Asuper-Earth is a type ofexoplanet with a mass higher thanEarth's, but substantially below those of the Solar System'sice giants,Uranus andNeptune, which are 14.5 and 17.1 times Earth's mass respectively.[1] The term "super-Earth" refers only to the mass of the planet, and so does not imply anything about the surface conditions orhabitability. The alternative term "gas dwarfs" may be more accurate for those at the higher end of the mass scale, although "mini-Neptunes" is a more common term.
In general, super-Earths are defined by theirmasses. The term does not imply temperatures, compositions, orbital properties, habitability, or environments. While sources generally agree on an upper bound of 10Earth masses[1][3][4] (~69% of the mass ofUranus, which is the Solar System's giant planet with the least mass), the lower bound varies from 1[1] or 1.9[4] to 5,[3] with various other definitions appearing in the popular media.[5][6][7] The term "super-Earth" is also used by astronomers to refer to planets bigger than Earth-like planets (from 0.8 to 1.2 Earth-radius), but smaller thanmini-Neptunes (from 2 to 4 Earth-radii).[8][9] This definition was made by theKepler space telescope personnel.[10]
Some authors further suggest that the term super-Earth might be limited to rocky planets without a significant atmosphere, or planets that have not just atmospheres but also solid surfaces or oceans with a sharp boundary between liquid and atmosphere, which the four giant planets in the Solar System do not have.[11] Planets above 10 Earth masses are termedmassive solid planets,[12]mega-Earths,[13][14] orgas giant planets,[15] depending on whether they are mostly made of rock and ice or mostly gas.
The first super-Earths were discovered by Aleksander Wolszczan andDale Frail around thepulsarPSR B1257+12 in 1992. The two outer planets (Poltergeist andPhobetor) of the system have masses approximately four times Earth—too small to be gas giants.
The first super-Earth around amain-sequence star was discovered by a team underEugenio Rivera in 2005. It orbitsGliese 876 and received the designationGliese 876 d (two Jupiter-sized gas giants had previously been discovered in that system). It has an estimated mass of 7.5 Earth masses and a very short orbital period of about 2 days. Due to the proximity of Gliese 876 d to its host star (ared dwarf), it may have a surface temperature of 430–650kelvin[16] and be too hot to support liquid water.[17]
In April 2007, a team headed byStéphane Udry based in Switzerland announced the discovery of two new super-Earths within theGliese 581 planetary system,[18] both on the edge of thehabitable zone around the star where liquid water may be possible on the surface. WithGliese 581c having a mass of at least 5 Earth masses and a distance fromGliese 581 of 0.073astronomical units (6.8 million mi, 11 million km), it is on the "warm" edge of the habitable zone around Gliese 581 with an estimated mean temperature (without considering effects from an atmosphere) of −3 degrees Celsius with analbedo comparable toVenus and 40 degrees Celsius with an albedo comparable to Earth. Subsequent research suggested Gliese 581c had likely suffered arunaway greenhouse effect like Venus.
Two further possible super-Earths were discovered in 2006:OGLE-2005-BLG-390Lb with a mass of 5.5 Earth masses, which was found bygravitational microlensing, andHD 69830 b with a mass of 10 Earth masses.[1]
The smallest super-Earth found as of 2008 wasMOA-2007-BLG-192Lb. The planet was announced by astrophysicist David P. Bennett for the internationalMOA collaboration on June 2, 2008.[19][20] This planet has approximately 3.3 Earth masses and orbits abrown dwarf. It was detected by gravitational microlensing.
In June 2008, European researchers announced the discovery of three super-Earths around the starHD 40307, a star that is only slightly less massive than theSun. Planets have at least the following minimum masses: 4.2, 6.7, and 9.4 times Earth's. The planets were detected by theradial velocity method by theHARPS (High Accuracy Radial Velocity Planet Searcher) inChile.[21]
In addition, the same European research team announced a planet 7.5 times the mass of Earth orbiting the starHD 181433. This star also has a Jupiter-like planet that orbits it every three years.[22]
PlanetCOROT-7b, with a mass estimated at 4.8 Earth masses and an orbital period of only 0.853 days, was announced on 3 February 2009. The density estimate obtained for COROT-7b points to a composition including rocky silicate minerals similar to that of the Solar System's four inner planets, a new and significant discovery.[23] COROT-7b, discovered right afterHD 7924 b, is the first super-Earth discovered that orbits amain sequence star that isG class or larger.[24]
The discovery ofGliese 581e with aminimum mass of 1.9 Earth masses was announced on 21 April 2009. It was at the time the smallest extrasolar planet discovered around a normal star and the closest in mass to Earth. Being at an orbital distance of just 0.03 AU and orbiting its star in just 3.15 days, it is not in the habitable zone,[25] and may have 100 times more tidal heating than Jupiter's volcanic satelliteIo.[26]
A planet found in December 2009,GJ 1214 b, is 2.7 times as large as Earth and orbits a star much smaller and less luminous than the Sun. "This planet probably does have liquid water," said David Charbonneau, a Harvard professor of astronomy and lead author of an article on the discovery.[27] However, interior models of this planet suggest that under most conditions it does not have liquid water.[28]
By November 2009, a total of 30 super-Earths had been discovered, 24 of which were first observed by HARPS.[29]
Discovered on 5 January 2010, a planetHD 156668 b with aminimum mass of 4.15Earth masses, is the least massive planet detected by theradial velocity method.[30] The only confirmed radial velocity planet smaller than this planet is Gliese 581e at 1.9 Earth masses (see above). On 24 August, astronomers using ESO's HARPS instrument announced the discovery of a planetary system with up to seven planets orbiting a Sun-like star,HD 10180, one of which, although not yet confirmed, has an estimated minimum mass of 1.35 ± 0.23 times that of Earth, which would be the lowest mass of any exoplanet found to date orbiting a main-sequence star.[31] Although unconfirmed, there is a 98.6% probability that this planet does exist.[32]
TheNational Science Foundation announced on 29 September the discovery of a fourth super-Earth (Gliese 581g) orbiting within the Gliese 581 planetary system. The planet has a minimum mass 3.1 times that of Earth and a nearly circular orbit at 0.146 AU with a period of 36.6 days, placing it in the middle of the habitable zone where liquid water could exist and midway between the planets c and d. It was discovered using the radial velocity method by scientists at the University of California at Santa Cruz and the Carnegie Institution of Washington.[33][34][35] However, the existence of Gliese 581 g has been questioned by another team of astronomers, and it is currently listed as unconfirmed atThe Extrasolar Planets Encyclopaedia.[36]
On 2 February, theKepler Space Observatory mission team released alist of 1235 extrasolar planet candidates, including 68 candidates of approximately "Earth-size" (Rp < 1.25Re) and 288 candidates of "super-Earth-size" (1.25Re <Rp < 2Re).[10][37] In addition, 54 planet candidates were detected in the "habitable zone." Six candidates in this zone were less than twice the size of the Earth [namely: KOI 326.01 (Rp=0.85Re), KOI 701.03 (Rp=1.73Re), KOI 268.01 (Rp=1.75Re), KOI 1026.01 (Rp=1.77Re), KOI 854.01 (Rp=1.91Re), KOI 70.03 (Rp=1.96Re) – Table 6][10] A more recent study found that one of these candidates (KOI 326.01) is in fact much larger and hotter than first reported.[38] Based on the latest Kepler findings, astronomerSeth Shostak estimates "within a thousand light-years of Earth" there are "at least 30,000 of these habitable worlds."[39] Also based on the findings, the Kepler Team has estimated "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone.[40]
On 17 August, a potentially habitable super-EarthHD 85512 b was found using the HARPS as well as a three super-Earth system82 G. Eridani.[41] On HD 85512 b, it would be habitable if it exhibits more than 50% cloud cover.[42][43] Then less than a month later, a flood of 41 new exoplanets, including 10 super-Earths, were announced.[44]
On 5 December 2011, the Kepler space telescope discovered its first planet within the habitable zone or "Goldilocks region" of its Sun-like star.Kepler-22b is 2.4 times the radius of the Earth and occupies an orbit 15% closer to its star than the Earth to the Sun. This is compensated for, however, as the star, with a spectral typeG5V, is slightly dimmer than the Sun (G2V). Thus, surface temperatures would still allow liquid water on its surface.
On 5 December 2011, the Kepler team announced that they had discovered 2,326 planetary candidates, of which 207 are similar in size to Earth, 680 are super-Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. Compared to the February 2011 figures, the number of Earth-size and super-Earth-size planets increased by 200% and 140% respectively. Moreover, 48 planet candidates were found in the habitable zones of surveyed stars, marking a decrease from the February figure; this was due to the more stringent criteria in use in the December data.
In 2011, a density of55 Cancri Ae was calculated which turned out to be similar to Earth's. At the size of about 2 Earth radii, it was the largest planet until 2014, which was determined to lack a significant hydrogen atmosphere.[46][47]
On 20 December 2011, the Kepler team announced the discovery of the first Earth-size exoplanets, Kepler-20e and Kepler-20f, orbiting a Sun-like star,Kepler-20.
PlanetGliese 667 Cb (GJ 667 Cb) was announced by HARPS on 19 October 2009, together with 29 other planets, whileGliese 667 Cc (GJ 667 Cc) was included in a paper published on 21 November 2011. More detailed data on Gliese 667 Cc were published in early February 2012.
In September 2012, the discovery of two planets orbitingGliese 163[48] was announced.[49][50] One of the planets,Gliese 163 c, about 6.9 times the mass of Earth and somewhat hotter, was considered to be within thehabitable zone.[49][50]
On 7 January 2013, astronomers from theKepler space observatory announced the discovery ofKepler-69c (formerlyKOI-172.02), anEarth-likeexoplanet candidate (1.5 times the radius of Earth) orbiting astar similar to theSun in thehabitable zone and possibly a "prime candidate to hostalien life".[51]
In April 2013, using observations by NASA's Kepler mission team led byWilliam Borucki, of the agency's Ames Research Center, found five planets orbiting in the habitable zone of a Sun-like star,Kepler-62, 1,200 light years from Earth. These new super-Earths have radii of 1.3, 1.4, 1.6, and 1.9 times that of Earth. Theoretical modelling of two of these super-Earths,Kepler-62e andKepler-62f, suggests both could be solid, either rocky or rocky with frozen water.[52]
On 25 June 2013, three "super Earth" planets have been found orbiting a nearby star at a distance where life in theory could exist, according to a record-breaking tally announced by the European Southern Observatory. They are part of a cluster of as many as seven planets that circleGliese 667C, one of three stars located a relatively close 22 light years from Earth in the constellation of Scorpio, it said. The planets orbit Gliese 667C in the so-called Goldilocks Zone — a distance from the star at which the temperature is just right for water to exist in liquid form rather than being stripped away by stellar radiation or locked permanently in ice.[citation needed]
In May 2014, previously discoveredKepler-10c was determined to have the mass comparable to Neptune (17 Earth masses). With the radius of 2.35R🜨, it is currently the largest known planet likely to have a predominantly rocky composition.[53] At 17 Earth masses, it is well above the 10 Earth mass upper limit that is commonly used for the term 'super-Earth' so the termmega-Earth has been proposed.[14] However, in July 2017, more careful analysis of HARPS-N and HIRES data showed that Kepler-10c was much less massive than originally thought, instead around 7.37 (6.18 to 8.69)M🜨 with a mean density of 3.14 g/cm3. Instead of a primarily rocky composition, the more accurately determined mass of Kepler-10c suggests a world made almost entirely of volatiles, mainly water.[54]
On 6 January 2015, NASA announced the 1000th confirmedexoplanet discovered by the Kepler space telescope. Three of the newly confirmed exoplanets were found to orbit withinhabitable zones of their relatedstars: two of the three,Kepler-438b andKepler-442b, are near-Earth-size and likely rocky; the third,Kepler-440b, is a super-Earth.[55]
On 30 July 2015,Astronomy & Astrophysics said they found a planetary system with three super-Earths orbiting a bright, dwarf star. The four-planet system, dubbedHD 219134, had been found 21 light years from Earth in the M-shaped northern hemisphere ofconstellation Cassiopeia, but it is not in thehabitable zone of its star. The planet with the shortest orbit isHD 219134 b, and is Earth's closest known rocky, and transiting, exoplanet.[56][57][58]
In February 2016, it was announced thatNASA'sHubble Space Telescope had detectedhydrogen andhelium (and suggestions ofhydrogen cyanide), but nowater vapor, in theatmosphere of55 Cancri e, the first time the atmosphere of a super-Earthexoplanet was analyzed successfully.[59]
In August 2016, astronomers announced the detection ofProxima b, anEarth-sizedexoplanet that is in thehabitable zone of thered dwarfstarProxima Centauri, the closest star to theSun.[60] Due to its closeness toEarth,Proxima b may be a flyby destination for a fleet of interstellarStarChip spacecraft currently being developed by theBreakthrough Starshot project.[60]
In February 2018, K2-141b, a rockyultra-short period planet (USP) super-Earth, with a period of 0.28 days orbiting the host star K2-141 (EPIC 246393474) was reported.[61] Another super-Earth,K2-155d, is discovered.[62]
In July 2018, a potential discovery of40 Eridani b was announced.[63] At 16 light-years it was the closest super-Earth at the time of discovery, and its star was the second-brightest hosting a super-Earth.[64][63][a]
In July 2019, the discovery ofGJ 357 d was announced. Thirty-one light-years from the Solar System, the planet is at least 6.1M🜨.
In 2021, the exoplanetG 9-40 b was discovered.
In 2022, the discovery of a super-Earth around the red dwarf starRoss 508 was reported. Part of the planet'selliptical orbit takes it within thehabitable zone.[66]
On 31 January 2024NASA reported the discovery of a super-Earth calledTOI-715 b located in thehabitable zone of a red dwarf star about 137 light-years away.[67][68]
On 23 October 2025, the super-EarthGJ 251 c, a rocky exoplanet four times massive as Earth, was discovered in the Gemini constellation by a team of scientists and researchers fromPennsylvania State University.
TheSolar System contains no known super-Earths. Earth is the largestterrestrial planet in the Solar System, and all larger planets have both at least 14 times the mass of Earth and thick gaseous envelopes without well-defined rocky or watery surfaces; that is, they are eithergas giants orice giants, not terrestrial planets.
It has been proposed that the presence ofgas giants, namely Jupiter, might have prevented the inward migration of primordial super-Earths into the inner Solar System.[69] This could explain why the Solar System does not possess a super-Earth close tothe Sun, unlike more than half of theplanetary systems studied byKepler.[70] TheGrand tack hypothesis has also been used to explain this lack of hot super-Earths within the Solar System.[71] The fact that there are barely anyasteroids orplanetesimals inside the orbit ofMercury led some astronomers believing that a super-Earth might have formed in proximity to the Sun, cleared its neighborhood and got destroyed by the Sun.[72]
In January 2016, the existence of a hypothetical super-Earth ninth planet in the Solar System, referred to asPlanet Nine, was proposed as an explanation for the orbital behavior of sixtrans-Neptunian objects, but it is speculated to also be an ice giant like Uranus or Neptune.[73][74] A refined model in 2019 constrains it to around five Earth masses;[75] planets of this mass are probablyMini-Neptunes.[76]
Due to the larger mass of super-Earths, their physical characteristics may differ from Earth's; theoretical models for super-Earths provide four possible main compositions according to their density: low-density super-Earths are inferred to be composed mainly of hydrogen and helium (mini-Neptunes); super-Earths of intermediate density are inferred to either have water as a major constituent (ocean planets), or have a denser core enshrouded with an extended gaseous envelope (gas dwarf or sub-Neptune). A super-Earth of high density is believed to be rocky and/or metallic, like Earth and the other terrestrial planets of the Solar System. A super-Earth's interior could be undifferentiated, partially differentiated, or completely differentiated into layers of different composition. Researchers at Harvard Astronomy Department have developed user-friendly online tools to characterize the bulk composition of the super-Earths.[78][79] A study onGliese 876 d by a team aroundDiana Valencia[1] revealed that it would be possible to infer from a radius measured by thetransit method of detecting planets and the mass of the relevant planet what the structural composition is. For Gliese 876 d, calculations range from 9,200 km (1.4 Earth radii) for a rocky planet and very large iron core to 12,500 km (2.0 Earth radii) for a watery and icy planet. Within this range of radii the super-Earth Gliese 876 d would have a surfacegravity between 1.9g and 3.3g (19 and 32 m/s2). However, this planet is not known to transit its host star.
The limit between rocky planets and planets with a thick gaseous envelope is calculated with theoretical models. Calculating the effect of the active XUV saturation phase of G-type stars over the loss of the primitive nebula-captured hydrogen envelopes in extrasolar planets, it's obtained that planets with a core mass of more than 1.5 Earth-mass (1.15 Earth-radius max.), most likely cannot get rid of their nebula captured hydrogen envelopes during their whole lifetime.[80] Other calculations point out that the limit between envelope-free rocky super-Earths and sub-Neptunes is around 1.75 Earth-radii, as 2 Earth-radii would be the upper limit to be rocky (a planet with 2 Earth-radii and 5 Earth-masses with a mean Earth-like core composition would imply that 1/200 of its mass would be in a H/He envelope, with an atmospheric pressure near to 2.0 GPa or 20,000 bar).[81] Whether or not the primitive nebula-captured H/He envelope of a super-Earth is entirely lost after formation also depends on the orbital distance. For example, formation and evolution calculations of theKepler-11 planetary system show that the two innermost planets Kepler-11b and c, whose calculated mass is ≈2M🜨 and between ≈5 and 6 M🜨 respectively (which are within measurement errors), are extremely vulnerable to envelope loss.[82] In particular, the complete removal of the primordial H/He envelope by energetic stellar photons appears almost inevitable in the case of Kepler-11b, regardless of its formation hypothesis.[82]
If a super-Earth is detectable by both the radial-velocity and the transit methods, then both its mass and its radius can be determined; thus its average bulk density can be calculated. The actual empirical observations are giving similar results as theoretical models, as it's found that planets larger than approximately 1.6 Earth-radius (more massive than approximately 6 Earth-masses) contain significant fractions of volatiles or H/He gas (such planets appear to have a diversity of compositions that is not well-explained by a single mass-radius relation as that found in rocky planets).[83][84] After measuring 65 super-Earths smaller than 4 Earth-radii, the empirical data points out that Gas Dwarves would be the most usual composition: there is a trend where planets with radii up to 1.5 Earth-radii increase in density with increasing radius, but above 1.5 radii the average planet density rapidly decreases with increasing radius, indicating that these planets have a large fraction of volatiles by volume overlying a rocky core.[85][86][87] Another discovery about exoplanets' composition is that about thegap or rarity observed for planets between 1.5 and 2.0 Earth-radii, which is explained by a bimodal formation of planets (rocky super-Earths below 1.75 and sub-Neptunes with thick gas envelopes being above such radii).[9]
Additional studies, conducted with lasers at theLawrence Livermore National Laboratory and theOMEGA laboratory at theUniversity of Rochester, show that the magnesium-silicate internal regions of the planet would undergo phase changes under the immense pressures and temperatures of a super-Earth planet, and that the different phases of this liquid magnesium silicate would separate into layers.[88]
Further theoretical work by Valencia and others suggests that super-Earths would be more geologically active than Earth, with more vigorousplate tectonics due to thinner plates under more stress. In fact, their models suggested that Earth was itself a "borderline" case, just barely large enough to sustain plate tectonics.[89] These findings were corroborated by van Heck et al., who determined that plate tectonics may be more likely on super-Earths than on Earth itself, assuming similar composition.[90] However, other studies determined that strongconvection currents in the mantle acting on strong gravity would make the crust stronger and thus inhibit plate tectonics. The planet's surface would be too strong for the forces ofmagma to break the crust into plates.[91]
New research suggests that the rocky centres of super-Earths are unlikely to evolve into terrestrial rocky planets like the inner planets of the Solar System because they appear to hold on to their large atmospheres. Rather than evolving into a planet composed mainly of rock with a thin atmosphere, the small rocky core remains engulfed by its large hydrogen-rich envelope.[92][93]
Theoretical models show that Hot Jupiters and Hot Neptunes can evolve by hydrodynamic loss of their atmospheres to Mini-Neptunes (as it could be the super-EarthGJ 1214 b),[94] or even to rocky planets known aschthonian planets (after migrating towards the proximity of their parent star). The amount of the outermost layers that is lost depends on the size and the material of the planet and the distance from the star.[82] In a typical system, a gas giant orbiting 0.02 AU around its parent star loses 5–7% of its mass during its lifetime, but orbiting closer than 0.015 AU can mean evaporation of the whole planet except for its core.[95][96]
The low densities inferred from observations imply that a fraction of the super-Earth population has substantial H/He envelopes, which may have been even more massive soon after formation.[97] Therefore, contrary to the terrestrial planets of the solar system, these super-Earths must have formed during the gas-phase of their progenitorprotoplanetary disk.[98]
Since the atmospheres,albedo andgreenhouse effects of super-Earths are unknown, the surface temperatures are unknown and generally only an equilibrium temperature is given. For example, theblack-body temperature of the Earth is 255.3K (−18 °C or 0 °F ).[99] It is thegreenhouse gases that keep the Earth warmer. Venus has a black-body temperature of only 184.2 K (−89 °C or −128 °F ) even though Venus has a true temperature of 737 K (464 °C or 867 °F ).[99] Though the atmosphere of Venus traps more heat than Earth's, NASA lists the black-body temperature of Venus based on the fact that Venus has an extremely high albedo (Bond albedo 0.90,Visual geometric albedo 0.67),[99] giving it a lower black body temperature than the more absorbent (loweralbedo) Earth.
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.[100] That said, super-Earth magnetic fields are yet to be detected observationally.
According to one hypothesis,[101] super-Earths of about two Earth masses may beconducive to life. The higher surface gravity would lead to a thicker atmosphere, increased surface erosion and hence a flatter topography. The result could be an "archipelago planet" of shallow oceans dotted with island chains ideally suited forbiodiversity. A more massive planet of two Earth masses would also retain more heat within its interior from its initial formation much longer, sustainingplate tectonics (which is vital for regulating thecarbon cycle and hence theclimate) for longer. The thicker atmosphere and stronger magnetic field would also shield life on the surface against harmfulcosmic rays.[101]
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