An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of 716 such planets have been identified as of December 22, 2011.[1] It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars.[2] It follows that tens of billions of exoplanets must exist in the Milky Way Galaxy alone.
For centuries, many philosophers and scientists supposed that extrasolar planets existed. But there was no way of knowing how common they were or how similar they might be to the planets of our Solar System. Various detection claims made starting in the nineteenth century were all eventually rejected by astronomers. The first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[3] The first confirmed detection of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Due to improved observational techniques, the rate of detections has increased rapidly since then.[1] Some exoplanets have been directly imaged by telescopes, but the vast majority have been detected through indirect methods such as radial velocity measurements.[1]
Most known exoplanets are giant planets believed to resemble Jupiter or Neptune. That reflects a sampling bias, since massive planets are easier to observe. Some relatively lightweight exoplanets, only a few times more massive than Earth, are known as well; statistical studies now indicate that they actually outnumber giant planets.[4][5] Planetary-mass objects also exist that orbit brown dwarfs or that "float free" in space, although strictly speaking the term "planet" may not be applicable to them.
The discovery of extrasolar planets has intensified interest in the possibility of extraterrestrial life.[6] Several giant planets are now known that orbit in their star's habitable zone. Among the candidates are Gliese 581 d[7][8] and HD 85512 b.[9] In December 2011, NASA confirmed that 600-light-year distant Kepler-22b, at 2.4 times the radius of Earth, is potentially the closest match to Earth in terms of both size and temperature. [10][11] Afterwards, on December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e[12] and Kepler-20f,[13] orbiting a Sun-like star, Kepler-20.[14][15][16]
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| “ | This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own. | ” |
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—Giordano Bruno[17] |
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In the sixteenth century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. He was burned at the stake by the Roman Inquisition in 1600, though his views on astronomy were not the main reason for his condemnation.[18]
In the eighteenth century the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centers of similar systems, they will all be constructed according to a similar design and subject to the dominion of One." [19]
Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 Capt. W. S. Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[20] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[21] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[22] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[23] Astronomers now generally regard all the early reports of detection as erroneous.[24]
In 1991 Andrew Lyne, M. Bailes and S.L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[25] The claim briefly received intense attention, but Lyne and his team soon retracted it.[26]
The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang.[27] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990 additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[28] but subsequent work in 1992 again raised serious doubts.[29] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[30]
In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[3] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, namely the nearby G-type star 51 Pegasi.[31] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet passed in front of it.
Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters," since theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters are a minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[32] Other multiple planetary systems were found subsequently.
As of December 22, 2011, a total of 716 confirmed exoplanets are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late 1980s.[1] That count includes 584 planets in planetary systems and 86 planets within multiple planetary systems. A system has been discovered in which a planet orbits around two stars, which orbit around each other.[33]
As of February 2011, NASA's Kepler mission had identified 1,235 unconfirmed planetary candidates associated with 997 host stars, based on the first four months of data from the space-based telescope,[5][34] including 54 that may be in the habitable zone.[35][36][37] Six candidates in this zone were thought to be smaller than twice the size of Earth,[34] though a more recent study found that one of the candidates is likely much larger and hotter than first reported.[38][39]
Planets are extremely faint light sources compared to their parent stars. At visible wavelengths, they usually have less than a millionth of their parent star's brightness. It is difficult to detect such a faint light source, and furthermore the parent star causes a glare that tends to wash it out. It is necessary to block the light from the parent star in order to reduce the glare, while leaving the light from the planet detectable; doing so is a major technical challenge.[40]
For the above reasons, telescopes have directly imaged no more than about thirty exoplanets as of November 2011. Several approaches have been studied for blocking the light from the parent star. One technique, recently demonstrated by a team of researchers from the Jet Propulsion Laboratory, uses a vector vortex coronagraph. The researchers are hopeful that many new planets may be imaged using this technique.[41][42] Another promising approach is nulling interferometry.[43]
All exoplanets that have been directly imaged are both large (more massive than Jupiter) and widely separated from their parent star. Most of them are also very hot, so that they emit intense infrared radiation; the images have then been made at infrared rather than visible wavelengths, to reduce the problem of glare from the parent star. An exception is the exoplanet Fomalhaut b, observed at visible wavelengths by the Hubble Space Telescope. That planet was found to be surprisingly bright in visible light, possibly because it is surrounded by a large disk of reflective material that may be a satellite system in the process of formation.
Though direct imaging may become more important in the future, the vast majority of known extrasolar planets have only been detected through indirect methods. The following are the indirect methods that have proven useful:
Most confirmed extrasolar planets have been found using ground-based telescopes. However, many of the methods can work more effectively with space-based telescopes that avoid atmospheric haze and turbulence. COROT (launched December 2006) and Kepler (launched March 2009) are the two currently active space missions dedicated to searching for extrasolar planets. Hubble Space Telescope and MOST have also found or confirmed a few planets. The Gaia mission, to be launched in March 2013, will use astrometry to determine the true masses of 1000 nearby exoplanets.
The official definition of "planet" used by the International Astronomical Union (IAU) only covers the Solar System and thus does not apply to exoplanets.[47][48] As of April 2011, the only definitional statement issued by the IAU that pertains to exoplanets is a working definition issued in 2001 and modified in 2003.[49] That definition contains the following criteria:
- Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our solar system.
- Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located.
- Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).
This article follows the above working definition. Therefore it only discusses planets that orbit stars or brown dwarfs. (There have also been several reported detections of planetary-mass objects that do not orbit any parent body.[50] Some of these may have once belonged to a star's planetary system before being ejected from it; the term "rogue planet" is sometimes applied to such objects.)
However, it should be noted that the IAU's working definition is not universally accepted. One alternate suggestion is that planets should be distinguished from brown dwarfs on the basis of formation. It is widely believed that giant planets form through core accretion, and that process may sometimes produce planets with masses above the deuterium fusion threshold;[51][52] massive planets of that sort may have already been observed.[53] This viewpoint also admits the possibility of sub-brown dwarfs, which have planetary masses but form like stars from the direct collapse of clouds of gas.
Also, the 13 Jupiter-mass cutoff does not have precise physical significance. Deuterium fusion can occur in some objects with mass below that cutoff. The amount of deuterium fused depends to some extent on the composition of the object.[54] The Extrasolar Planets Encyclopaedia includes objects up to 25 Jupiter masses, saying: "The fact that there is no special feature around 13 MJup in the observed mass spectrum reinforces the choice to forget this mass limit.",[55] and the Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity"[56]
The standard for naming exoplanets is an extension of the one used by the Washington Multiplicity Catalog (WMC) for multiple-star systems.[57] This section will therefore start by briefly discussing the WMC standard, which has been adopted by the International Astronomical Union.[57]
Under that standard, the brightest member of a system receives the letter "A." Distinct components not contained within "A" are labeled "B", "C", etc. Sub-components are designated by one or more suffixes with the primary label, starting with lowercase letters for the 2nd hierarchical level and then numbers for the 3rd.[58] For example, if there is a triple star system in which two stars orbit each other closely while a third star is in a more distant orbit, the two closely orbiting stars would be considered a component with two subcomponents. They would receive the designations Aa and Ab, while the third star would receive the designation B. (Note that, for historical reasons, this standard is not always strictly followed. For example, the three members of the Alpha Centauri triple star system are conventionally referred to as Alpha Centauri A, B and C while the formal standard would give their designations as Alpha Centauri Aa, Ab and B respectively.)
Following an extension of the above standard, an exoplanet's name is normally formed by taking the name of its parent star and adding a lowercase letter. The first planet discovered in a system is given the designation "b" and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size.
For instance, in the 55 Cancri system the first planet - 55 Cancri b - was discovered in 1996; two additional farther planets were simultaneously discovered in 2002 with the nearest to the star being named 55 Cancri c and the other 55 Cancri d; a fourth planet was claimed (its existence was later disputed) in 2004 and named 55 Cancri e despite lying closer to the star than 55 Cancri b; and the most recently discovered planet, in 2007, was named 55 Cancri f despite lying between 55 Cancri c and 55 Cancri d.[59] As of September 2011 the highest letter in use is "h", for the planet HD 10180 h.[1]
If a planet orbits one member of a binary star system, then an uppercase letter for the star will be followed by a lowercase letter for the planet. Examples are 16 Cygni Bb[60] and HD 178911 Bb.[61] Planets orbiting the primary or "A" star should have 'Ab' after the name of the system, as in HD 41004 Ab.[62] However, the "A" is sometimes omitted; for example the first planet discovered around the primary star of the Tau Boötis binary system is usually called simply Tau Boötis b.[63]
If the parent star is a single star, then it may still be regarded as having an "A" designation, though the "A" is not normally written. The first exoplanet found to be orbiting such a star could then be regarded as a secondary sub-component that should be given the suffix "Ab." For example, 51 Peg Aa is the host star in the system 51 Peg; and the first exoplanet is then 51 Peg Ab. Since most exoplanets are in single star systems, the implicit "A" designation was simply dropped, leaving the exoplanet name with the lower-case letter only: 51 Peg b.
A few exoplanets have been given names that do not conform to the above standard. For example, the planets that orbit the pulsar PSR 1257 are often referred to with capital rather than lowercase letters. Also, it should be noted that the underlying name of the star system itself can follow several different systems. In fact, some stars (such as Kepler-11) have only received their names due to their inclusion in planet-search programs.
Hessman et al. state that the implicit system for exoplanet names utterly failed with the discovery of circumbinary planets.[57] They note that the discovers of the two planets around HW Virginis tried to circumvent the naming problem by calling them "HW Vir 3" and "HW Vir 4", i.e. the latter is the 4th object – stellar or planetary – discovered in the system. They also note that the discovers of the two planets around NN Serpentis were confronted with multiple suggestions from various official sources and finally chose to use the designations "NN Ser c" and "NN Ser d."
The proposal of Hessman et al. starts with the following two rules:
They note that under these two proposed rules all of the present names for 99% of the planets around single stars are preserved as informal forms of the IAU sanctioned provisional standard. They would rename Tau Bootis b formally as Tau Boötis Ab, retaining the prior form as an informal usage (using Rule 2, above).
To deal with the difficulties relating to circumbinary planets, the proposal contains two further rules:
They submit that the new form using parentheses is the best for known circumbinary planets and has the desirable effect of giving these planets identical sub-level hierarchical labels and stellar component names which conform to the usage for binary stars. They say that it requires the complete renaming of only two exoplanetary systems: The planets around HW Virginis would be renamed HW Vir (AB) b & (AB) c, while those around NN Serpentis would be renamed NN Ser (AB) b & (AB) c. In addition the previously known single circumbinary planets around PSR B1620-26 and DP Leonis) can almost retain their names (PSR B1620-26 b and DP Leonis b) as unofficial informal forms of the "(AB)b" designation where the "(AB)" is left out.
The discoverers of the circumbinary planet around Kepler-16 followed Hessman et al.'s proposed naming scheme when naming the body Kepler-16 (AB)-b, or simply Kepler-16b when there is no ambiguity.[64]
Another nomenclature, often seen in science fiction, uses Roman numerals in the order of planets' positions from the star. (This was inspired by an old system for naming moons of the outer planets, such as "Jupiter IV" for Callisto.) But such a system has proven impractical for scientific use. To use our solar system as an example, Jupiter would most likely be the first planet discovered, and Saturn the second; but, as the terrestrial planets would not be easily detected, Jupiter and Saturn would be called "Sol I" and "Sol II" in this nomenclature, but would need to be renamed "Sol V" and "Sol VI" when the four terrestrial planets (Mercury, Venus, Earth, Mars) were discovered later. In contrast, under the current system, when the terrestrial planets were found, Jupiter and Saturn would remain "Sol b" and "Sol c" and not need renaming.
Finally, several planets have received unofficial names comparable to those of planets in the Solar System: notably Osiris (HD 209458 b), Bellerophon (51 Pegasi b), and Methuselah (PSR B1620-26 b). W Lyra of the Max Planck Institute for Astronomy has suggested names mostly drawn from Roman-Greek mythology for the 403 extrasolar planet candidates known as of October 2009.[65] But the International Astronomical Union (IAU) currently has no plans to assign names of this sort to extrasolar planets, considering it impractical.[66]
Planet-search programs have discovered planets orbiting a substantial fraction of the stars they have looked at. However the overall proportion of stars with planets is uncertain because not all planets can yet be detected. The radial-velocity method and the transit method (which between them are responsible for the vast majority of detections) are most sensitive to large planets in small orbits. Thus many known exoplanets are "hot Jupiters": planets of Jovian mass or larger in very small orbits with periods of only a few days. It is now estimated that 1% to 1.5% of sunlike stars possess such a planet, where "sunlike star" refers to any main-sequence star of spectral classes F, G, or K without a close stellar companion.[67] It is further estimated that 3% to 4.5% of sunlike stars possess a giant planet with an orbital period of 100 days or less, where "giant planet" means a planet of at least 30 Earth masses.[68]
The proportion of stars with smaller or more distant planets is less certain. It is known that small planets (of roughly Earth-like mass or somewhat larger) are more common than giant planets. It also appears that there are more planets in large orbits than in small orbits. Based on this, it is estimated that perhaps 20% of sunlike stars have at least one giant planet while at least 40% may have planets of lower mass.[68][69][70][2]
Whatever the proportion of stars with planets, the total number of exoplanets must be very large. Since our own Milky Way Galaxy has at least 200 billion stars, it must also contain tens or hundreds of billions of planets.
Most known exoplanets orbit stars roughly similar to our own Sun, that is, main-sequence stars of spectral categories F, G, or K. One reason is that planet search programs have tended to concentrate on such stars. But in addition, statistical analysis indicates that lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to detect.[68][71] Observations using the Spitzer Space Telescope indicate that stars of spectral category O, which are much hotter than our Sun, produce a photo-evaporation effect that inhibits planetary formation.[72]
Ordinary stars are composed mainly of the light elements hydrogen and helium. They also contain a small proportion of heavier elements such as iron, and this fraction is referred to as a star's metallicity. Stars of higher metallicity are much more likely to have planets, and the planets they have tend to be more massive than those of lower-metallicity stars.[67] It has also been shown that stars with planets are more likely to be deficient in lithium.[73]
Many planetary systems are not as placid as the solar system, and have extreme orbital parameters and strongly interacting orbits, so that Kepler's laws do not hold in such systems.[74]
Most known extrasolar planet candidates have been discovered using indirect methods and therefore only some of their physical and orbital parameters can be determined. For example, out of the six independent parameters that define an orbit, the radial-velocity method can determine four: semi-major axis, eccentricity, longitude of periastron, and time of periastron. Two parameters remain unknown: inclination and longitude of the ascending node.
Many exoplanets have orbits with very small semi-major axes, and are thus much closer to their parent star than any planet in our own solar system is to the Sun. This is mainly due to observational selection: the radial-velocity method is most sensitive to planets with small orbits. Astronomers were initially very surprised by these "hot Jupiters", but it is now clear that most exoplanets have much larger orbits, some located in habitable zones with temperature potentially suitable for liquid water and life.[68] It appears plausible that in most exoplanetary systems, there are one or two giant planets with orbits comparable in size to those of Jupiter and Saturn in our own solar system. Giant planets with substantially larger orbits are now known to be rare, at least around Sun-like stars.[75]
The eccentricity of an orbit is a measure of how elliptical (elongated) it is. Most exoplanets with orbital periods of 20 days or less have near-circular orbits, i.e. very low eccentricity. That is believed to be due to tidal circularization: reduction of eccentricity over time due to gravitational interaction between two bodies. By contrast, most known exoplanets with longer orbital periods have quite eccentric orbits. (As of July 2010, 55% of such exoplanets have eccentricities greater than 0.2 while 17% have eccentricities greater than 0.5.[1]) This is not an observational selection effect, since a planet can be detected about equally well regardless of the eccentricity of its orbit. The prevalence of elliptical orbits is a major puzzle, since current theories of planetary formation strongly suggest planets should form with circular (that is, non-eccentric) orbits.[24] The prevalence of eccentric orbits may also indicate that our own solar system is unusual, since all of its planets except for Mercury have near-circular orbits.[67]
However, it is suggested that some of the high eccentricity values reported for exoplanets may be overestimates, since simulations show that many observations are also consistent with two planets on circular orbits. Reported observations of single planets in moderately eccentric orbits have about a 15% chance of being a pair of planets.[76] This misinterpretation is especially likely if the two planets orbit with a 2:1 resonance. One group of astronomers has concluded that "(1) around 35% of the published eccentric one-planet solutions are statistically indistinguishable from planetary systems in 2:1 orbital resonance, (2) another 40% cannot be statistically distinguished from a circular orbital solution" and "(3) planets with masses comparable to Earth could be hidden in known orbital solutions of eccentric super-Earths and Neptune mass planets."[77]
A combination of astrometric and radial velocity measurements has shown that some planetary systems contain planets whose orbital planes are significantly tilted relative to each other, unlike our own Solar System.[78] Research has now also shown that more than half of hot Jupiters have orbital planes substantially misaligned with their parent star's rotation. A substantial fraction even have retrograde orbits, meaning that they orbit in the opposite direction from the star's rotation.[79] Andrew Cameron of the University of St Andrews stated, "The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin."[80] Rather than a planet's orbit having been disturbed, it may be that the star itself flipped early in their system's formation due to interactions between the star's magnetic field and the planet-forming disc.[81]
A system has been discovered in which two planets may share the same orbit (but later data revision indicates they might be in a 2:1 resonance, not in the same orbit). Such co-orbital planets are thought to be the origin of the impact that produced the Earth-Moon system because models suggest the collision was low-speed.[82] Another system has been discovered in which a planet orbits around two suns, which orbit around each other. The planet is comparable to Saturn in mass and size and is on a nearly circular 229-day orbit around its two stars. The stars have an eccentric 41-day orbit.[33]
When a planet is found by the radial-velocity method, its orbital inclination i is unknown and can range from 0 to 90 degrees. The method is unable to determine the true mass (M) of the planet, but rather gives a lower limit for its mass M sini. In a few cases an apparent exoplanet may be a more massive object such as a brown dwarf or red dwarf. However the probability of a small value of i (say less than 30 degrees, which would give a true mass at least double the observed lower limit) is relatively low (1-(√3)/2 ≈ 13%) and hence most planets will have true masses fairly close to the observed lower limit.[68] Furthermore, if the planet's orbit is nearly perpendicular to the line of vision (i.e. i close to 90°), the planet can also be detected through the transit method. The inclination will then be known, and the planet's true mass can be found. Also, astrometric observations and dynamical considerations in multiple-planet systems can sometimes provide an upper limit to the planet's true mass.
As of September 2011, all but 50 of the many known exoplanets have more than ten times the mass of Earth.[1] Many are considerably more massive than Jupiter, the most massive planet in the Solar System. However, these high masses are in large part due to an observational selection effect: all detection methods are more likely to discover massive planets. This bias makes statistical analysis difficult, but it appears that lower-mass planets are actually more common than higher-mass ones, at least within a broad mass range that includes all giant planets. In addition, the discovery of several planets only a few times more massive than Earth, despite the great difficulty of detecting them, indicates that such planets are fairly common.[67]
The results from the first 43 days of the Kepler mission "imply that small candidate planets with periods less than 30 days are much more common than large candidate planets with periods less than 30 days and that the ground-based discoveries are sampling the large-size tail of the size distribution".[5]
One can estimate the temperature of an exoplanet based on the intensity of the light it receives from its parent star. For example, the planet OGLE-2005-BLG-390Lb is estimated to have a surface temperature of roughly -220°C (roughly 50 K). However, such estimates may be substantially in error because they depend on the planet's usually unknown albedo, and because factors such as the greenhouse effect may introduce unknown complications. A few planets have had their temperature measured by observing the variation in infrared radiation as the planet moves around in its orbit and is eclipsed by its parent star. For example, the planet HD 189733b has been found to have an average temperature of 1205±9 K (932±9°C) on its dayside and 973±33 K (700±33°C) on its nightside.[83]
If a planet is detectable by both the radial-velocity and the transit methods, then both its true mass and its radius can be found. The planet's density can then be calculated. Planets with low density are inferred to be composed mainly of hydrogen and helium, while planets of intermediate density are inferred to have water as a major constituent. A planet of high density is believed to be rocky, like Earth and the other terrestrial planets of the Solar System.
Spectroscopic measurements can be used to study a transiting planet's atmospheric composition.[84] Water vapor, sodium vapor, methane, and carbon dioxide have been detected in the atmospheres of various exoplanets in this way. The technique might conceivably discover atmospheric characteristics that suggest the presence of life on an exoplanet, but no such discovery has yet been made.
Another line of information about exoplanetary atmospheres comes from observations of orbital phase functions. Extrasolar planets have phases similar to the phases of the Moon. By observing the exact variation of brightness with phase, astronomers can calculate particle sizes in the atmospheres of planets.
Stellar light is polarized by atmospheric molecules; this could be detected with a polarimeter. So far, one planet has been studied by polarimetry.
Many unanswered questions remain about the properties of exoplanets. One puzzle is that many transiting exoplanets are much larger than expected given their mass, meaning that they have surprisingly low density. Several theories have been proposed to explain this observation, but none have yet been widely accepted among astronomers.[85] Another question is how likely exoplanets are to possess moons and possibly magnetospheres. No such moons and magnetospheres have yet been detected, but they may be fairly common.
Perhaps the most interesting question about exoplanets is whether they might support life. Several planets do have orbits in their parent star's habitable zone, where it should be possible for liquid water to exist and for Earth-like conditions to prevail. Most of those planets are giant planets more similar to Jupiter than to Earth; if any of them have large moons, the moons might be a more plausible abode of life. Discovery of Gliese 581 g, thought to be a rocky planet orbiting in the middle of its star's habitable zone, was claimed in September 2010 and, if confirmed,[86] it could be the most "Earth-like" extrasolar planet discovered to date.[87] But the existence of Gliese 581 g has been questioned or even discarded by other teams of astronomers; it is listed as unconfirmed at The Extrasolar Planets Encyclopaedia.[86] Subsequently, though, the super-earth Kepler-22b was confirmed to be in the habitable zone of its parent star, Kepler-22, the first planet of its size confirmed to be in this zone.[88]
Various estimates have been made as to how many planets might support simple life or even intelligent life. For example, Dr. Alan Boss of the Carnegie Institution of Science estimates there may be a "hundred billion" terrestrial planets in our Milky Way Galaxy, many with simple life forms. He further believes there could be thousands of civilizations in our galaxy. Recent work by Duncan Forgan of Edinburgh University has also tried to estimate the number of intelligent civilizations in our galaxy. The research suggested there could be thousands of them.[89] Apart from the scenario of an extraterrestrial civilization that is emitting powerful signals, the detection of life at interstellar distances is a tremendously challenging technical task that may not be feasible for many years, even if such life is commonplace.
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