K2-137b
The confirmed exoplanet with the shortest known orbital period.
K2-137b, also known as EPIC 228813918 b, is a rocky exoplanet rich in iron and extremely hot. It orbits the red dwarf star K2-137, located in the constellation Virgo roughly 99 parsecs from Earth. The planet holds a Guinness World Record for the shortest orbital period among confirmed planets, circling its star once every 0.1797 days at a mere 0.0058 AU. (The candidate KOI-1843.03 has a period four minutes shorter, but remains unconfirmed.)
In terms of radius, K2-137b is a sub-Earth. The discovery paper estimates its size at about 0.949 times Earth’s radius, similar to Venus. A later study by Adams et al. (2021) gives a smaller value of 0.532 Earth radii, slightly larger than Mars; if correct, this would make it one of the smallest exoplanets found. Despite its small size, its mass falls between 1.01 and 2.80 times that of Earth, comparable to some super-Earths. Because the planet has not been torn apart by tidal forces from its star, iron must make up at least a certain percentage of its mass. Tidal stretching gives it an aspect ratio between 1.21 and 1.66. Its equilibrium temperature is roughly 1471 K, hot enough to melt silicate minerals. While K2-137b itself shows no signs of evaporation, such iron-rich ultra-short-period planets may be the higher-mass counterparts to disintegrating worlds like Kepler-1520b.
The host star, K2-137, is a red dwarf of spectral type M3V. Smith et al. (2018) report its mass, radius, and effective temperature, while Adams et al. (2021) give a substantially smaller mass but a slightly hotter temperature.
Several formation scenarios could explain K2-137b’s high iron content and extremely short orbit. It may have been stripped of its outer silicate layers through high-velocity giant impacts, similar to theories for Mercury’s iron-rich composition. Such collisions are thought to be common among ultra-short-period planets due to orbital speeds reaching 270 km/s for K2-137b, compared to Earth’s 29.8 km/s. Alternatively, the planet might have formed from iron-rich material near the inner edge of the protoplanetary disk, where iron condenses at higher temperatures than enstatite, and photophoresis could separate iron grains from silicate grains based on differences in thermal conductivity.
Quick Facts
- Discoverer
- Smith et al.
- Discovered
- November 2017 (accepted)
- Discovery Site
- Kepler Space Observatory
- Discovery Method
- Transit method
- Apsis
- astron
- Eccentricity
- 0
- Star
- K2-137
Facts from the source article.
Lore & Background
K2-137b is a sub-Earth in radius, with estimates ranging from similar to Venus (0.949 Earth radii) to slightly larger than Mars (0.532 Earth radii). If the smaller value is correct, it would be one of the smallest exoplanets discovered. Despite its small size, its mass is between 1.01 and 2.80 times that of Earth, comparable to some super-Earths. The planet has not been disrupted by tidal forces from its host star, implying that iron makes up at least a significant fraction of its mass. It is tidally stretched to an aspect ratio between 1.21 and 1.66. Its equilibrium temperature of about 1471 K is hot enough to melt silicate minerals.
Several formation hypotheses exist for K2-137b's iron-rich composition and short orbit. It may have been stripped of its outer silicate layers through high-velocity giant impacts, similar to theories for Mercury's high iron content; such collisions are projected to be common among ultra-short-period planets due to orbital speeds reaching 270 km/s. Alternatively, it could have formed from iron-rich material near the inner edge of the protoplanetary disk, where iron condenses at higher temperatures than enstatite and photophoresis separates iron grains from silicate grains. A third possibility is that the planet sits at the Roche limit and has been slowly losing its silicate crust and mantle through Roche lobe overflow as its orbit decays.
Reader's Guide
K2-137b is significant as the confirmed exoplanet with the shortest orbital period, a record recognized by Guinness World Records, though the candidate KOI-1843.03 has a period four minutes shorter. Its extreme proximity to its star and high iron content make it a key object for studying the formation and evolution of ultra-short-period planets. The planet itself shows no signs of evaporation, but it may represent a higher-mass counterpart to disintegrating planets such as Kepler-1520b. The uncertainty in its radius—ranging from Venus-like to Mars-like—highlights the challenges in characterizing such small worlds. Its mass range, overlapping with super-Earths despite its sub-Earth radius, suggests a dense, iron-rich interior. The multiple formation scenarios—giant impacts, iron-rich disk material, or Roche lobe overflow—illustrate the competing processes that can produce such an extreme world. K2-137b thus serves as a natural laboratory for testing theories of planetary composition, tidal evolution, and orbital dynamics in the innermost regions of planetary systems.
Did You Know?
- K2-137b holds the Guinness World Record for the shortest orbital period of any confirmed planet.
- Its orbital period is 0.1797 days, or about 4.3 hours.
- The planet's equilibrium temperature of 1471 K is hot enough to melt silicate minerals.
- It orbits at just 0.0058 AU from its red dwarf star.
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