K2-38b
A dense, iron-rich super-Earth in a 4-day orbit.
K2-38b, also known as EPIC 204221263 b, is a dense, rocky exoplanet that orbits very close to a Sun-like star. It ranks among the densest exoplanets discovered. The planet is about 55% wider than Earth, with a diameter near 20,000 km, but it packs roughly 12 times Earth's mass—around 7.2 × 10^25 kg, slightly less than Uranus. This suggests a composition heavy in iron and an extremely strong surface gravity. It lies within K2 Campaign 2, in the constellation Scorpius.
**Characteristics**
**Mass, radius, and temperature** K2-38b is a massive rocky world, significantly larger and heavier than Earth. Its radius of 1.55 Earth radii sits close to the 1.6-radius threshold where planets typically accumulate thick hydrogen-helium envelopes and become mini-Neptunes. Instead, K2-38b is a very dense terrestrial planet. Early estimates indicated it might be almost entirely iron, with a mass of about 12.0 Earth masses and a density near 17.5 g/cm³, making it one of the densest exoplanets known. Later measurements in 2020 revised the mass downward to about 7.3 Earth masses, leading to less extreme constraints on its composition.
**Orbit** The planet follows a very tight orbit around its host star. It completes one full orbit in just 4 days, at a distance of roughly 0.05 AU. For comparison, Mercury takes 88 days to orbit the Sun at 0.38 AU. The orbit’s eccentricity is slightly higher than that of Mars.
**Host star** The parent star, K2-38, is a G2 main-sequence star similar to the Sun. It has a mass of 1.10 solar masses and a radius of 1.07 solar radii, with a temperature of 5757 K. Its age is unknown. The Sun, by contrast, has a temperature of 5778 K and is about 4.5 billion years old. K2-38 has a visual magnitude of 11.39, making it far too dim to see without a telescope.
Quick Facts
- Discovery Site
- Kepler Space Observatory
- Discovered
- 2016
- Discovery Method
- Transit
- Apsis
- astron
- Semimajor
- 0.049940.00048 · 0.00049 AU
- Eccentricity
- 0.1970.067 · 0.060
- Period
- 4.01593 (± 0.0005) d
- Inclination
- 88.36 0.17 · 0.15
- Star
- K2-38
- Density
- 11.05.8 · 3.7 g cm / −3
- Surface Grav
- 3.081.2 · 0.74 g
- Single Temperature
- 1266 K
Facts from the source article.
Lore & Background
K2-38b was discovered in 2016 and later characterized by follow-up observations. The planet has a radius of 1.55 Earth radii, close to the 1.6 Earth radii threshold where planets typically accumulate thick hydrogen-helium atmospheres and become mini-Neptunes. Instead, K2-38b is a very dense terrestrial planet. Initial measurements suggested a mass of about 12.0 Earth masses and a density of about 17.5 g/cm³, making it one of the densest exoplanets ever discovered. However, measurements in 2020 resulted in a lower mass of 7.3 Earth masses, leading to less extreme constraints on its composition.
The planet orbits its host star every 4 days at a distance of about 0.05 AU, far closer than Mercury's orbit around the Sun. Its orbital eccentricity is slightly larger than that of Mars. The host star, K2-38, is a G2 main-sequence star similar to the Sun, with a temperature of 5757 K and an unknown age. It has a visual magnitude of 11.39, making it far too dim to be seen without a telescope. The system lies in the constellation Scorpius and was observed during K2 Campaign 2.
Reader's Guide
K2-38b is significant as one of the densest exoplanets ever found, challenging models of planetary formation and composition. Its high density suggests an iron-rich interior, possibly with a large metallic core, and its radius near the mini-Neptune boundary makes it a key object for studying the transition between rocky super-Earths and gas-enveloped planets. The initial mass estimate of 12.0 Earth masses and density of 17.5 g/cm³ placed it among the most extreme known worlds, though later measurements reduced the mass to 7.3 Earth masses, introducing uncertainty about its exact makeup. The planet's very short orbital period of 4 days and close proximity to its Sun-like star also make it a valuable target for atmospheric and tidal studies. Its legacy lies in demonstrating that massive, iron-rich rocky planets can exist in extremely tight orbits, and in highlighting the ongoing refinement of exoplanet characterization as observational techniques improve.
Did You Know?
- K2-38b is about 55% larger than Earth but about 12 times more massive.
- Its initial density estimate of 17.5 g/cm³ made it one of the densest exoplanets ever discovered.
- The planet orbits its star every 4 days at a distance of 0.05 AU.
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