K2-18 c
K2-18 c
confirmed planet • updated: 2017-12-19
K2-18 c orbits the star K2-18, which is smaller and cooler than our Sun. Scientists found it in 2017 using the radial velocity method at La Silla Observatory.
This page summarizes a catalog entry. If a measurement is missing, it is not shown or guessed. Some values can differ slightly between studies; when that happens, we describe the range rather than picking a favorite without evidence.
Scientific context
Scientific context: This profile layers interpretation on top of archival measurements. Modeled bands appear where direct detections (like spectra or transits) are not listed.
What we can’t claim: surface conditions, biology, or breathable atmosphere without direct spectra.
K2-18 c has a minimum mass of about 7.5 Earth masses and orbits its star every 9 days. Its orbit is very stretched out (eccentricity of 0.47), meaning it gets much closer and farther from the star during its orbit. The planet’s equilibrium temperature is estimated at 363 K (about 90°C or 194°F), but this depends on unknown factors like atmosphere and reflectivity. Because radial velocity gives a minimum mass (m sin i), the true mass could be higher if the orbit is tilted.
Glossary (plain English)
- AU: the average Earth–Sun distance.
- Semi-major axis: the planet’s average distance from its star.
- Eccentricity: how oval the orbit is (0 = circle).
- Radial velocity: finding a planet by measuring a star’s tiny “wobble.”
- m·sin i: a minimum mass estimate; the true mass can be higher if the orbit is tilted.
- Equilibrium temperature: a rough estimate from starlight alone, not a surface reading.
K2-18 c was first reported in 2017 using the Radial Velocity method. The discovery is linked to observations from La Silla Observatory.
Radial-velocity detections come from subtle shifts in the star’s spectrum as it wobbles under the planet’s gravity. The first mass value is often a minimum (m·sin i) unless the orbit’s tilt is known.
The catalog lists an orbital period of about 8.96 days, a semi-major axis near 0.060 AU. The orbit’s eccentricity is 0.47, which describes how stretched the orbit is.
A higher eccentricity can imply stronger past interactions or migration, and it can matter for long‑term stability in multi‑planet systems. Because this is a multi‑planet system, stability is ultimately tested with dynamical (N‑body) fits; catalogs can update as models improve.
The archive reports a mass scale of 0.0 MJ.
For radial‑velocity work this is commonly m·sin i (a minimum mass) because the orbital tilt is unknown. A rough radius estimate is shown for UI completeness, but it should not be treated as a measurement.
Why “m·sin i” shows up on RV planets
K2-18 c orbits K2-18.
A temperature near 3457 K places it on the cool side of the main sequence. The system is about 124.0 light‑years away.
Several key parameters are not present in this single catalog row. Missing fields don’t mean the science is unknown—only that this particular snapshot doesn’t carry the values.
In this case the most noticeable gaps are: radius. As new observations arrive, archives often refresh these entries (and sometimes revise earlier numbers).
Scientists keep revisiting systems like this because each new instrument pass can tighten uncertainties: better timing improves the orbit, better spectra improves the star, and better follow‑up can confirm or refute competing solutions. Even when a planet is well‑established, refined stellar properties can shift the inferred planet size, temperature, and habitability context.