The star
The star
K2-138
- Type
- G8 V
- Surface
- ≈ 5,356 K
- Size
- 0.86 × the Sun
- Mass
- 0.94 × the Sun
- Brightness
- 0.517 × the Sun
- Habitable zone
- 0.553–1.32 AU
The planets
K2-138
K2-138 b
- Year
- 2.35 days
- Distance from star
- 0.0338 AU
- Mass
- 3.1 Earths
- Size
- 1.5 × Earth
- Temperature
- ≈ 1157 K (+884 °C)
- Found
- 2017, transit (K2)
K2-138
K2-138 c
- Year
- 3.56 days
- Distance from star
- 0.0446 AU
- Mass
- 6.3 Earths
- Size
- 2.3 × Earth
- Temperature
- ≈ 1012 K (+739 °C)
- Found
- 2017, transit (K2)
K2-138
K2-138 d
- Year
- 5.40 days
- Distance from star
- 0.0589 AU
- Mass
- 7.9 Earths
- Size
- 2.4 × Earth
- Temperature
- ≈ 888 K (+615 °C)
- Found
- 2017, transit (K2)
K2-138
K2-138 e
- Year
- 8.26 days
- Distance from star
- 0.0782 AU
- Mass
- 13 Earths
- Size
- 3.4 × Earth
- Temperature
- ≈ 757 K (+484 °C)
- Found
- 2017, transit (K2)
K2-138
K2-138 f
- Year
- 12.76 days
- Distance from star
- 0.104 AU
- Mass
- 1.6 Earths
- Size
- 2.9 × Earth
- Temperature
- ≈ 653 K (+380 °C)
- Found
- 2017, transit (K2)
K2-138
K2-138 g
- Year
- 41.97 days
- Distance from star
- 0.231 AU
- Mass
- 4.3 Earths
- Size
- 3 × Earth
- Temperature
- ≈ 445 K (+172 °C)
- Found
- 2021, transit (K2)
Masses found by the wobble method are minimums, because the orbit's tilt is unknown. Temperatures are equilibrium estimates for a bare planet, before any atmosphere.
How we know
The shadow
The planet passes in front of its star and dims it by a fraction of a percent, every orbit. The dip gives the planet's size and year.
Sources
- Planet and star parameters: NASA Exoplanet Archive, Planetary Systems Composite Parameters, retrieved 1 Nov 2026. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with NASA under the Exoplanet Exploration Program.
- Position: HYG Database v4.1 (CC BY-SA 4.0).
- Habitable zone: a simple estimate from the star's brightness (√L × 0.95 to 1.67 AU), shown as a guide.