K2-38b

K2-38b, also designated EPIC 204221263 b, is a massive rocky exoplanet closely orbiting a Sun-like star and is one of the densest planets ever found. Discovered in 2016 by Crossfield et al. and later characterized by Sinukoff et al., K2-38b is a rocky super-Earth about 55% larger than Earth but about 12 times more massive, indicating a composition rich in iron and an extremely high surface gravity. The planet is within K2 Campaign 2, in the constellation Scorpius.

K2-38b
Discovery[1]
Discovery siteKepler Space Observatory
Discovery date2016
Transit
Orbital characteristics
0.04994+0.00048
−0.00049 AU
Eccentricity0.197+0.067
−0.060[2]
4.01593 (± 0.0005)[3] d
Inclination88.36 +0.17
−0.15[2]
StarK2-38
Physical characteristics
Mean radius
1.54±0.14[2] R⊕
Mass7.3+1.1
−1.0[2] M⊕
Mean density
11.0+5.8
−3.7 g cm−3
3.08++1.2
−0.74 g
Temperature1,266 K (993 °C; 1,819 °F)[2]

    Characteristics

    Mass, radius, and temperature

    K2-38b is a massive rocky exoplanet significantly larger and more massive than Earth. It has a radius of 1.55 R⊕,[3] close to the 1.6 R⊕ limit where planets would begin to accumulate thick hydrogen-helium atmospheres and become something similar to a Mini-Neptune. However, K2-38b is instead a very dense terrestrial planet. Initially it was believed planet is made almost entirely of iron,[1] with a mass of about 12.0 M⊕ and a density of about 17.5 g/cm3.[3] This made it one of the densest exoplanets ever discovered. Measurement in 2020 have resulted in lower mass of 7.3+1.1
    −1.0    [2] M⊕, and less extreme constraints on composition though.

    Orbit

    K2-38b has a very tight orbit around its host star. The planet takes just 4 days to complete a single orbit at a distance of about 0.05 AU. For comparison, Mercury orbits every 88 days at 0.38 AU from the Sun. The eccentricity of K2-38's orbit is slightly larger than that of Mars.[2]

    Host star

    The parent star K2-38 is a G2 main-sequence star, similar to our own Sun. It is 1.10 R☉ and 1.07 M☉, with a temperature of 5757 K and an unknown age. For comparison, the Sun has a temperature of 5778 K and is about 4.5 billion years old.[3]

    The visual magnitude of K2-38, or how bright it appears to the human eye, is 11.39. Therefore, it is far too dim to be seen without a telescope.[1]

    See also

    References
    1. Sinukoff, Evan; et al. (2016). "Eleven Multiplanet Systems from K2 Campaigns 1 and 2 and the Masses of Two Hot Super-Earths". The Astrophysical Journal. 827 (1). 78. arXiv:1511.09213. Bibcode:2016ApJ...827...78S. doi:10.3847/0004-637X/827/1/78.
    2. Characterization of the K2-38 planetary system. Unraveling one of the densest planets known to date, 2020, arXiv:2007.01081
    3. "K2-38 b CONFIRMED PLANET OVERVIEW PAGE". NASA Exoplanet Archive. Retrieved 2018-01-20.
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