Kepler-20e
First sub-Earth-radius exoplanet found around a Sun-like star.
Kepler-20e is an exoplanet that orbits the star Kepler-20. It is the first planet smaller than Earth ever discovered circling a star like the Sun. Located second-closest to its star after Kepler-20b, the planet is far too hot for liquid water or life as we know it, with a surface temperature of about 1400 degrees Fahrenheit (1040 K). A year there lasts only 6.098 days, because its orbit is extremely close to the star—just 0.0507 astronomical units away. The orbit is tilted 87.50°, which allows astronomers to observe it transiting from our solar system. Of the five planets in the system, Kepler-20e has the lowest maximum orbital eccentricity, though it can still reach 0.28.
The planet is likely entirely rocky and probably lacks an atmosphere. It is tidally locked, always showing the same face to its star, much like the Moon does to Earth. This creates a stark temperature contrast between its permanent day and night sides. Astronomers believe the planet is geologically active, driven by its formation and strong gravitational interactions with its host star. Artistic depictions often show active volcanoes on both the day and night sides.
Quick Facts
- Discoverer
- Kepler team
- Discovered
- 20 December 2011
- Discovery Method
- Transit (Kepler Mission)
- Apsis
- astron
- Semimajor
- 0.0507 AU
- Eccentricity
- <0.28
- Period
- 6.098493 d
- Inclination
- 87.50 · +0.33 · -0.34
- Star
- Kepler-20 (KOI-070)
- Single Temperature
- 1040 K
Facts from the source article.
Lore & Background
Kepler-20e was announced on 20 December 2011 along with the other four planets in the Kepler-20 system. Its orbit has a semimajor axis of 0.0507 AU and a period of 6.098 days, with an inclination of 87.50° that makes its transits observable from the Solar System. The planet has a high maximum eccentricity of 0.28, the lowest in that system. Kepler-20e is likely entirely rocky and without an atmosphere. It is tidally locked, always showing the same side to its host star, which could produce large temperature differences between its permanent night and day sides. Astronomers think the planet is likely geologically active, due to its formation process and strong gravitational interactions with its host star; artistic depictions show active volcanoes on both the night and day sides.
Reader's Guide
Kepler-20e holds a foundational place in exoplanet science as the first planet smaller than Earth discovered orbiting a star other than the Sun. Its detection demonstrated that Earth-sized worlds exist around Sun-like stars, opening a new frontier in the search for potentially habitable planets. Despite its own uninhabitable surface temperature of 1040 K, the planet's rocky composition and tidal locking provide a natural laboratory for studying geological activity under extreme stellar irradiation. The system's architecture—five planets in close orbits—offers insights into planetary formation and orbital dynamics, with Kepler-20e's relatively low eccentricity among its siblings hinting at a more settled orbital history. Its discovery, announced in December 2011 alongside the other four Kepler-20 planets, marked a milestone in the Kepler mission's ability to detect small transiting worlds. The planet's likely lack of atmosphere and permanent day-night divide make it a key object for understanding how close-in rocky planets evolve, especially regarding volcanic activity driven by tidal forces and internal heat.
Did You Know?
- Kepler-20e is the first planet smaller than Earth found orbiting a Sun-like star.
- A year on Kepler-20e lasts only about 6 days.
- Its surface temperature of around 1400 °F (1040 K) is far too hot for liquid water.
More in Transiting Exoplanets, Part 4 1-24
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