K2-137b (also designatedEPIC 228813918 b) is a hot,iron-rich,rockyexoplanet around thered dwarf K2-137 in theconstellationVirgo about 99 parsecs (320 light-years) fromEarth. It orbits the star very rapidly, with anorbital period of 0.1797 days (4.31 h), at a distance of just 0.0058 AU (870,000 km). It has received theGuinness World Record for the shortest orbital period for a confirmed planet, though the planetary candidateKOI-1843.03 has a period four minutes shorter.[5]
K2-137b is a sub-Earth in terms of radius. The discovery paper gives an estimate of0.89±0.09R🜨,[1] similar toVenus (0.949R🜨[6]), whereas Adams et al. (2021) provides a smaller value of0.64±0.10R🜨,[3] slightly larger thanMars (0.532R🜨[7]). If the latter is true, this would be one of thesmallest exoplanets discovered so far. Despite the small size, it is somewhere between 1.01 and 2.80 times as massive as Earth, comparable to somesuper-Earths. Because it has yet to bedisrupted bytidal forces from the host star, it can be inferred that iron makes up at least42±5 % of its mass. Still, the planet is tidally stretched to anaspect ratio of between 1.21 and 1.66.[4] Itsequilibrium temperature is approximately 1,471 K (1,198 °C; 2,188 °F),[1] hot enough to meltsilicate minerals.[8]
While K2-137b itself displays no signs ofevaporation, such iron-rich USP planets may be higher-mass counterparts to disintegrating planets e.g.,Kepler-1520b.[4]
The planet orbits a red dwarf star, K2-137, with thespectral type M3V. Smithet al. (2018)[1] gives it a mass of0.463±0.052 M☉, a radius of0.442±0.044 R☉, and aneffective temperature of3492±70 K, while Adams et al. (2021)[3] presents a substantially smaller mass of0.29±0.06 M☉ but a slightly hotter temperature of3697±109 K.
Several hypotheses exist as to how K2-137b became such an iron-rich, short-period planet. It could have been stripped of its outer silicate layers through high-velocitygiant impacts, much like howMercury came to have a high iron content according to several theories. Such collisions are projected to be common among USP planets due to highorbital speeds, reaching 270 km/s for K2-137b compared to the 29.8 km/s[9] of Earth. On the other hand, it may have formed out of iron-rich material. Matter near the inner edge of theprotoplanetary disk could be enriched in iron due to ironcondensing at higher temperatures thanenstatite andphotophoresis separating iron grains from silicate grains by the difference inthermal conductivity. Finally, it is possible that the planet is situated at theRoche limit and has been slowly losing the silicatecrust andmantle toRoche lobe overflow as its orbit slowlydecays.[4]
^Williams, David R. (25 November 2020)."Venus Fact Sheet". NASA Goddard Space Flight Center.Archived from the original on 11 May 2018. Retrieved15 April 2021.
^Williams, David (2018)."Mars Fact Sheet". NASA Goddard Space Flight Center. Archived fromthe original on 17 March 2020. Retrieved22 March 2020.
