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 (nearly 20,000 km wide) but about 12 times more massive (around 7.2*10^25 kg, a bit less than Uranus) 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 site	Kepler Space Observatory
Discovery date	2016
Detection method	Transit
Orbital characteristics
Semi-major axis	0.04994+0.00048 −0.00049 AU
Eccentricity	0.197+0.067 −0.060[2]
Orbital period (sidereal)	4.01593 (± 0.0005)[3] d
Inclination	88.36 +0.17 −0.15[2]
Star	K2-38
Physical characteristics
Mean radius	1.54±0.14[2] R🜨
Mass	7.3+1.1 −1.0[2] M🜨
Mean density	11.0+5.8 −3.7 g cm−3
Surface gravity	3.08+1.2 −0.74 g
Temperature	1,266 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

• Mega-Earth