Extrasolar planet
Planet Fomalhaut b (inset against Fomalhaut's interplanetary dust cloud) imaged by the Hubble Space Telescope's coronagraph (NASA photo)
The star HR 8799 (center blob) with infrared images of planets HR 8799d (bottom), HR 8799c (upper right), and HR 8799b (upper left)
2MASS J044144 is a brown dwarf with a companion about 5-10 times the mass of Jupiter. It is not clear whether this companion object is a sub-brown dwarf or a planet.
Coronagraphic image of AB Pictoris showing a companion (bottom left), which is either a brown dwarf or a massive planet. The data was obtained on March 16, 2003 with NACO on the VLT, using a 1.4 arcsec occulting mask on top of AB Pictoris.

An extrasolar planet, or exoplanet, is a planet outside the Solar System. As of November 17, 2011, 702 extrasolar planets (in 577 planetary systems and 82 multiple-planet systems) have been identified.[1]

A substantial fraction of stars have planetary systems - data from the HARPS mission indicates that this includes more than half of all Sun-like stars.[2] Data from the Kepler mission has been used to estimate that there are at least 50 billion planets in our own galaxy.[3] There also exist planetary-mass objects that orbit brown dwarfs and others that orbit the galaxy directly just as stars do, although it is unclear if either type should be labeled as a "planet".

Extrasolar planets became an object of investigation in the 19th century. Many supposed that they existed, but there was no way of knowing how common they are or how similar they are to the planets of our solar system. The first confirmed detection was in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[4] The first confirmed detection of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet, 51 Pegasi b, was found in a four-day orbit around the nearby G-type star 51 Pegasi. The frequency of detections has increased since then.[1]

Many exoplanets are detected through radial velocity observations and other indirect methods rather than sensor imaging.[1] Most are giant planets resembling Jupiter; this partly reflects a sampling bias, as more massive planets are easier to observe. Several relatively lightweight exoplanets, only a few times more massive than Earth, were detected and projections suggest that these outnumber giant planets.[5][6]

The discovery of extrasolar planets has intensified interest in the possibility of extraterrestrial life.[7] As of September 2011, possible candidates as potentially habitable exoplanets are Gliese 581 d[8][9] and HD 85512 b.[10]

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,[6][11] including 54 that may be in the habitable zone.[12][13][14] Six candidates in this zone were thought to be smaller than twice the size of Earth,[11] though a more recent study found that one of the candidates is likely much larger and hotter than first reported.[15]

History of detection

Early speculations

This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.

—Giordano Bruno[16]

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 for his ideas by the Roman Inquisition.[17]

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." [18]

Retracted discoveries

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.[19] 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.[20] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[21] 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.[22] Astronomers now generally regard all the early reports of detection as erroneous.[23]

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.[24] The claim briefly received intense attention, but Lyne and his team retracted it.[25]

Confirmed discoveries

The first published, confirmed discovery was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and S. Yang.[26] 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 were widely sceptical for several years about this and other similar observations. Some of the possible planets might instead have been brown dwarfs, intermediate in mass between planets and stars. The following year, however, additional observations were published that supported the existence of a planet orbiting Gamma Cephei,[27] though subsequent work in 1992 raised serious doubts.[28] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[29]

In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around another pulsar, PSR 1257+12.[4] 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 an ordinary main-sequence star (51 Pegasi).[30] 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.

As of November 17, 2011, 702 confirmed exoplanets (in 577 planetary systems and 82 multiple-planet systems) are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late 1980s.[1] The first system to have more than one planet detected was PSR 1257+12; the first confirmed to have multiple planets orbiting a main-sequence star was Upsilon Andromedae. Among the known exoplanets are four pulsar planets orbiting two separate pulsars. Infrared observations of circumstellar dust disks suggest millions of comets in several extrasolar systems. A system has been discovered in which a planet orbits around two suns, which orbit around each other.[31]

Detection methods

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.

Direct image of exoplanets around the star HR8799 using a vector vortex coronagraph on a 1.5m portion of the Hale telescope

For the above reasons, telescopes have directly imaged no more than about ten exoplanets. This has only been possible for planets that are especially large (usually much larger than Jupiter) and widely separated from their parent star. Most of the directly imaged planets have also been very hot, so that they emit intense infrared radiation; the images have then been made at infrared rather than visible wavelengths, in order to reduce the problem of glare from the parent star.

A team of researchers from NASA's Jet Propulsion Laboratory demonstrated a technique for blocking a star's light with a vector vortex coronagraph, thus enabling direct detections to be made more easily. The researchers are hopeful that many new planets may be imaged using this technique.[32][33] Another promising approach is nulling interferometry.[34]

At the moment, however, the vast majority of known extrasolar planets have only been detected through indirect methods. The following are the indirect methods that have proven useful:

As a planet orbits a star, the star also moves in its own small orbit around the system's center of mass. Variations in the star's radial velocity — that is, the speed with which it moves towards or away from Earth — can be detected from displacements in the star's spectral lines due to the Doppler effect. Extremely small radial-velocity variations can be observed, down to roughly 1 m/s. This has been by far the most productive method of discovering exoplanets. It has the advantage of being applicable to stars with a wide range of characteristics.
  • Transit method
If a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors. This has been the second most productive method of detection, though it suffers from a substantial rate of false positives and confirmation from another method is usually considered necessary.
  • Transit Timing Variation (TTV)
TTV is a variation on the transit method where the variations in transit of one planet can be used to detect another. The first planetary candidate found this way was exoplanet WASP-3c, using WASP-3b in the WASP-3 system by Rozhen Observatory, Jena Observatory, and Toruń Centre for Astronomy.[35] The new method can potentially detect Earth sized planets or exomoons.[35]
Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Planets orbiting the lensing star can cause detectable anomalies in the magnification as it varies over time. This method has resulted in only a few planetary detections, but it has the advantage of being especially sensitive to planets at large separations from their parent stars.
Astrometry consists of precisely measuring a star's position in the sky and observing the changes in that position over time. The motion of a star due to the gravitational influence of a planet may be observable. Because that motion is so small, however, this method has not yet been very productive at detecting exoplanets.
  • Pulsar timing
A pulsar (the small, ultradense remnant of a star that has exploded as a supernova) emits radio waves extremely regularly as it rotates. If planets orbit the pulsar, they will cause slight anomalies in the timing of its observed radio pulses. Four planets have been detected in this way, around two different pulsars. The first confirmed discovery of an extrasolar planet was made using this method.
  • Timing of eclipsing binaries
If a planet has a large orbit that carries it around both members of an eclipsing double star system, then the planet can be detected through small variations in the timing of the stars' eclipses of each other. As of December 2009, two planets have been found by this method.
Disks of space dust surround many stars, and this dust can be detected because it absorbs ordinary starlight and re-emits it as infrared radiation. Features in the disks may suggest the presence of planets.

Most extrasolar planet candidates were 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.

Definition

The official definition of "planet" used by the International Astronomical Union (IAU) only covers the Solar System and thus takes no stance on exoplanets.[36][37] 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.[38] This 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, sometimes called "rogue planets," that do not orbit any parent body.[39] Some of these may have once belonged to a star's planetary system before being ejected from it.)

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;[40][41] massive planets of that sort may have already been observed.[42] 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.

The 13 Jupiter-mass cutoff is a rule of thumb rather than something of precise physical significance. Larger objects will burn most of their deuterium and smaller ones will burn only a little, and the 13 MJ value is somewhere in between. The amount of deuterium burnt also depends not only on mass but on the composition of the planet — specifically, on the amount of helium and deuterium present in the object's interior, and on the amount of heavier elements in the atmosphere, which determines the opacity and, therefore, the radiative cooling rate.[43] 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.",[44] 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"[45]

Nomenclature

History

A history of the systems used in the scientific literature for naming exoplanets is set out by Hessman et al. (2010).[46] They first note that the naming of stars in multiple systems under the provisional working standard adopted during the XXIV General Assembly of the International Astronomical Union (IAU) is that of the Washington Mulitplicity Catalog (WMC), which is itself a refinement of the Washington Double Star Catalog.[47] The brightest component is called "A" with distinct components not contained within "A" 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. When it came to naming exoplanets the simplest solution was to name planets around single stars using a variation of this standard. If the host system is the "A" component, then the first exoplanet was considered to be the secondary sub-component and should have been 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. This meant that researchers from the exoplanetary community have adopted what Hessman et al. refer to as the "lower-case b" nomenclature, i.e. without the reference to the primary component.

Under the WMC[47] additional components are labeled with the next available letter in the alphabet. This has been followed by the extrasolar planet community, with the addition that if multiple planets are discovered at the same time, the closest one to the star gets the next letter, followed by the farther planets in order. 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.[48] As of September 2011 the highest letter in use is "h", for the planet HD 10180 h.[1]

Under the WMC[47] 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 for planets orbiting secondary or "B" stars are 16 Cygni Bb[49] and HD 178911 Bb,[50] and Hessman et al. note that all the planets around such secondary stars have to that date been correctly named. Planets orbiting the primary or "A" star should have 'Ab' after the name of the system, as in HD 41004 Ab.[51] However, they note that the name given to the first planet discovered around the primary star of the Tau Boötis binary system was Tau Boötis b.[52]

Hessman et al. state that the implicit system for exoplanet names utterly failed with the discovery of circumbinary planets. 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", i.e. implicitly "NN Ser Ac" and "NN Ser Ad" with the central very close binary system composed of "NN Ser Aa" and "NN Ser Ab". They (Hessman et al.) state that this solution conflicted with standard usage; the official alternatives being either to declare "NN Ser Aa+Ab" as one dynamical component with the exoplanets "NN Ser B" and "NN Ser C" orbiting around it, or to adopt the standard usage of "NN Ser A" and "NN Ser B" for the close binary stars, leaving the planets as "NN Ser C" and "NN Ser D".

2010 Proposal

Hessman et al.'s proposed extrasolar planet naming convention contains four rules, the first two of which are —

Rule 1. The formal name of an exoplanet is obtained by appending the appropriate suffixes to the formal name of the host star or stellar system. The upper hierarchy is defined by upper-case letters, followed by lower-case letters, followed by numbers, etc. The naming order within a hierarchical level is for the order of discovery only. (This rule corresponds to the present provisional WMC naming convention.)
Rule 2. Whenever the leading capital letter designation is missing, this is interpreted as being an informal form with an implicit "A" unless otherwise explicitly stated. (This rule corresponds to the present exoplanet community usage for planets around single stars.)

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 a further rule: Rule 3. As an alternative to the nomenclature standard in Rule 1, a hierarchical relationship can be expressed by concatenating the names of the higher order system and placing them in parentheses, after which the suffix for a lower order system is added. This rule, they state, permits keeping the lower-case b notation even when the previous hierarchical naming would suggest the use of a different suffix. They give as a fictitious example an exoplanet in a circumbinary orbit around the close binary system CT Men. In principle, the exoplanet could be named with any of the following conventions: CT Men B, the 'second' part of the system otherwise consisting of the two stars CT Men Aa+Ab but potentially containing another stellar system CT MenC with a totally different dynamical status; CT Men C, the third body in the system otherwise consisting of the two stars CT MenA+B, placing the circumbinary exoplanet on the same hierarchy as the two stars it orbits; or CT Men (AB)b, the 'second' dynamical part of the system otherwise consisting of the two stars CT MenA+B.

To decide which form to choose they propose: Rule 4. When in doubt (i.e. if a different name has not been clearly set in the literature), the hierarchy expressed by the nomenclature should correspond to dynamically distinct (sub-)systems in order of their dynamical relevance. The choice of hierarchical levels should be made to emphasize dynamical relationships, if known.

They submit that the new form using parentheses is the best for known circumbinary planets and has the nice side-effect of giving these kinds of planets identical sub-level hierarchical labels and stellar component names which conform to the usage within the very close binary community. 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, and 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 specifically 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.[53]

Other naming systems

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). The International Astronomical Union (IAU) currently has no plans to assign names of this sort to extrasolar planets, considering it impractical.[54] However, W Lyra of the Max Planck Institute for Astronomy considered this logic flawed, and suggested names mostly drawn from Roman-Greek mythology for the 403 extrasolar planet candidates known at October 2009.[55]

General properties

Number of stars with planets

Most of the discovered extrasolar planets lie within 300 light years of the Solar System.

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 roughly Jupiter-like mass 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.[56] 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.[57]

The proportion of stars with smaller or more distant planets remains difficult to estimate. Extrapolation suggests that small planets (of roughly Earth-like mass) 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.[57][58][59]

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 billions of planets, if not hundreds of billions of them.

Characteristics of planet-hosting stars

The Morgan-Keenan spectral classification

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.[57][60] Recent 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.[61]

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.[56] It has also been shown that stars with planets are more likely to be deficient in lithium.[62]

Orbital parameters

Scatterplot showing masses and orbital periods of all extrasolar planets discovered through 2010-10-03, with colors indicating method of detection:
  transit
  timing
  direct imaging
  pulsar timing
For reference, Solar System planets are marked as gray circles. The horizontal axis plots the log of the mass, while the vertical axis plots the log of the semi-major axis.

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 (or, at least, most high-mass exoplanets) have much larger orbits, some located in habitable zones with temperature potentially suitable for liquid water and life.[57] 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.[63]

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.[23] The prevalence of eccentric orbits may also indicate that our own solar system is somewhat unusual, since all of its planets except for Mercury have near-circular orbits.[56] 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.[64] 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."[65]

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.[66] 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.[67] 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."[68] 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.[69]

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.[70]

And a 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.[31]

Mass distribution

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.[57] 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.[56]

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".[6]

Temperature and composition

Comparison of sizes of planets with different compositions

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.[71]

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.[72] 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.

Unanswered questions

Artist's impression of Upsilon Andromedae d, a gas giant planet that lies in the habitable zone: if it has large moons they may be able to support liquid water

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.[73] 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,[74] it could be the most "Earth-like" extrasolar planet discovered to date.[75] 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.[74]

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.[76] 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.

See also

Lists

Classifications

Systems

Habitability and life

Astronomers

  • Geoffrey Marcy – co-discoverer with R. Paul Butler and Debra Fischer of more exoplanets than anyone else
  • R. Paul Butler – co-discoverer with Geoffrey Marcy and Debra Fischer of more exoplanets than anyone else
  • Debra Fischer – co-discoverer with Geoffrey Marcy and R. Paul Butler of more exoplanets than anyone else
  • Aleksander Wolszczan – co-discoverer of PSR B1257+12 B and C, the first ever discovered exoplanets, with Dale Frail
  • Dale Frail – co-discoverer of PSR B1257+12 B and C, the first ever discovered exoplanets, with Aleksander Wolszczan
  • Michel Mayor – co-discoverer of 51 Pegasi b, the first ever discovered exoplanet orbiting a Sun-like star, with Didier Queloz
  • Didier Queloz – co-discoverer of 51 Pegasi b, the first ever discovered exoplanet orbiting a Sun-like star, with Michel Mayor
  • Stephane Udry – co-discoverer of Gliese 581 c, the most Earth-like planet
  • David Charbonneau − co-discoverer of HD 209458b, the first transiting exoplanet, and GJ 1214 b, a transiting super-Earth

Observatories and methods

Missions

Current

Under development

  • Gaia mission – launch in March 2013, not including a risk margin of six months.[77] An Astrometry mission that will also measure the orbits and inclinations of a thousand extrasolar planets accurately, determining their true masses.

Canceled

Proposed

  • PEGASE – for launch between 2010–2012
  • TESS – NASA studied but declined to select for flight. Private funding is now being sought for launch around 2013–2014[79]
  • PLATO – for launch in 2017
  • New Worlds Mission – for launch in 2019
  • EChO

Books

Websites

References

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