![Kepler-138 b: 0.64 Times Earth](https://cdn.slatesource.com/2/f/1/2f14a376-1a43-45e0-97d0-3ed119a9652f.webp)

# Kepler-138 b: 0.64 Times Earth

- [Made in Slatesource](https://slatesource.com/@steph/kepler-138-b-0-64-times-earth)
- By [Steph](https://slatesource.com/@steph)
- Created on Sep 19, 2026

## Terrestrial around Kepler-138

It is 0.64 times Earth's radius, closest in size to Mars, and 0.1 times Earth's mass, at 1.70 g/cm3, less dense than Earth. A year there lasts 10.31 days, on an orbit 5 times closer to its star than Mercury is to the Sun. Its equilibrium temperature is 452 K, hotter than any desert. The host star Kepler-138 runs at 3841 K with 0.54 times the Sun's mass.

journey·3 Planets

# Kepler-138 from the inside out

Orbit

/

Planet

"terrestrial, 452 K.

"super-Earth, 410 K.

"super-Earth, 345 K.

Radius

0.64 Earth radii

Mass

0.1 Earth masses

Equilibrium temperature

452 K (measured)

One year

10.31 days

Orbit radius

0.0753 AU

Host star

Kepler-138 at 3841 K

Distance

218 light-years

Starlight blocked

0.012% at each transit

**0.64x** Earth: smaller than Venus

What you get from the shadow

Block a known fraction of a known star and you have measured a width.

Found by Kepler in 2014. Kepler watched a single field continuously, so it found the planets that come round often. A world with a long year simply never repeated within the mission.

Kepler-138 has 4 known planets. Systems like this are the ones where architecture can be studied instead of guessed at.

Two independent measurements sit behind this page: a dip in starlight for the size, and something else entirely for the mass. Together they make a density.

[NASA Exoplanet Archive: source data, including the values not used here](https://exoplanetarchive.ipac.caltech.edu/overview/Kepler-138%20b?utm_source=slatesource)