Amega-Earth ormassive solid planet[1][2] is a proposedneologism for a massive terrestrialexoplanet that is at least ten times themass ofEarth (M🜨). Mega-Earths would be substantially more massive thansuper-Earths (terrestrial and ocean planets with masses around5–10 M🜨). The term "mega-Earth" was coined in 2014, whenKepler-10c was revealed to be aNeptune-mass planet with a density considerably greater than that of Earth.[3] However, it has since been determined to be a typical volatile-rich planet weighing just under half that mass.[4]
Mega-Earths or comparable objects may exist as remnant cores of evaporatedgas giants orwhite dwarfs,[5][6][7] and may also form aroundmassive stars andsupermassive black holes asblanets for the latter.[1][8]
Kepler-10c was the first exoplanet to be classified as a mega-Earth. At the time of its discovery, it was believed to have a mass around 17 times that of Earth (M🜨) and a radius around 2.3 times Earth's (R🜨), giving it a high density that implied a mainly rocky composition. However, several follow-up radial velocity studies produced different results for Kepler-10c's mass, all much below the original17 M🜨 estimate. In 2017, a more careful analysis using data from multiple different telescopes and spectrographs found that Kepler-10c is more likely around7.4 M🜨, making it a typical volatile-richmini-Neptune and not a mega-Earth.[9][4]
K2-56b, also designatedBD+20594b, is a much more likely mega-Earth,[10] with about16 M🜨 and 2.2 R🜨. At the time of its discovery in 2016, it had the highest chance of being rocky for a planet its size, with a posterior probability that it is dense enough to be terrestrial at about 0.43. For comparison, at the time the corresponding probability for Kepler-10c was calculated as 0.1, and as 0.002 for Kepler-131b.[11]
Kepler-145b [fr] is one of the most massive planets classified as mega-Earths, with a mass of37.1 M🜨 and a radius of 2.65 R🜨, so large that it could belong to a sub-category of mega-Earths known as "supermassive terrestrial planets" (SMTP). It likely has an Earth-like composition of rock and iron without any volatiles. A similar mega-Earth,K2-66b, has a mass of about21.3 M🜨 and a radius of about 2.49 R🜨, and orbits a subgiant star. Its composition appears to be mainly rock with a small iron core and a relatively thin steam atmosphere.[12]
Kepler-277b andKepler-277c are a pair of planets orbiting the same star, both thought to be mega-Earths with masses of about87.4 M🜨 and64.2 M🜨, and radii of about 2.92 R🜨 and 3.36 R🜨, respectively.[13]
PSR J1719−1438 b may be one of the most massive mega-Earths ever known, with a mass of about330 M🜨 and a radius less than 4 R🜨, slightly more massive but smaller thanJupiter. It is apulsar planet which is most likely composed largely of crystalline carbon but with adensity far greater thandiamond.[6][14] However, as it is a likely remnant core of a former white dwarf companion ofPSR J1719−1438, it is instead considered an ultra-low-mass carbon white dwarf or object per some definitions.[15][16]
The discovery of mega-Earths had challenged planetary formation theories.[17] Formation mechanisms and the occurrence of such objects remain subjects of ongoing research and debate.[17]
A 2007 study had suggested the possibility of hypothetical solid planets up tothousands of M🜨 forming around massive stars (B-type andO-type stars; 5–120 M☉).[1] The hypothesis proposed that theprotoplanetary disk around such stars would contain enough heavy elements, and that highUV radiation and strongwinds couldphotoevaporate the gas in the disk, leaving just the heavy elements. For comparison, Neptune's mass equals17 M🜨, Jupiter has318 M🜨. The most massive of those objects were assumed to be up to approximately4,000 M🜨 (or13 MJ) per the said upper mass limit used in theIAU's working definition of an exoplanet.[1] However, this limit has been debated due to no precise physical significance, with many exoplanet catalogs including objects with heavier masses, such as up to60 MJ.[18]
Despite the suggestion of the possibility of massive solid planets, it lacks supporting evidence for planetary formation theories and was primarily based on simulating mass-radius relationships for rocky planets, without investigating whether planetary formation theories support the existence of such objects.[1] The 2007 study acknowledged that such massive exoplanets are not yet known to exist.[1] More recent research has shown that the ratio of protoplanetary disk mass to stellar mass decreases rapidly for massive stars with initial masses above 10 M☉, falling to less than10−4.[19] Furthermore, no protoplanetary disks have been observed around O-type stars to date.[19]
Given these considerations, the formation and existence of massive solid planets around massive stars remain speculative and require further research and observational evidence.
