star, since these are ideal conditions for direct detection. An example is the star Fomalhaut, around which the Hubble Space Telescope was able to produce an image of a wide disc of protoplanetary matter, with a massive planetary body - now named Dagon - orbiting within it.
Direct imaging like this is rare, because detecting a planet against the overwhelming light of its parent star is extraordinarily difficult. More usually, the existence and nature of extrasolar planets is inferred from observations of the star itself. Planets orbiting a star will have various tiny but detectable effects on that star, and by observing these effects it is possible to deduce the mass and orbit of the planets involved. There are numerous techniques for doing this, but some of the more common include:
Perhaps the most intuitive approach to exoplanet-hunting is to look for the effects of a planet's passing in front of its star. During a transit like this, the star's light dims and then brightens again in a characteristic pattern that reveals the presence of a planet. This is the method used with great success by the Kepler spacecraft, but it does have one significant limitation: the plane of a star's system must be so oriented that its planet or planets pass across its face as seen from Earth. Despite this restriction, the method has located hundreds of planetary systems, especially within the small area of sky monitored by Kepler.
As a planet follows its orbit around a star, its motion is dominated by the gravity of that star, but it also exerts a fractional gravitational force of its own. Exoplanets can therefore be detected by looking for a telltale 'wobble' in a star caused by a planet's tiny gravitational effect. This process can be achieved through very precise observations of a star's actual position (termed astrometry), but more usually the measurements are made by following the patterns within a star's spectrum, whose minute variations can reveal its motion relative to the detector. This spectral radial velocity method has been the source of most exoplanet discoveries to date.
Sometimes the presence of a planet can cause detectable variations in the brightness of its parent star. For example, a massive planet can cause its star to stretch into a slightly ellipsoidal shape, and as the star rotates its distorted form will result in regular changes in its brightness as seen from Earth, in turn revealing the presence of an orbiting planet.
There are various phenomena that oscillate with highly regular timing, such as a pulsar or a pulsating variable star, and the presence of a planet will cause anomalies in the otherwise precise pulsation pattern. It was this technique that revealed the presence of bodies in orbit around the pulsar now known as Lich, one of the earliest exoplanetary discoveries.
When two stars are precisely aligned with one another, the path of the light from the more distant star is bent by the gravitational field of the nearer star in a characteristic and predictable way known as gravitational lensing. If the foreground star has a planet, then the fine-scale 'microlensing' of the light will be affected, revealing the planet's presence. This gravitational microlensing technique has the drawback that it requires exact alignments of stars, which are necessarily brief effects, but it brings the advantage that it can detect exoplanets over much greater distances than other current techniques.
Common Types of Extrasolar Planets
Numerous varieties of exoplanet have been found, but those discovered to date broadly divide into the same two basic types that are found within the Solar System: rocky terrestrial planets like Earth or Mars, and gas giant planets like Jupiter or Saturn (in terms of mass, the dividing line between the two types is generally placed at ten times the mass of Earth). At present most of the extrasolar planets found have been gas giants, but this is mainly due to the fact that the more massive a planet is, the easier it is for most detection methods to find it. As techniques are refined, smaller and less massive rocky planets are being found more and more commonly.
One planet type found in many extrasolar systems that is unknown in the Solar System appears when a gas giant planet orbits very closely around its parent star. Planets of this kind are known as 'hot Jupiters', being planets with masses comparable to Jupiter, but circling their star well within the radius of Earth's orbit around the Sun, typically taking just a few days to complete an orbit. Dimidium or 51 Pegasi b is an example of a hot Jupiter, with a mass about half that of Jupiter itself, orbiting its star Helvetios (51 Pegasi) in just four days, at a distance closer to the star than Mercury is to the Sun.
At the other end of the scale, less massive planets are commonly referred to as 'Earth-mass' planets. This application is not precise: an Earth-mass exoplanet might be up to five times the actual mass of Earth. It is possible for rocky terrestrial planets to exist with a mass even greater than this (for example, Gliese 876 d is a rocky world with some seven times Earth's mass). Really massive terrestrial planets like this are referred to as 'super Earths'.
There is a natural interest in locating extrasolar planets that are capable of supporting life, and one of the markers for this is a planet's orbit relative to its habitable zone. This is the zone around a star where water can exist in liquid form, a key element in the chemistry of life. Planets closer to their star than the inner edge of this zone are typically too hot for water to exist as a liquid, and beyond the zone's outer edge water can exist only as ice. The distance and size of the habitable zone varies from star to star: for energetic giant stars it can be many times the diameter of Earth's orbit, while for weak red dwarf stars, planets need to follow a very close orbit in order to occupy this zone. The idea of the habitable zone is something of a simplification (for example, water exists in liquid form on several of the moons of Jupiter and Saturn, which lie far outside the Sun's habitable zone) but it provides a useful rule-of-thumb for detecting potentially life-bearing planets.
A narrower, more quantitative pointer towards a planet's suitability for life - at least life as it exists on Earth - is the Earth Similarity Index or ESI. This takes account of a range of factors, including a planet's temperature, as well as factors such as its size and density (and thus its surface gravity). The result is a figure ranging from 0 to 1, where an index of 1 would be an exact twin of Earth, at least under the ESI parameters. In general an ESI of greater than 0.8 is considered 'Earth-like' (though this is meant in very broad terms: Mars on this scale would measure only very slightly under this 0.8 value).
On this scale the most Earth-like planet so far discovered would potentially be Gliese 581 g, the fourth planet of the red dwarf Gliese 581 in Libra. The existence of this planet is not yet definitively confirmed, but if it does exist its parameters would give it an Earth Similarity Index of 0.89. This planet - assuming its discovery is confirmed - is rather more massive than Earth, and orbits much closer to its star, but since that star is a faint red dwarf, the orbit of Gliese 581 g would place it within the habitable zone.
The highest confirmed ESI value is only slightly lower, at a value of 0.88 for the exoplanet Kepler-438 b. This is another planet in a close orbit around a red dwarf in its habitable zone, in this case the parent star Kepler-438 lies some 470 light years from the Sun in the constellation of Lyra. Though Kepler-438 b is extremely similar to Earth in many respects, there are reasons to think it might not have conditions conducive to life. First, it is tidally locked to its star (that is, it always shows the same face, like the Moon to Earth) giving rise to extreme environments on its surface. Second, Kepler-438 is known to be an active flare star, subjecting its planet to regular bursts of intense radiation.
The most Earth-like planet in orbit around a Sun-like star is much more distant from the Sun, at some 1,400 light years. This is Kepler-452 b, the only known planet of the star Kepler-452, which is a yellow dwarf with the same spectral classification, G2V, as the Sun. Its planet Kepler-452 b orbits within the star's habitable zone, and has an Earth Similarity Index of 0.83, being rather larger and more massive than Earth itself.
Factors such as the habitable zone or the Earth Similiarity Index give some clues about whether a planet might be suitable for life, but they do not in themselves reveal whether life exists on any given planet. One way to take this next step is through spectroscopy, using any of various methods to isolate the chemical composition of an exoplanet's atmosphere. For example, the relatively high proportion of oxygen in Earth's atmosphere originated with the photosynthesis of ancient cyanobacteria, so finding a similarly high oxygen content in another planet's atmosphere would point to at least primitive photosynthetic life. In principle, it might even be possible to detect features such as the emissions of an industrial society on an extrasolar planet, though possibilities like this are rather more speculative.
An even more direct means of detecting advanced extraterrestials would be to find examples of astronomically-scaled engineering projects in extrasolar systems. No such example has been found, though the topic has come to the fore with the discovery of strange phenomena in the system KIC 8462852, informally known as 'Tabby's Star'. This star shows brief patterns of dimming that seem to show that its light is being occluded to a significant degree, but these patterns are not periodic in nature (and so cannot be the result of a planet in a natural orbit). The reason for this behaviour is not currently understood; suggestions include the possibility that it might be caused by swarms of cometary fragments, or a giant planet with an extraordinary ring structure, but the possibility has also been raised that these patterns might be caused by gigantic artificial structures orbiting the star. At present the actual phenomenona at work in this system remain unexplained, and are very far from providing direct evidence for any kind of alien activity. They do, however, help to illustrate the kind of effects that might point to an extraterrestrial civilization at work in another stellar system.
Some Significant Exoplanetary systems
Closest to the Sun
The closest extrasolar planet to the Sun is Proxima b, a rocky world like Earth but rather more massive. In orbit around the Sun's nearest stellar neighbour, the red dwarf Proxima Centauri, Proxima b is as close to the Solar System as an extrasolar planet could possibly be, at a distance of just under 4.2 light years. Proxima b orbits just 0.05 AU from its parent star (well within Mercury's orbital distance from the Sun) but because of Proxima's low energy output this orbit places the planet inside the habitable zone of its star.
Most Distant from the Sun
The most distant extrasolar planets are detected using the gravitational microlensing effect, which is particularly effective at observing over extreme distances, though it has a transitory nature that makes confirmation difficult. This technique has discovered tentative evidence of planets millions of light years away, beyond the Milky Way Galaxy, but the nature of technique makes confirmation in these cases essentially impossible. The most distant confirmed exoplanet currently known is SWEEPS-11 b, a hot Jupiter orbiting a dwarf star in the central regions of the Galaxy. Sources differ on this planet's precise distance from the Sun, but most agree on a figure approaching 28,000 light years.
Identifying the largest and most massive extrasolar planets is difficult, because really massive planets can be difficult to distinguish from brown dwarfs, which are properly substellar objects rather than planets as such (and indeed some brown dwarfs have planets of their own). Generally speaking, a body with a mass greater than about thirteen times that of Jupiter is considered a brown dwarf, though this is not a hard limit, and true planets may exist beyond this mass. There are many bodies known above this limit (for example WASP-81 c has a mass of some 57 times that of Jupiter, and is probably a brown dwarf). The body DENIS-P J082303.1-491201 b is commonly listed as the most massive extrasolar planet at 29 times Jupiter's mass, though this too may be a brown dwarf. Taking the somewhat arbitrary 'thirteen Jupiter mass' limit into account the most massive known planet would be HD 217786 b, a gas giant orbiting a star in Pisces, which lies almost exactly on the brown dwarf threshold.
The most massive exoplanets are not necessarily the largest, because less dense bodies can be remarkably large while remaining - comparatively - low in mass. At the extreme of this phenomenon is HD 100546 b, which is projected to have a diameter more than six times that of Jupiter, while being far less massive than many similar planets. This body is an outlier, and probably belongs to the brown dwarf class. Generally it is exceptionally rare for extrasolar planets to exceed twice Jupiter's diameter, with the largest currently known (excepting likely brown dwarfs) is probably HAT-P-67 b, with a diameter 2.1 times that of Jupiter. Despite its size this object, which orbits a bright yellow star in the northern parts of Hercules, has a mass just a third that of Jupiter, and thus appears to be a true planet with a very low density.
Smallest and Least Massive
The nature of the techniques used to detect exoplanets mean that it can be difficult to measure low mass planets with certainty, and there are several candidates. Among these is Draugr (one of the planets of the pulsar Lich, among the first extrasolar planets found), with a mass roughly twice that of Earth's Moon. Currently the most likely candidate for smallest exoplanet is probably Kepler-37 b, a planet whose dimensions are not known precisely, but which calculations suggest is probably about a third the diameter of Earth (and thus comparable with Earth's Moon).
The busiest stellar system found to date belongs to the Sun-like yellow dwarf HD 10180, some 127 light years from the Solar System in the constellation of Hydrus. The HD 10180 system is known to contain at least seven planets, and possibly as many as nine, with most of those planets packed into tight orbits around the star less than half Earth's distance from the Sun. These are massive planets, generally about ten times the mass of Earth, with the outer planet HD 10180 h approaching the mass of Saturn.
Another system with at least seven planets is the first to be catalogued by the Transiting Planets and Planetesimals Small Telescope project, better known by its catalogue number TRAPPIST-1. This is a tiny, cool red dwarf star in the constellation Aquarius, around which seven extrasolar planets have been identified. All seven planets appear to be rocky worlds, ranging from 0.8 Earth diameters to 1.1, and following very close orbits around their parent star. Because of the low luminosity of the TRAPPIST-1 red dwarf, these close orbits place three of the seven planets within its habitable zone, though their habitability is like reduced by the frequent flare activity of the dwarf star.
Because of the way current detection methods work, it is far easier to find planets in orbit around a star, and the overwhelming majority of exoplanets so far discovered fall into that category. These are not, however, the only possible type of extrasolar planet; there are also so-called 'rogue' planets that wander through the Galaxy independently of any star. These rogue planets originate either by being ejected from their home star systems, or by forming independently of any star. Theoretically, these rogue planets are thought to be extremely numerous, probably numbering in the billions in the Milky Way Galaxy alone. By their nature, though, they are extremely difficult to locate. Only a handful of candidate objects have so far been identified (and some of these are thought to be rogue brown dwarfs rather than true planets). An example of a likely rogue planet is the body catalogued as PSO J318.5-22, an object with some six or seven times the mass of Jupiter, floating through space about eighty light years from the Sun in the constellation of Capricornus.