Extrasolar planet (extrasolar planet or exoplanet)solar systemOutsideplanet。However, when we usually mention exoplanets, we pay more attention to those located outside the solar systemAround othersfixed starrevolutionOf the planet, less mentionedRogue planet And surroundingCompact starRevolving planets, etc.
For thousands of years, astronomers have been circulating the conjecture of exoplanets.In the early 1990s, this assumption became a reality. Swiss astronomers discovered the first exoplanet orbiting a sun like starPegasus 51b[1]。Since then, exoplanets have become a hot topic in astronomy. Since 2002, there have been dozens of new exoplanets discovered every year;stayKepler space telescope After the launch, the number increased rapidly.There are also many methods to detect exoplanets, such as radial velocity method, occultation method, direct imaging method, micro gravitational lens method, etc.As of November 16, 2020, we have found 3234Planetary system4374 exoplanets and 2550 exoplanet candidates from 2365 planetary systems.[2]
With the continuous discovery of exoplanets, we have also made assumptions and studies on their physical properties, composition, evolution mechanism, etc., and found some special objects, such asHot Jupiter，Hot Neptune ，Super EarthEtc.These discoveries have further deepened our understanding of the universe, provided us with an opportunity to study the evolution of planetary systems and hope to discover "Earth 2.0", and also made the discovery of extraterrestrial life possible.
International Astronomical UnionThe term "planet" defined by (IAU) in 2006 only includes the solar system, so it is not applicable to exoplanets.[3]However, the IAU also has a definition of exoplanets, which was promulgated in 2001 and revised in 2003.[4]It is described as follows:
Real quality is lower thandeuteriumLower limit of mass required for nuclear fusion (for similar sunMetal abundanceAnd its mass is 13 times that of Jupiter), and the celestial body that revolves around the star or stellar remains is called“planet”。Its lower mass/scale limit is the same as the lower mass/scale limit defined for planets used in the solar system.[4]
Substellar objects whose actual mass exceeds the lower limit of mass required for deuteron fusion are called“Brown dwarf”。[4]
Free floating objects in young star clusters whose mass is lower than the lower limit of mass required for deuteron fusion are not called "planets", but "sub brown dwarf" (or other more appropriate names).[4]
In fact, with the continuous discovery of new exoplanets, this definition also has its limitations.Some astronomers suggest that exoplanets should be distinguished from brown dwarfs and sub brown dwarfs according to the mechanism of planet formation.
On the one hand, deuteron fusionthresholdIt is not possible to accurately distinguish objects with similar mass (about several to dozens of times the mass of Jupiter) formed by different formation mechanisms.The accretion process in the process of planet formation will form a rocky core, so that the final giant planet may exceed the lower limit mass of deuteron fusion.[5]For example, the SOPHIE stepped grating spectrometer in France found an exoplanet with a mass of 14.3 times that of Jupiter in 2009.[6]There are sub brown dwarfs in the stars. Although they are formed by the direct collapse of nebula, their mass is within 13 times the mass of Jupiter.[7]Such objects were discovered as early as 1995.[8]
On the other hand, the threshold of deuteron fusion itself is a mass range, about 10-15 times the mass of Jupiter, and 13 times the mass of Jupiter is not an accurate value[9]。In the "Encyclopedia of Extrasolar Planets" database, we can see extrasolar planets with a mass of more than 15 Jupiter masses.[2]
Naming method
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Schematic diagram of exoplanet nomenclature[10]
The naming of exoplanets is based on the supplement of the Washington Multiparity Catalog (WMC) naming system and adopted by the International Astronomical Union.In the WMC naming system, the brightest star or the star system with the brightest star is named with the letter label "A", and other stars/star systems not included in the star system "A" are named with "B", "C", etc.Lower level star systems or stars are suffixed on the basis of one or more previous main labels, the second level is sequential lowercase letters, and the third level is sequential numbers.If there is a three-star system, in which two stars revolve closely around each other, and the system composed of these two stars is brighter than the third star, and the system revolves around the third star in a distant orbit, then these two stars that revolve closely will be named Aa and Ab, and the distant star will be named B.[10]However, due to historical reasons, we do not always follow this standard.
To expand the above criteria, we usually add lowercase letters after the name of the parent star to name exoplanets.The first planet found in the planetary system will be named "b" (the parent star is considered to be "a"), and the planets discovered later will be named in lowercase alphabetical order.If multiple planets are found in the same system at the same time, they will be named in lowercase alphabetical order according to the distance from the planet to the parent star.[10]Of course, there are still some special circumstances.
In addition, the International Astronomical Union (IAF) conducted a global collection of exoplanet naming in 2014 and 2019.The exoplanets on the shortlist will be given a new name in addition to the original number.[11-12]For example, HD 173416 b, the first extrasolar planet discovered by Chinese astronomers, won a beautiful name in 2019“Wangshu”。[13]
Discovery History
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Historical speculation
As early as in ancient Greece, some scholars put forward the hypothesis of exoplanets, but this concept andDemocritusThe atomic hypothesis is too advanced.
In the 16th century, Italian scientistsBrunoDevelopedCopernicusOfHeliocentric theory, onOn Infinity, the Universe and the WorldThe prediction of exoplanets is proposed in.He believed that the stars in the sky, like our sun, would be surrounded by planets, and these planets could also breed their own life.However,Geocentric theoryIt still maintains its inertia for thousands of years. This thought, like Copernicus' heliocentric theory, is regarded as heresy.[14]
In 1687,NewtonPublished his epoch-making works《Mathematical Principles of Natural Philosophy》, which mentioned the same possibility.By analogy with the planets around the sun, Newton's mind came up with a spectacular scene of planets around other stars similar to the solar system.Since then, the conjecture of exoplanets has been accepted by more and more people.[14]
Modern exploration
In 1855, William Stephen Jacob, an Indian born British astronomer, announced that a planet like object was suspected to have been found in the Ophiuchus 70 binary system.[15]Subsequently, American astronomer Thomas Jefferson Jackson See confirmed this discovery in 1896, and calculated the orbital period of this object to be 36 years.[16]But recently astronomers generally believe that this discovery is wrong.
In 1917, Adriaan van Maanen, a Dutch American astronomer, discovered a white dwarf with heavy element spectral lines whose spectrum was "polluted" and named itFan Ma Nanxing（van Maanen's Star）。He attributed the spectral pollution to a faint F-type star around the white dwarf.In fact, the latest theory believes that this pollution may come from exoplanets: the perturbation of exoplanets to asteroids may cause asteroids to crash into white dwarfs, thus causing spectral pollution.Therefore, this became the first indirect evidence of observing exoplanets in history.[17]
In 1953, the British astronomer Peter Berners Fellgett summarized the experience of predecessors and proposed a new spectrometer to improve the measurement accuracy of Doppler apparent velocity at that time.[18]In 1967, Roger F. Griffin, a British astronomer, put this idea into practice and successfully established an optical television directional velocity spectrometer,[19]Later, he improved with another Professor Rita E. Griffin, making the observation accuracy of the spectrometer better than 100m/s[20]That is to say, the speed change of 100m/s in the visual direction can be observed.Later, thanks to the efforts of astronomers, this figure has been further improved[21]And laid a solid foundation for the later discovery of exoplanets by using the radial velocity method.
The darkness before dawn.
In 1988, a Canadian exoplanet exploration team announced the discovery of the first exoplanet orbiting a sun like star.They observedShaowei increases eightIt is speculated that this change may be caused by a planet whose mass is 1-9 times that of Jupiter.[22]However, due to the low confidence of the observation data, the discovery was not widely recognized, and the honor of discovering the first exoplanet was also missed by the exoplanet exploration team.It was not until 2002 that astronomers confirmed the discovery of this exoplanet by using more accurate radial velocity measurements and named itShaowei Increases Eight Ab（Gamma Cephei Ab）。[23]
HD 114762 b Imagination
In 1989, the exoplanet exploration team led by American astronomer David W. Latham found a celestial body orbiting the star HD 114762, which was later namedHD 114762 b。They speculated that the lower limit of the mass of the object was 11 times the mass of Jupiter, and the orbital period was 84 days. It might be a brown dwarf or a gas giant.[24]This discovery was confirmed by other astronomers in 1991[25]At that time, the object was also regarded as the first exoplanet found orbiting a sun like star.Unfortunately, with the further study of HD 114762b, astronomers measured the mass of the object more accurately (more than 13.5 times the mass of Jupiter), and believed that it was not an exoplanet butBrown dwarf。[26]
The first discovery of exoplanets
Pulsar PSR B1257+12 and its exoplanet image
In 1992, two radio astronomers - the United StatesArecibo Observatory(Arecibo Observatory)Alexander Wolfson(Alexander Wolszczan) andNational Radio ObservatoryDale APSR B1257+12The two planets in motion are respectively namedPSR B1257+12 BandPSR B1257+12 C。According to their calculations, the mass of PSR B1257+12B is about 3.4 times that of the Earth, 0.36 astronomical units away from the parent star;The mass of PSR B1257+12 C is about 2.8 times that of the Earth, 0.47 astronomical units away from the parent star.[27]This is the first time that humans have clearly confirmed the discovery of extrasolar planets, and it is also the first discovery later known asSuper EarthThe celestial body of.
Imagination of Pegasus 51b
In 1995, an astronomer at the Geneva Observatory, SwitzerlandMichel Mayor(Michel Mayor) andDidier Queiroz(Didier Queloz) announced the discovery of the first exoplanet orbiting a sun like starPegasus 51b（51 Pegasi b）。Its parent star51 Pegasi It is 50.45 light years away from the earth, with spectral type G2IV.Using the radial velocity method, two astronomers calculated that Pegasus 51b is about 8 million kilometers away from the parent star, and its mass is about 0.5-2 times the mass of Jupiter.Compared with the solar system, this is equivalent to placing Jupiter within the orbit of Mercury.[28]Later, astronomers called this kind of objects with Jupiter's mass, orbits very close to the parent star, and high surface temperature asHot Jupiter。
Due to the epoch-making discovery of extrasolar planets orbiting sun like stars, Michelle Mayor and Didier Queiroz and cosmologists from Princeton University in the United StatesJames Peebles(James Peebles) shared the 2019 Nobel Prize in Physics.This discovery set off a revolution in astronomy, and the awarding words of the Nobel Prize also wrote: "Their discovery has forever changed our understanding of the world."[29]
Detection method
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Compared with the host star, exoplanets have very low luminosity, which is generally difficult to be detected directly and easily blocked by the light of the parent star.Therefore, astronomers have proposed various indirect exoplanet detection methods and achieved some success.Different detection methods also affect the observation results, resulting in certain selectivity and bias, that is, some types of exoplanets are easier to find.
Apparent velocity method
The Radial Velocities (RV) method usesDoppler effectThe method of detecting exoplanets by observing the periodic changes of stellar spectra caused by the rotation of planets and stars.stayKepler space telescope Before launch, this is the most effective way to identify exoplanets, the first exoplanet orbiting a sun like starPegasus 51bIt was discovered by this method.
In planetary systems, planets and stars move in circles around the common center of mass.This causes the stars to sometimes move towards us and sometimes away from us in the view of observers on Earth.Due to the Doppler effect of light, starsOpposing motionIts spectrum will occurblue shift；When the star moves away from each other, its spectrum will redshift.The periodic changes of stellar spectra also correspond to the orbital periods of exoplanets.According to the degree of red shift or blue shift of spectral lines, we can infer the change of the radial velocity of stars, so as to find exoplanets and estimate their mass.[30]When the mass of the planet is far less than that of the star, we can deduce the radial velocity
Is the planet orbit relative to the ecliptic planeRail inclination。[31]
Variation Diagram of Visual Velocity of Pegasus 51b in One Cycle[28]
The formula shows that the lower limit of the mass of exoplanets should be measured by the apparent velocity method
That is, because the angle between the orbital plane and the ecliptic plane of the exoplanets around the parent star, the measurement results are lower than the true results.Therefore, the radial velocity method is suitable for detecting exoplanets with large mass and small orbital period.Moreover, this method has nothing to do with the distance between the star and the earth, but in fact, the detection equipment is required to have a highSignal-to-noise ratio。[32]Otherwise, the spectral noise will submerge the weak signal of spectral displacement.Therefore, using the radial velocity method, we can usually only find small mass exoplanets near stars closer to the solar system.
Modern radial velocity spectrometers have reached an accuracy of about 1m/s, such as the high-precision radial velocity planet finder (HARPS) installed by the 3.6m telescope of the La Silla Observatory in Chile (affiliated to the European Southern Observatory)[32]。Taking the sun as an example, the Earth is 1AU away from the sun and contributes 0.09 m/s to the daily radial velocity;Jupiter is 5.20 AU away from the sun and contributes 12.7 m/s.[33]So assuming there is another distant solar system, we can find Jupiter but not enough to find Earth.
By November 2020, we have discovered more than 900 exoplanets through the radial velocity method.[2]Among them, HARPS, the Anglo Australian Planetary Search Project (AAPS), which is based on advanced optical fiber, has made great contributionsStep grating(AFOE) automatic planetary probe (APF), etc.For the ESPRESSO project under preparation, the estimated apparent velocity measurement accuracy is 0.10m/s;There is also the CODEX project, whose estimated apparent velocity measurement accuracy is 0.02m/s[33]Therefore, we can expect to find more terrestrial planets and super Earths.
Occultation method
Transit Photometry, also known as transit method and transit method, determines the existence of exoplanets by observing the subtle changes in star luminosity when exoplanets cross the star surface in the visual direction.The first exoplanet discovered by occultation method isHD 209458b, a hot Jupiter discovered in 1999.[34]along withKepler space telescope The method of occultation has become the method to find the most exoplanets.
By observing the subtle changes in the brightness of the target star, we canPhotorefractive curveThe discovery of exoplanets in.In the line of sight direction, when exoplanets pass through the star surface, the brightness of the star will have a weak decline, forming a "groove" on the light curve.[35]This principle iseclipsePrinciple of.Just like when a solar eclipse occurs, the moon blocks the light from the sun, or does it happenVenus transitWe can see a small black spot on the sun.However, when exoplanets are discovered by occultation method, this small photometric change is not enough for naked eye observation, but requires high-precision astronomical equipment.
Venus transit/NASA
However, just like eclipses and transits, occultation requires an appropriate orbit. Only when stars, exoplanets and observers are nearly in the same straight line can we observe the subtle changes in the light curve.And we also need to determine that the reason for this change is the occultation of exoplanets, not other astronomical phenomena.Therefore, we need to observe the target star for a long time and observe the periodic light curve to determine the discovery of exoplanets.[35]In addition, this exoplanet needs to block enough light to make the instrument respond.Therefore, occultation method is suitable for exoplanets with small semi major diameter, large volume and small inclination angle.This is why most exoplanets detected by occultation are hot Jupiter.Due to the large number of stars in the universe, occultation is still very common.
Schematic diagram of occultation method
Combined with the radial velocity method, we can speculate the true mass and density of stars, and further understand the physical structure of planets.[32]By November 2020, we have discovered more than 3100 exoplanets through occultation.[2]Among them, CNES（CNES）LeadingConvective Rotation and Planetary Transit Satellite(COROT) (launched in 2006, now retired) andNASALeadingKepler space telescope (Launched in 2009, now retired) has made great contributions to the discovery of exoplanets by occultation method, which also makes the number of exoplanets discovered by occultation method unprecedented.
Astrometry
Astrometry usesCelestial mechanicsandAstrometryTo discover exoplanets.This method has a long history. From the observation in ancient Greece to Kepler Newton, astrometry has made great progress.This method is also widely used——NeptuneThe discovery of is attributed to this method.
In planetary systems, planets and stars move in circles around the common center of mass.This makes it seem to observers on Earth that the path of the stars is not a straight line, but "wave forward" under the influence of planetary gravity.By observing the position and velocity of stars for a long time, we can use astrometry to calculate the parameters of planets that affect their operation, including mass, revolution period, orbital inclination, eccentricity, etc.[36]
However, other stars are far away from usvoluntarilywithAngular secondCome and calculate.Even the largestBarnard star, and Barnard is only 6 light-years away from Earth.Not to mention the stars farther away from us.Therefore, astrometry requires extremely strict observation accuracy, and can only find exoplanets with long revolution period, large mass and close to the Earth.
Schematic diagram of exoplanets and parent stars
Although astrometry was proposed very early, it was not until 2002 that the exoplanet Gliese 876b, previously discovered by the radial velocity method, was successfully verified and its mass was calculated[37], and the first exoplanet HD 176051b about 1.5 times the mass of Jupiter was discovered independently until 2010[38]。As of November 2020, only 12 exoplanets have been discovered by astrometry, and 10 of them are more than 13 times the mass of Jupiter, which is likely to be brown dwarfs.[2]Launched in October 2013European Space Agency（ESO）Gaia Space Telescope(GAIA) has conducted a large-scale sky survey, or will use the observation data to find a large number of massive exoplanets.[39]
Direct imaging
As the name implies, Direct Imaging is to directly image exoplanets.
For generalMain sequence starIn terms ofStepan Boltzmann lawandWien's displacement law, calculate itsthermal radiationMainly focused onNear infraredTo the ultraviolet band, the peak is between the visible and ultraviolet bands.However, exoplanets do not have sufficient and stable energy sources. Generally, the temperature is low, and the main concentration area and peak value of thermal radiation are in the infrared band.So in exoplanetsradiant fluxIn larger cases, we can completely observe the infrared band and distinguish the two.In addition, exoplanets need to be far enough away from their parent stars, and it seems to observers on Earth that they must at least reach the resolution limit of telescopes
, the unit is angular second, λ is the observation wavelength, D is the telescope aperture, and both are in microns.
On the other hand, the observation also requires higher instruments, requiring a coronagraph to block the light from stars, and the observation system needs to maintain a lower temperature to reduce the generation of infrared radiation.In addition, for ground-based telescopes, the background radiation (from the atmosphere and the earth) at about 300K will also have a greater impact on the observation.[40]
We generally use the direct imaging method to search for young Jupiter like planets with temperatures between 600-2000K, and the peak wavelength of their thermal radiation is between 1.4-4.8 μ m.Such planets are generally far away from their parent stars and can be distinguished, and their surface area is large and their radiation flux is large enough to be observed in the near-infrared to mid infrared bands.In addition, for observers on Earth, the distance between exoplanets and their parent stars needs to be more than 0.1-0.3 angular seconds.This is about 30 meters from the earthSecond gapThe exoplanet is 3-9 away from the parent starAstronomical unit。[40]
The first exoplanet detected by direct imaging is2M1207bIn 2004Very large telescope(VLT) discovery.[41]As of November 2020, there are also more than 100 exoplanets discovered by direct imaging, most of which are giant planets dozens of times the mass of Jupiter.[2]NASAOfHubble Space Telescope, HawaiianKeck Observatory as well asEuropean Southern ObservatoryTelescope arrays located in Chile and other regions all participate in the search for exoplanets by direct imaging.[32]
Micro gravitational lens method
Gravitational microlensing is also a method to detect exoplanets by measuring the photometric changes of stars, but its principle is different from that of occultation.[42]
Gravitational lensyesEinsteinGeneral relativityAn optical effect predicted.Since space-time will be distorted near a massive object, the light will bend when passing near a massive object.If there is a massive celestial body on the line from the observer to the light source, the observer will see one or more images formed due to the bending of light, which is called gravitational lens phenomenon.If the foreground object has a small mass and the deflection of light is small, it will be difficult to distinguish multiple images generated at this time. The visual effect is that the brightness of the background star has been significantly enhanced.When the current scene star passes through a background star with an exoplanet, the brightness of the background star will increase and a peak will appear on the light curve.The mass of exoplanets is smaller than that of foreground stars, and the resulting peaks will be smaller, but it can still be observed that a smaller peak is superimposed on a certain position of the original light curve.We can then determine whether there are exoplanets by the second peak generated by the light curve.[42]
Light variation curve of micro gravitational lens of exoplanet OGLE-2005-390L b[43]
However, due to the contingency of the event that the foreground object passes through the background star, the use of micro gravitational lens method also has contingency and non repeatability.This has a great impact on the accuracy of exoplanet detection.Also due to the contingency of this event, the micro gravitational lens method does not have a strong selection effect on extrasolar planets, and various extrasolar planets may be observed.Therefore, the micro gravitational lens method is expected to find small planets with mass between Mercury and Mars[42]We can also find exoplanets whose orbits are far away from the stars, and also detect planetary systems that are far away from the earth, so that we can also detect those that do not revolve around the starsRogue planet 。[32]
From 1991 to 1992, astronomers first proposed that exoplanets can be detected by micro gravitational lens method.[44-45]But it was not until 2002 that Polish astronomers developed a viable technology,[46]Then in 2004, exoplanets were detected for the first time by using the micro gravitational lens method.[47]The figure shows the light variation curve of an exoplanet OGLE-2005-390Lb discovered by the micro gravitational lens method in 2005.[43]By November 2020, the number of exoplanets discovered by this method has reached more than 100.[2]
Chronometry
Timing is to observe some fixed period disturbances to detect exoplanets. It is generally divided into three kinds: pulsar timing, Stellar Pulsations Timing and Transit Timing.As of November 2020, there are more than 40 exoplanets discovered by timing method.[2]
Pulse period variation diagram of pulsar PSR B1257+12[27]
The pulsar timing method is used to observe exoplanets around pulsars.PulsaryesSupernova explosionThe debris behind is a kind of high-speed rotationneutron starIt has extremely stable electromagnetic pulse period and is used as a "cosmic clock" for time calibration.When there are exoplanets near a pulsar, its pulse period will change to some extent, which is consistent with the revolution period of the planet.This method has high accuracy, which can detect objects with the mass of Mercury, and also detect exoplanets far away from pulsars.[48]However, the number of pulsars is small, and there are not many exoplanets discovered by this method.The most representative is the first exoplanet discoveredPSR B1257+12 BandPSR B1257+12 C。[27]
The pulsar timing method is used to observe exoplanets near pulsar.Pulsating variable starIt is a special kind of stars, which will expand and contract periodically.Similar to the pulsar timing method, when there are exoplanets around the pulsar variable star, the dragging of planets will also modulate the pulsing period of the parent star, so that we can detect exoplanets.[49]In 2007, the first exoplanet V391 Pegasi b was discovered by using the pulsating variable star timing method.[50]
Imagination of extrasolar planets discovered by TDV
The transit timing method is used to observe the multi planet system where occultation has been found, including transit time variations (TTV) and transit duration variations (TDV).When there are multiple planets in the exoplanet system, other planets will perturb the transiting planets, so that the transiting time changes (corresponding to TTV) or the duration of the transiting stars changes (corresponding to TDV).By precisely measuring these changes, we can infer the parameters of other planets that exert influence.[51]Kepler Space Telescope discovered the exoplanet Kepler-19c in 2011 for the first time by using transit timing method.[52]
Other methods
In addition, there are some less commonly used exoplanet detection methods, such as the Relativistic Beaming method[53], Polarimetry[54], Radio Observation[55], Reflection/Emission Modulations[56]wait.Of course, there are few exoplanets discovered by these methods, and some methods are still theoretical.
classification
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Logarithmic mass radius relationships of stars and different types of planets[57]
Generally speaking, we divide the planets intoterrestrial planet、Jupiter like planet(gas giant) and Neptune like（Ice giant）Three categories.The three types of planets are approximately linearly correlated on the mass radius logarithmic curve, but there are obvious differences among them.[57]
Earth like planets are mainly composed of rocks, metals and silicates, with solid surfaces and similar internal structures.[58]Its surface often hasstructureandvolcanoThe interior has undergone differentiation, forming the structure of core, mantle and crust.[59]
The composition of Jupiter like planets is similar to that of the sun, mainly hydrogen and helium, and hydrogen and helium on the outer surface exist in the form of gas.The Jupiter like planet does not necessarily have a solid surface, and its atmosphere transits directly to the liquid surface.Jupiter like planets generally have a core of rock or rock ice[59](even without rocky core)[58]The outer layer is the metal hydrogen helium intermediate layer, and the outer layer is molecular hydrogen and helium.[59]
Neptune like planet is similar to Jupiter like planet, but its main composition is ice (water, methane, ammonia, etc., that is, some heavier elements, such as oxygen, carbon, nitrogen, etc.), rather than hydrogen and helium.[58]There may be a solid or liquid rock ice core inside, an ice mantle (liquid ice material) outside, and molecular hydrogen and helium outside but containing more ice material.[59]
Special type
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Super Earth
Super Earth is the most exoplanet discovered so far, with a mass between Earth and Neptune, and a typical orbital period of less than 100 days.[60]The definition of super earth is only related to the mass of exoplanets, which may be earth like planets or rock cores wrapped in gaseous shells.And even if it is composed of rocks, it may be that the core of the gaseous shell has been denuded by the stellar wind.[60]
Super Earth
For super earth and earth like exoplanets with mass close to the earth, habitability is an unavoidable topic.so-calledLivable zoneIt is the area around the star where liquid water is allowed to exist.For life on the earth, sufficient water, suitable atmospheric composition and thickness, suitable light and temperature are necessary conditions for survival and development.Some astrobiologists put forward the so-called "Superhabitable Planet", and the selection criteria are as follows:[61]
The planet is between 5 billion and 8 billion years old
Not more than 1.5 times the mass of the earth, about 10% larger than the earth
The average temperature is 5 ℃ higher than the earth
The atmosphere is wet and contains 25-30% oxygen. The rest is mainly inert gases (such as nitrogen)
The distribution of sea and land is reasonable, with a large number of shoals and islands
There are large satellites (1-10% of the planet's mass) at a proper distance (10-100 of the planet's radius)
With plate tectonics or similar geological/geochemical cycle mechanism, and a strong protective magnetic field
Kepler 452b vs. Earth
These conditions are harsh, and many conditions are difficult to detect, so there are few exoplanets that meet the standard.Discovered in 2015Kepler 452b, known as "Earth 2.0", its revolution period (385 days) is similar to the Earth, its radius is about 60% larger than the Earth, the average surface temperature is about - 8 ℃, and it revolves around a main sequence star 1400 light-years away from the Earth and consistent with the solar spectral type (G2V type).[62]
Hot JupiterIt is a kind of gaseous planet that is very close to the parent star and has a mass close to Jupiter.[63]Through occultation method and radial velocity method, we have found hundreds of hot Jupiter.Their masses are generally between 0.36-11.8 Jupiter masses, and their orbital periods are between 1.3-111 days.[64]Most hot jupiters have low eccentricity of their orbital orbits, which may be due to the fact that their orbits are very close to the stars and are affected by the strong tidal perturbation of the stars.Also due toTidal forceThe role of hot JupiterTidal locking State, that is, always with a fixed side toward the parent star.[65]The observation surface of hot Jupiter has an atmosphere with dense smoke clouds, strong optical scattering effect and obvious vertical layered structure.The surface temperature of hot Jupiter is very high due to its close proximity to the parent star.[66]At the same time, the stellar wind from the star will strip the surface atmosphere of hot Jupiter, making its mass continuously lost.
The discovery of hot Jupiter poses a serious challenge to the theory of planet formation.According to previous theories, such massive gas giants can only form inAccretion diskIn the place far away from the parent star, there is sufficient gas, dust and ice, which can quickly form the core and continuously accumulate gas, forming a gas giant.The mainstream explanation for the origin of hot Jupiter is the migration theory.According to this theory, hot Jupiter, like other gas giants, formed outside the frost line, but in its later evolution, its gauge band moved inward to a place close to the star, and finally formed a stable short period orbit.[67]Another explanation is that the formation of hot Jupiter has nothing to do with gas giants, but is formed by super Earth accretion gas.[68]
Hot Neptune
Gliese 436 b Imagination
Hot Neptune It is an exoplanet with a mass similar to Neptune and very close to its parent star,[69]It usually contains a large number of gases, mainly hydrogen and helium.Like hot Jupiter, hot Neptune is formed in two ways: if it moves from orbit, it will have more ice inside;[70]If it is directly formed locally, there will be more internal metals and high-temperature resistant materials.[71]It was discovered in 2004 and Gliese 436b confirmed in 2007 was the first hot Neptune detected.[72]Its observation shows that its atmosphere is beingStellar windPeel off to form a huge hydrogen cloud similar to a comet structure.[73]
Rogue planet
Imagination of wandering planets
Rogue Planet, also known as Interstellar Planet and Free floating Planet, is a kind of planet that does not revolve around any star and roams in the interstellar space alone.They are probably planets thrown out of the original planetary system due to the disturbance of other celestial bodies.
So far, the most effective method for detecting stray planets is the micro gravitational lens method.In 2011, astronomers from Japan and New Zealand estimated that the number of stray planets in the Milky Way would be twice as many as the number of stars, that is, more than 200 billion.[74]However, due to the contingency of the observation method, the number of stray planets we actually observed was only a few dozen.
Latest information
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On March 21, 2022, the National Aeronautics and Space Administration (NASA) said that with a batch of 65 newly confirmed exoplanets being recorded in the institution's exoplanet archive, the number of confirmed exoplanets in the solar system exceeded 5000.[75-76]
