Exoplanet

An exoplanet (UK: /ˈɛk.soʊˌplæn.ɪt/, US: /ˌɛk.soʊˈplæn.ɪt/)[4] or extrasolar planet is a planet outside the Sun's Solar System. The first evidence of an exoplanet was noted in 1917, but was not recognized as such.[5] The first scientific detection of an exoplanet was in 1988; it was confirmed to be an exoplanet in 2012. The first confirmed detection occurred in 1992. As of 1 January 2019, there are 3,946 confirmed planets in 2,945 systems, with 650 systems having more than one planet.[6]

There are many methods of detecting exoplanets. The High Accuracy Radial Velocity Planet Searcher (HARPS) has discovered about a hundred exoplanets since 2004, while the Kepler space telescope, since 2009, has found more than two thousand. Kepler has also detected a few thousand[7][8] candidate planets,[9][10] of which up to 40% may be false positives.[11] In several cases, multiple planets have been observed around a star.[12] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][13][14] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[15]

The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[16][17] about 30 times the mass of Jupiter, although according to some definitions of a planet, it is too massive to be a planet and may be a brown dwarf instead. There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit. Some are so far out that it is difficult to tell whether they are gravitationally bound to the star. Almost all of the planets detected so far are within the Milky Way. Nonetheless, evidence suggests that extragalactic planets, exoplanets further away in galaxies beyond the local Milky Way galaxy, may exist.[18][19] The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.[20]

The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[21]

Besides exoplanets, there are also rogue planets, which do not orbit any star. These tend to be considered separately, especially if they are gas giants, in which case they are often counted as sub-brown dwarfs, like WISE 0855−0714.[22] The rogue planets in the Milky Way possibly number in the billions (or more).[23][24]

Planets everywhere (artist’s impression)
Artist's impression of how commonly planets orbit the stars in the Milky Way[1]
Histogram Chart of Discovered Exoplanets as of 2017-11-26
Discovered exoplanets each year as of 26 November 2017[2]
Exoplanet Comparison TrES-3 b
Size comparison of Jupiter and the exoplanet TrES-3b. TrES-3b has an orbital period of only 31 hours[3] and is classified as a Hot Jupiter for being large and close to its star, making it one of the easiest planets to detect by the transit method.
Distribution of exoplanets by distance
NASA histogram chart of confirmed exoplanets by distance

Nomenclature

The unusual exoplanet HIP 65426b — SPHERE's firs
Exoplanet HIP 65426b is the first discovered planet around star HIP 65426.[25]

The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the designation is normally formed by taking the name or, more commonly, designation of its parent star and adding a lower case letter.[26] The first planet discovered in a system is given the designation "b" (the parent star is considered to be "a") 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. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.

History of detection

For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed,[27] but there was no way of detecting them or of knowing their frequency or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers. The first evidence of an exoplanet was noted as early as 1917, but was not recognized as such.[5] The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[28] The first confirmation 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. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[29]

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 (1584)[30]

In the sixteenth century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.

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 centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[31]

In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[32]

Discredited claims

Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen 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.[33] 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.[34] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[35] 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.[36] Astronomers now generally regard all the early reports of detection as erroneous.[37]

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.[38] The claim briefly received intense attention, but Lyne and his team soon retracted it.[39]

Confirmed discoveries

444226main exoplanet20100414-a-full
The three known planets of the star HR8799, as imaged by the Hale Telescope. The light from the central star was blanked out by a vector vortex coronagraph.
Brown dwarf 2M J044144 and planet
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.

As of 1 January 2019, a total of 3,946 confirmed exoplanets are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late 1980s.[6] 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 of the University of Victoria and the University of British Columbia.[40] 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,[41] but subsequent work in 1992 again raised serious doubts.[42] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[43]

The Star AB Pictoris and its Companion - Phot-14d-05-normal
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 16 March 2003 with NACO on the VLT, using a 1.4 arcsec occulting mask on top of AB Pictoris.

On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[28] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[44] These pulsar planets are thought 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 somehow survived the supernova and then decayed into their current orbits.

On 6 October 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.[45][46] 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 host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited 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", because 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 make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[47] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[48]

On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[49][50][51] Prior to these results, most confirmed planets were gas giants comparable in size to Jupiter or larger as they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[49]

On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[52]

On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the Virgo constellation.[53] This exoplanet “Wolf 503b” is twice the size of earth and was discovered orbiting a type of star known as an “Orange Dwarf”. Wolf 503b completes one orbit in as fast as six days due to its close proximity to the star. It is also orbiting especially close to its host star which gravitational pull is causing it to complete an orbit so swiftly. This exoplanet is relatively close to Earth and its host star shines extremely bright. Wolf 503b is the only exoplanet that can be found near this Fulton Gap and that is this large of size. Scientist have noticed a large gap known as the Fulton Gap, where rare large planet sizes are discovered.[54][53]

Astronomers who study exoplanets have found thousands of exoplanets in our galaxy. Wolf 503b is so important because of how close it is to Earth, giving it convenient accessibility for extended studies through the Kepler space Telescope. The "orange dwarf” star that Wolf 503b is orbiting is a bright star. Scientist state that orange dwarf stars have a lifespan of three times longer than the Sun. Wolf 503b has a strong influence on its orange dwarf host star. Due to Wolf 503b’s large size, it has a gravitational influence on its host star. Under the Fulton Gap studies, this opens up a new fields for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[53]

Candidate discoveries

As of June 2017, NASA's Kepler mission had identified more than 5,000 planetary candidates,[55] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[7][8][56]

ExoplanetPopulations-20170616
Exoplanet populations
SmallPlanetsComeInTwoSizes-20170619
Small planets come in two sizes
KeplerHabitableZonePlanets-20170616
Kepler habitable zone planets

Methodology

ALMA Discovers Trio of Infant Planets ALMA Discovers Trio of Infant Planets
Measuring the flow of gas within a protoplanetary disc allows the detection of exoplanets.[59]

About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[60]

Formation and evolution

Planets may form within a few to tens (or more) of millions of years of their star forming.[61][62][63][64][65] The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming [66] to planetary systems of over 10 Gyr old.[67] When planets form in a gaseous protoplanetary disk,[68] they have hydrogen envelopes that cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen is eventually lost to space. This means that even terrestrial planets may start off with large radii if they form early enough.[69][70][71] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[72]

Eccentricity

Of the many exoplanets discovered, most have a higher orbital eccentricity than planets in the Solar System. Exoplanets found with low orbital eccentricity, near circular orbits, are almost all very close to their star and are tidally locked to the star. In contrast, seven out of eight planets in the Solar System have near-circular orbits. The exoplanets discovered show that the Solar System, with its unusually low eccentricity, is rare.[73] One theory attributes this low eccentricity to the high number of planets in the Solar System; another suggests it arose because of its unique asteroid belts. A few other multiplanetary systems have been found, but none resemble the Solar System. The Solar System has unique planetesimal systems, which led the planets to have near-circular orbits. The exoplanet systems discovered have either no planetesimal systems or one very large one. Low eccentricity is needed for habitability, especially advanced life.[74] High multiplicity planet systems are much more likely to have habitable exoplanets.[75][76]

Planet-hosting stars

Morgan-Keenan spectral classification
The Morgan-Keenan spectral classification
Artist’s impression of exoplanet orbiting two stars
Artist’s impression of exoplanet orbiting two stars.[77]

There is at least one planet on average per star.[12] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[78]

Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[79][80] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets.

Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star host planets. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[81]

Some planets orbit one member of a binary star system,[82] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[83] and one in the quadruple system Kepler-64.

General features

Color and brightness

Color HD 189733b vs solar system
This color–color diagram compares the colors of planets in the Solar System to exoplanet HD 189733b. The exoplanet's deep blue color is produced by silicate droplets, which scatter blue light in its atmosphere.

In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[84][85] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[86] and Kappa Andromedae b, which if seen up close would appear reddish in color.[87]

The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[88]

The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[89][90][91]

For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[92]

There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[92]

Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[92]

Magnetic field

In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[93][94]

Interaction between a close-in planet's magnetic field and a star can produce spots on the star in a similar way to how the Galilean moons produce aurorae on Jupiter.[95] Auroral radio emissions could be detected with radio telescopes such as LOFAR.[96][97] The radio emissions could enable determination of the rotation rate of a planet which is difficult to detect otherwise.[98]

Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[99][100]

Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[101]

In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[102][103]

Plate tectonics

In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[104][105] with one team saying that plate tectonics would be episodic or stagnant[106] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[107]

If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[108][109]

Volcanism

Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[110][111]

Rings

The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[112][113]

The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[114]

The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[115]

Moons

In December 2013 a candidate exomoon of a rogue planet was announced.[116] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[117]

Atmospheres

Cloudy versus clear atmospheres on two exoplanets
Clear versus cloudy atmospheres on two exoplanets.[118]

Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[119]

KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[120] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[121]

In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14×106 km (9×106 mi) long.[122]

In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[123][124] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.

Insolation pattern

Tidally locked planets in a 1:1 spin–orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[125] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[126] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[127]

As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[60] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[60] For example, molecular oxygen (O
2
) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means.[128] Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.[129][130]

See also

Notes

  1. ^ a b For the purpose of this 1 in 5 statistic, "Sun-like" means G-type star. Data for Sun-like stars was not available so this statistic is an extrapolation from data about K-type stars
  2. ^ a b For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii
  3. ^ For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).
  4. ^ About 1/4 of stars are GK Sun-like stars. The number of stars in the galaxy is not accurately known, but assuming 200 billion stars in total, the Milky Way would have about 50 billion Sun-like (GK) stars, of which about 1 in 5 (22%) or 11 billion would be Earth-sized in the habitable zone. Including red dwarfs would increase this to 40 billion.

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Further reading

  • Boss, Alan (2009). The Crowded Universe: The Search for Living Planets. Basic Books. ISBN 978-0-465-00936-7 (Hardback); ISBN 978-0-465-02039-3 (Paperback).
  • Dorminey, Bruce (2001). Distant Wanderers. Springer-Verlag. ISBN 978-0-387-95074-7 (Hardback); ISBN 978-1-4419-2872-6 (Paperback).
  • Jayawardhana, Ray (2011). Strange New Worlds: The Search for Alien Planets and Life beyond Our Solar System. Princeton, NJ: Princeton University Press. ISBN 978-0-691-14254-8 (Hardcover).
  • Perryman, Michael (2011). The Exoplanet Handbook. Cambridge University Press. ISBN 978-0-521-76559-6.
  • Seager, Sara, ed. (2011). Exoplanets. University of Arizona Press. ISBN 978-0-8165-2945-2.
  • Villard, Ray; Cook, Lynette R. (2005). Infinite Worlds: An Illustrated Voyage to Planets Beyond Our Sun. University of California Press. ISBN 978-0-520-23710-0.
  • Yaqoob, Tahir (2011). Exoplanets and Alien Solar Systems. New Earth Labs (Education and Outreach). ISBN 978-0-9741689-2-0 (Paperback).
  • van Dishoeck, Ewine F.; Bergin, Edwin A.; Lis, Dariusz C.; Lunine, Jonathan I. (2014). "Water: From Clouds to Planets". Protostars and Planets VI. Protostars and Planets Vi. p. 835. arXiv:1401.8103. Bibcode:2014prpl.conf..835V. doi:10.2458/azu_uapress_9780816531240-ch036. ISBN 978-0-8165-3124-0.

External links

ARIEL

The Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL), is a space observatory planned for launch in 2028 as the fourth medium-class mission of the European Space Agency's Cosmic Vision programme. The mission is aimed at observing at least 1,000 known exoplanets using the transit method, studying and characterising the planets' chemical composition and thermal structures.

Carbon planet

A carbon planet is a theoretical type of planet that contains more carbon (Z = 6) than oxygen (Z = 8). Carbon is the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen.

Marc Kuchner and Sara Seager coined the term "carbon planet" in 2005 and investigated such planets following the suggestion of Katharina Lodders that Jupiter formed from a carbon-rich core.

Prior investigations of planets with high carbon-to-oxygen ratios include Fegley & Cameron 1987. Carbon planets could form if protoplanetary discs are carbon-rich and oxygen-poor. They would develop differently from Earth, Mars, and Venus, which are composed mostly of silicon–oxygen compounds. The theory is now built on reasonable scientific ideas and has gained support. Different planetary systems have different carbon-to-oxygen ratios, with the Solar System's terrestrial planets closer to being "oxygen planets". The exoplanet 55 Cancri e is a possible example of a carbon planet.

EChO

The Exoplanet Characterisation Observatory (EChO) was a proposed space telescope as part of the Cosmic Vision roadmap of the European Space Agency, and competed with four other missions for the M3 slot in the programme. On 19 February 2014 the PLATO mission was selected in place of the other candidates in the programme, including EChO.EChO would be the first dedicated mission to investigate exoplanetary atmospheres, addressing the suitability of those planets for life and placing the Solar System in context. EChO is intended to provide high resolution, multi-wavelength spectroscopic observations. It would measure the atmospheric composition, temperature and albedo of a representative sample of known exoplanets, constrain models of their internal structure and improve our understanding of how planets form and evolve. It will orbit around the L2 Lagrange point, 1.5 million km from Earth in the anti-sunward direction.

Exoplanet Data Explorer

The Exoplanet Data Explorer / Exoplanet Orbit Database lists extrasolar planets up to 24 Jupiter masses.

"We have retained the generous upper mass limit of 24 Jupiter masses in our definition of a “planet”, for the same reasons as in the Catalog: at the moment, any mass limit is arbitrary and will serve little practical function both because of the sin i ambiguity in radial velocity masses and because of the lack of physical motivation.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. A useful theoretical and rhetorical distinction is to segregate brown dwarfs from planets by their formation mechanism, but such a distinction is of little utility observationally."

Extragalactic planet

An extragalactic planet, also known as an extragalactic exoplanet, is a star-bound planet, or rogue planet, located outside of the Milky Way galaxy. Due to the huge distances to such worlds, they would be very hard to detect directly. However, indirect evidence suggests that such planets may exist. Nonetheless, the most distant known planets are SWEEPS-11 and SWEEPS-04, located in Sagittarius, approximately 27,710 light-years from the Sun, while the Milky Way is between 100,000–180,000 light years in diameter. This means that even galactic planets located farther than that distance have not been detected.

Extrasolar Planets Encyclopaedia

The Extrasolar Planets Encyclopaedia is an astronomy website, founded in Paris, France at the Meudon Observatory by Jean Schneider in February 1995, which maintains a database of all the currently known and candidate extrasolar planets, with individual pages for each planet and a full list interactive catalog spreadsheet. The main catalogue comprises databases of all of the currently confirmed extrasolar planets as well as a database of unconfirmed planet detections. The databases are frequently updated with new data from peer-reviewed publications and conferences.

In their respective pages, the Planets are listed along with their basic properties such as the year of planet’s discovery, mass, radius, orbital period, semi-major axis, eccentricity, inclination, longitude of periastron, time of periastron, maximum time variation, and time of transit, including all error range values.

The individual planet data pages also contain the data on the parent star such as Name, Distance (pc), Spectral Type, Effective Temperature, Apparent Magnitude V, Mass, Radius, Age, Right Asc. Coord., Decl. Coord. Even when they are known, not all of these figures are listed in the interactive spreadsheet catalog. And many missing planet figures that would simply require the application of Kepler's third law of motion are left blank. Most notably absent on all pages is the star's luminosity.

As of June 2011, the catalog aims to include objects up to 25 Jupiter masses, an increase on the previous inclusion criteria of 20 Jupiter masses.

Fast Infrared Exoplanet Spectroscopy Survey Explorer

Fast Infrared Exoplanet Spectroscopy Survey Explorer (FINESSE) is a NASA mission proposal for a space observatory operating in the Near-infrared spectrum for the Medium-Class Explorers program. The Principal Investigator is Mark Swain of the Jet Propulsion Laboratory in Pasadena, California.FINESSE was one of three Medium-Class Explorers (MIDEX) mission concepts that received $2 million to conduct a nine-month mission concept study in August 2017. The other two competing concepts are Arcus (an X-ray space observatory) and SPHEREx (a near-infrared space observatory). If selected, the mission would launch no earlier than 2022 and would last at least two years.

List of exoplanet extremes

The following are lists of extremes among the known exoplanets. The properties listed here are those for which values are known reliably.

List of exoplanet search projects

The following is a list of exoplanet search projects.

List of nearest exoplanets

There are 3,946 known exoplanets, or planets outside our solar system that orbit a star, as of January 1, 2019; only a small fraction of these are located in the vicinity of the Solar System. Within 10 parsecs (32.6 light-years), there are 56 exoplanets listed as confirmed by the NASA Exoplanet Archive. Among the over 400 known stars within 10 parsecs, 29 have been confirmed to have planetary systems; 51 stars in this range are visible to the naked eye, nine of which have planetary systems.

The first report of an exoplanet within this range was in 1998 for a planet orbiting around Gliese 876 (15.3 light-years (ly) away), and the latest as of 2017 is one around Ross 128 (11 ly). The closest exoplanet found is Proxima Centauri b, which was confirmed in 2016 to orbit Proxima Centauri, the closest star to our Solar System (4.25 ly). HD 219134 (21.6 ly) has six exoplanets, the highest number discovered for any star within this range. A planet around Fomalhaut (25 ly) was, in 2008, the first planet to be directly imaged.Most known nearby exoplanets orbit close to their star and have highly eccentric orbits. A majority are significantly larger than Earth, but a few have similar masses, including two planets (around YZ Ceti, 12 ly) which may be less massive than Earth. Several confirmed exoplanets are hypothesized to be potentially habitable, with Proxima Centauri b and three around Gliese 667 C (23.6 ly) considered the most likely candidates. The International Astronomical Union took a public survey in 2015 about renaming some known extrasolar bodies, including the planets around Epsilon Eridani (10.5 ly) and Fomalhaut.

List of potentially habitable exoplanets

This is a list of potentially habitable exoplanets and possible exoplanets. The list is based on estimates of habitability by the Habitable Exoplanets Catalog (HEC), and data from the NASA Exoplanet Archive. The HEC is maintained by the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo.Surface planetary habitability is thought to require orbiting at the right distance from the host star for liquid surface water to be present, in addition of various geophysical and geodynamical aspects, atmospheric density, radiation type and intensity, and the host star's plasma environment.

Lists of exoplanets

This is a list of exoplanets. as of 10 January 2019 there are 3,872 confirmed exoplanets. The majority of these planets were discovered by the Kepler spacecraft. In addition to the confirmed exoplanets, there are 2,425 potential exoplanets from its first mission that are yet to be confirmed, and 473 from its "Second Light" mission.For yearly lists on physical, orbital and other properties, as well as on discovery circumstances and other aspects, see § Specific exoplanet lists

Methods of detecting exoplanets

Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those reasons, very few of the extrasolar planets reported as of April 2014 have been observed directly, with even fewer being resolved from their host star.

Instead, astronomers have generally had to resort to indirect methods to detect extrasolar planets. As of 2016, several different indirect methods have yielded success.

NASA Exoplanet Archive

The NASA Exoplanet Archive is an online astronomical exoplanet catalog and data service that collects and serves public data that support the search for and characterization of extra-solar planets (exoplanets) and their host stars. It is part of the Infrared Processing and Analysis Center and is on the campus of the California Institute of Technology (Caltech) in Pasadena, CA. The archive is funded by NASA and was launched in early December 2011 by the NASA Exoplanet Science Institute as part of NASA's Exoplanet Exploration Program. In July 2017, the archive's collection of confirmed exoplanets surpassed 3,500.The archive's data include published light curves, images, spectra and parameters, and time-series data from surveys that aim to discover transiting exoplanets. The archive also develops Web-based tools and services to work with the data, particularly the display and analysis of transit data sets from the Kepler mission and COnvection ROtation and planetary Transits (CoRoT) mission, for which the Exoplanet Archive is the U.S. data portal. Other astronomical surveys and telescopes that have contributed data sets to the archive include SuperWASP, HATNet Project, XO, Trans-Atlantic Exoplanet Survey and KELT.

According to third-party web analytics provider SimilarWeb, the company's website has over 130,000 visits per month, as of January 2015.

NASA Star and Exoplanet Database

The NASA Star and Exoplanet Database (NStED) is an on-line astronomical stellar and exoplanet catalog and data service that collates and cross-correlates astronomical data and information on exoplanets and their host stars. NStED is dedicated to collecting and serving important public data sets involved in the search for and characterization of exoplanets and their host stars. The data include stellar parameters (such as positions, magnitudes, and temperatures), exoplanet parameters (such as masses and orbital parameters) and discovery/characterization data (such as published radial velocity curves, photometric light curves, images, and spectra).

The NStED collects and serves public data to support the search for and characterization of extra-solar planets (exoplanets) and their host stars. The data include published light curves, images, spectra and parameters, and time-series data from surveys that aim to discover transiting exoplanets. All data are validated by the NStED science staff and traced to their sources. NStED is the U.S. data portal for the CoRoT mission.

As of December 2011, SDtED is no longer in operation. Most data and services have been transferred to the NASA Exoplanet Archive.

Nexus for Exoplanet System Science

The Nexus for Exoplanet System Science (NExSS) initiative is a National Aeronautics and Space Administration (NASA) virtual institute designed to foster interdisciplinary collaboration in the search for life on exoplanets. Led by the Ames Research Center, the NASA Exoplanet Science Institute, and the Goddard Institute for Space Studies, NExSS will help organize the search for life on exoplanets from participating research teams and acquire new knowledge about exoplanets and extrasolar planetary systems.

PA-99-N2

PA-99-N2 is a microlensing event detected in the direction of the Andromeda Galaxy in 1999.

Sloan Digital Sky Survey

The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P. Sloan Foundation, which contributed significant funding.

Data collection began in 2000, and the final imaging data release covers over 35% of the sky, with photometric observations of around 500 million objects and spectra for more than 3 million objects. The main galaxy sample has a median redshift of z = 0.1; there are redshifts for luminous red galaxies as far as z = 0.7, and for quasars as far as z = 5; and the imaging survey has been involved in the detection of quasars beyond a redshift z = 6.

Data release 8 (DR8), released in January 2011, includes all photometric observations taken with the SDSS imaging camera, covering 14,555 square degrees on the sky (just over 35% of the full sky). Data release 9 (DR9), released to the public on 31 July 2012, includes the first results from the Baryon Oscillation Spectroscopic Survey (BOSS) spectrograph, including over 800,000 new spectra. Over 500,000 of the new spectra are of objects in the Universe 7 billion years ago (roughly half the age of the universe). Data release 10 (DR10), released to the public on 31 July 2013, includes all data from previous releases, plus the first results from the APO Galactic Evolution Experiment (APOGEE) spectrograph, including over 57,000 high-resolution infrared spectra of stars in the Milky Way. DR10 also includes over 670,000 new BOSS spectra of galaxies and quasars in the distant universe. The publicly available images from the survey were made between 1998 and 2009.

Transiting Exoplanet Survey Satellite

The Transiting Exoplanet Survey Satellite (TESS) is a space telescope for NASA's Explorers program, designed to search for exoplanets using the transit method in an area 400 times larger than that covered by the Kepler mission. It was launched on April 18, 2018 atop a Falcon 9 rocket. During its 2-year primary mission, it is expected to find more than 20,000 exoplanets, compared to about 3,800 exoplanets known when it launched. The first light image from TESS was taken on August 7, 2018, and released publicly on September 17, 2018.The primary mission objective for TESS is to survey the brightest stars near the Earth for transiting exoplanets over a two-year period. The TESS satellite uses an array of wide-field cameras to perform a survey of 85% of the sky. With TESS, it is possible to study the mass, size, density and orbit of a large cohort of small planets, including a sample of rocky planets in the habitable zones of their host stars. TESS will provide prime targets for further characterization by the James Webb Space Telescope, as well as other large ground-based and space-based telescopes of the future. While previous sky surveys with ground-based telescopes have mainly detected giant exoplanets, TESS will find a large number of small planets around the nearest stars in the sky. TESS records the nearest and brightest main sequence stars hosting transiting exoplanets, which are the most favorable targets for detailed investigations.TESS uses a novel highly-elliptical orbit around the Earth with an apogee approximately at the distance of the Moon and a perigee of 108,000 km. TESS orbits Earth twice during the time the Moon orbits once, a 2:1 resonance with the Moon. The orbit is expected to remain stable for a minimum of 10 years.

Led by the Massachusetts Institute of Technology with seed funding from Google, on April 5, 2013, it was announced that TESS, along with the Neutron Star Interior Composition Explorer (NICER), had been selected by NASA for launch.

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