Kepler-186 f
Kepler-186 f — the celebrated fifth world, announced in 2014 as the first Earth-size planet found in another star's habitable zone (1.17 Earth radii, on a ~130-day orbit receiving about 30% of the sunlight Earth gets). It is shown here honestly as a DISPUTED, likely-but-unconfirmed planet: validated in 2014, its confidence has been revised DOWN by later statistical re-analyses — to about 96% in 2019, and to roughly 80% (a 20% false-alarm probability) in a 2025 study that calls it marginal — both short of the strict bar used to certify a planet. It has never been independently mass-confirmed, because its faint red-dwarf host is too dim for the radial-velocity method. Its size is real (from transit); its mass and true appearance are unknown, so no density is quoted and the landscape shown is an artist's concept.
Key facts
Science
Physical
- Radius
- 7,454 km · 1.17× wider than Earth
Orbital
- Semi-major axis
- 0.432 AU
- Eccentricity
- 0.04
- Inclination
- 0.00 °
- Orbital period
- 129.9 days
- Perihelion
- 0.415 AU
- Aphelion
- 0.449 AU
- Mean orbital velocity
- 36.2 km/s
Composition
- Bulk composition
- An Earth-size world (1.17 Earth radii). Its size is measured from transits, but its mass has never been measured — its faint red-dwarf host is too dim for the radial-velocity method — so no density is quoted; it is presumed rocky. Its detection is disputed, and the doubt has grown over time: validated in 2014 as the first Earth-size planet in a star's habitable zone, its confidence was revised down to about 96% by a 2019 re-analysis (Burke et al.) and to about 80% — a 20% false-alarm probability — by a 2025 reassessment (Robnik & Seljak, PNAS) that lists it among the marginal/likely-false-alarm candidates. Both fall short of the strict threshold used to certify a planet, so it remains flagged controversial and unconfirmed. Never imaged; the surface shown is an artist's concept.
- Surface appearance
- Appearance based on NASA/JPL-Caltech's artist's concept. No spacecraft has imaged this world, so its real appearance is unknown — The surface shown is an artistic representation, shown only in Vision mode; Realism shows a plain spectral colour.
Radiation & temperature
- Insolation
- 0.221 S⊕ · 22% of the sunlight Earth receives
- Equilibrium temperature
- 177 K (-96 °C)
Discovery & history
- Method
- Transit detection
- Name
- Designated 'f' under the IAU convention that letters a star's planets 'b', 'c', 'd'... in the order they are found (the star is the unwritten 'a'); no IAU proper name has been assigned.
Announced on 17 April 2014 (Quintana et al.) as a landmark — the first Earth-size planet in a star's habitable zone — from four years of NASA Kepler transits. Its status has since been questioned, and the doubt has GROWN, not faded: a 2019 re-analysis of Kepler's small, long-period signals (Burke et al.) put its false-positive probability near 4% (~96% confidence), and a 2025 reassessment (Robnik & Seljak, PNAS), using a more rigorous false-alarm method, put it weaker still — a 20% false-alarm probability, about 80% confidence — explicitly listing it among the 'marginal or likely false alarms.' Both fall short of the strict under-1% bar for statistical validation. It was never mass-confirmed: Kepler-186 is far too faint for radial velocity, so the planet's mass is only estimated from its size. The NASA Exoplanet Archive still flags it controversial — which is why it appears here clearly labelled, not quietly dropped.
Images
Images from open archives · credit shown per image