K2-18 c
K2-18 c — the HOT INNER companion, NOT the famous biosignature planet (that is K2-18 b, further out). It is a smaller world detected only by radial velocity — a MINIMUM mass of about 7 Earths on a tight 9-day orbit, far too warm for liquid water. It has never transited, so it has never been imaged and its size is only a model estimate; no density is quoted. Its detection is disputed but has held up: the 9-day wobble was read as a planet (2017), challenged as stellar activity (2018), then independently recovered (2019, 2022). Shown here honestly as a likely-but-still-flagged world.
Key facts
Science
Physical
- Minimum mass (m·sin i)
- 4.17 × 10²⁵ kg · 6.99 M⊕
- Radial-velocity detection measures m·sin i; the true mass depends on the unknown orbital inclination.
- Radius
- Not measured
- No transit — the radius is a model estimate for a world of this minimum mass; the true size is unknown, so no density is quoted.
Orbital
- Semi-major axis
- 8,975,872 km
- From the NASA Exoplanet Archive (radial-velocity orbit)
- Eccentricity
- 0.2
- From the NASA Exoplanet Archive (radial-velocity orbit)
- Inclination
- 0.00 °
- From the NASA Exoplanet Archive (radial-velocity orbit)
- Orbital period
- 9.2 days
- From the NASA Exoplanet Archive (radial-velocity orbit)
- Perihelion
- 7,180,698 km
- Computed from the orbital elements
- Aphelion
- 10,771,047 km
- Computed from the orbital elements
- Mean orbital velocity
- 70.9 km/s
- Computed from the orbital elements
Composition
- Bulk composition
- Known only from radial velocity: a MINIMUM mass of about 7 Earths on a 9-day orbit, so it could be a large rocky super-Earth or a small sub-Neptune — it has never transited, so its true size and density are unknown. It is the HOT INNER companion, NOT the temperate biosignature planet K2-18 b. Its detection was disputed as stellar activity in 2018 but independently recovered in 2019 and 2022; the NASA Exoplanet Archive still flags it controversial. Never imaged.
- Cloud appearance
- Appearance based on NASA/JPL-Caltech's artist's concept. No spacecraft has imaged this world, so its real appearance is unknown — The banded clouds shown are a representation of its class, shown only in Visual mode; Realism shows a plain spectral colour.
Radiation & temperature
- Insolation
- 6.04 S⊕ · 6.04× the sunlight Earth receives
- Computed from K2-18's luminosity and the orbital distance (inverse-square; Earth's sunlight = 1).
- Equilibrium temperature
- 363 K (90 °C)
- Computed model value for its warm 9-day orbit (it receives about 7× the sunlight Earth does) — not measured.
Discovery & history
- Method
- Radial-velocity detection (stellar wobble)
- Name
- Designated 'c' under the IAU convention that letters a star's planets in the order they are found (the star is the unwritten 'a'); no IAU proper name has been assigned.
Reported in 2017 (Cloutier et al.) from HARPS radial velocities as a second planet, inside the orbit of K2-18 b. In 2018 a CARMENES team argued the 9-day signal was stellar activity rather than a planet — but Cloutier et al. (2019) and an independent line-by-line reanalysis (Radica et al. 2022) both recovered it, refining a minimum mass near 7 Earths and a 9.21-day period. The NASA Exoplanet Archive still flags it controversial, so it appears here plainly labelled — a real-but-contested world, and explicitly the hot inner sibling, not the famous biosignature planet b.
Images
Never imaged — this world is known only from the tiny gravitational tug it exerts on its star (radial velocity). An artist's impression of the whole system is on the system view; the planet's own appearance is unknown.