Exoplanet

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Size-exaggerated artist's conception showing the ratio of planets to stars in our galaxy
Artist's view gives an impression of how commonly planets orbit the stars in the Milky Way.[1]
Size comparison of Jupiter and the exoplanet TrES-3b
Size comparison of Jupiter and the exoplanet TrES-3b. TrES-3b has an orbital period of only 31 hours[2] 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.

An exoplanet (BrE /ˈek.səʊˌplæn.ɪt/ AmE /ˌek.soʊˈplæn.ɪt/)[3] or extrasolar planet is a planet that orbits a star other than the Sun. Starting in 1988, and as of 16 August 2016, there have been 3,501 exoplanets in 2,623 planetary systems and 592 multiple planetary systems confirmed.[4] HARPS (since 2004) has discovered about a hundred exoplanets while the Kepler space telescope (since 2009) has found more than two thousand. Kepler has also detected a few thousand[5][6] candidate planets,[7][8] of which about 11% may be false positives.[9] On average, there is at least one planet per star, with a percentage having multiple planets.[10] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c] Assuming there are 200 billion stars in the Milky Way,[d] one can hypothesize 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.[11]

The least massive planet known is Draugr (formally PSR B1257+12 A), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is DENIS-P J082303.1-491201 b,[12][13] about 29 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, but there have also been a few possible detections of extragalactic planets. The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri.[14]

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.[15]

Besides exoplanets, there are also rogue planets, which do not orbit any star and which tend to be considered separately, especially if they are gas giants, in which case they are often counted, like WISE 0855−0714, as sub-brown dwarfs.[16] The rogue planets in the Milky Way possibly number in the billions (or more).[17][18]

Nomenclature

The exoplanet naming convention is an extension of the system used for naming multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the name is normally formed by taking the name of its parent star and adding a lower case letter. 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 naming of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.

History of detection

For centuries philosophers and scientists suspected that extrasolar planets existed, 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 confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[19] 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.

Early speculations

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 centers of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[21]

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.[22]

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 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.[23] 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.[24] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[25] 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.[26] Astronomers now generally regard all the early reports of detection as erroneous.[27]

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.[28] The claim briefly received intense attention, but Lyne and his team soon retracted it.[29]

Confirmed discoveries

False-color, star-subtracted, direct image using a vortex coronagraph of 3 exoplanets around star HR8799
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.
Hubble image of brown dwarf 2MASS J044144 and its 5–10 Jupiter-mass companion, before and after star-subtraction
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 which is either a brown dwarf or a massive 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 16 March 2003 with NACO on the VLT, using a 1.4 arcsec occulting mask on top of AB Pictoris.

As of 16 August 2016, a total of 3,501 confirmed exoplanets are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late 1980s.[4] 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.[30] 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,[31] but subsequent work in 1992 again raised serious doubts.[32] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[33]

On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[19] 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.[34] 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.[35][36] 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 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", 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.[37] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[38]

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".[39][40][41] 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.[39]

On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[42]

Candidate discoveries

As of March 2014, NASA's Kepler mission had identified more than 2,900 planetary candidates, several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[5][6][43]

Plot of orbital period (in days) vs. radius relative to Earth's, as of January 2015 and 23 July 2015
Kepler mission – new exoplanet candidates – as of 23 July 2015.[44]

Methodology

The first exoplanet was detected on 6 October 1995, and was named 51 Pegasi b.[45] When an extrasolar planet is observed to transit their parent star, astronomers are able to assess some physical properties of the planet from an interstellar distance, including planetary mass and size, which in turn provide fundamental constraints on models of their physical structure.[46] Furthermore, such events afford the opportunity to study the dynamics and chemistry of its atmosphere.[46]

Statistical surveys and individual characterization are the keys to addressing the fundamental questions in exoplanetology.[47] As of August 2016, varying techniques have been used to discover 3,502 exoplanets.[48] Documenting the properties of a large sample exoplanets at various ages, orbiting their parent stars of various types, will contribute to increased understanding —or better models— of planetary formation (accretion), geological evolution, orbit migration,[47][49] and their potential habitability.[50] Characterizing the atmospheres of extrasolar planets is the new frontier in exoplanetary science.[51]

Detection techniques

About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques.[50] The following methods have proved successful for discovering a new planet or confirming an already discovered planet:[52]

  • Radial velocity
  • Gravitational microlensing
  • Direct imaging
  • Polarimetry
  • Astrometry
  • Transit photometry
    • Reflection/emission modulations
    • Light variations due to relativistic beaming
    • Light variations due to ellipsoidal variations
  • Timing variations
    • Pulsar timing
    • variable star timing
    • Transit timing variation method
    • Transit duration variation method
    • Eclipsing binary minima timing

Formation and evolution

Planets form within a few tens of millions of years of their star forming,[53][54][55] and there are stars that are forming today and other stars that are ten billion years old, so unlike the planets of the Solar System, which can only be observed as they are today, studying exoplanets allows the observation of exoplanets at different stages of evolution. When planets form 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 can start off with large radii.[56][57][58] 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.[59]

Planet-hosting stars

There is at least one planet on average per star.[10] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[60]

The Morgan-Keenan spectral classification system, showing size-and-color comparisons of M, K, G, F, A, B, and O stars
The Morgan-Keenan spectral classification

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.[61][62] 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.

Stars with a higher metallicity than the Sun are more likely to have planets, especially giant planets, than stars with lower metallicity.[63]

Some planets orbit one member of a binary star system,[64] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[65] and one in the quadruple system Kepler-64.

General features

Color and brightness

Color-color diagram comparing the colors of Solar System planets to exoplanet HD 189733b. HD 189733b reflects as much green as Mars and almost as much blue as Earth.
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.[66][67]

Visually, GJ 504 b would have a magenta color.[68]

Kappa Andromedae b if seen up close, would appear reddish in color.[69]

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.[70]

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.[71][72][73]

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.[74]

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.[74]

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.[74]

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.[75][76]

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.[77] Auroral radio emissions could be detected with radio telescopes such as LOFAR.[78][79] The radio emissions could enable determination of the rotation rate of a planet which is difficult to detect otherwise.[80]

Earth's magnetic field results from its flowing liquid metallic core, but in super-Earths the mass can produce high pressures with large viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Magnesium oxide, which is rocky on Earth, can 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.[81]

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.[82]

Plate tectonics

In 2007 two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[83][84] with one team saying that plate tectonics would be episodic or stagnant[85] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[86]

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.[87][88]

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.[89][90]

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.[91][92]

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.[93]

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.[94]

Moons

In December 2013 a candidate exomoon of a rogue planet was announced.[95] No exomoons have been confirmed so far.

Comet-like tails

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.[96] 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.[97]

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 14×10^6 km (9×10^6 mi) long.[98]

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.[99] 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.[100] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[101]

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.[50] 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).[50] 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.[102] 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.[103][104]

Cultural impact

On 9 May 2013, a congressional hearing by two United States House of Representatives subcommittees discussed "Exoplanet Discoveries: Have We Found Other Earths?", prompted by the discovery of exoplanet Kepler-62f, along with Kepler-62e and Kepler-62c. A related special issue of the journal Science, published earlier, described the discovery of the exoplanets.[104]

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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