Kepler-20f

Lore & Background

Kepler-20f was discovered using the transit method, in which the dimming of starlight as a planet crosses in front of its host star is measured. The planet orbits Kepler-20 every 19.58 days at a distance of 0.1387 AU, well inside the orbit of Mercury. Its orbit has a low eccentricity, similar to Mars in the Solar System.

The planet's radius is very close to Earth's, giving it a high probability (greater than 95%) of being a rocky world. However, its equilibrium temperature of about 705 K is far too hot for liquid water on the surface and hot enough to melt some metals. Despite this, its size suggests it may retain an atmosphere of water vapor. The mass is uncertain, estimated between 0.66 and 3.04 Earth masses depending on composition, with an Earth-like composition yielding about 1.2 Earth masses; radial velocity measurements provide an upper limit.

Kepler-20 is a Sun-like star with a metallicity nearly identical to the Sun's. It hosts four other known planets: Kepler-20b, c, d, and e, all of which orbit within the distance of Mercury from the Sun. The star has an apparent magnitude of 12.51, too dim to be seen with the naked eye.

Reader's Guide

Kepler-20f holds significance as one of the first exoplanets found with a radius nearly identical to Earth's, demonstrating that Earth-sized worlds exist in other systems. Its discovery, announced in December 2011 along with the other Kepler-20 planets, highlighted the Kepler mission's ability to detect small rocky planets. The planet's high equilibrium temperature and close orbit underscore that Earth-sized radius does not imply Earth-like conditions; its surface is inhospitable, with temperatures that could melt certain metals. The system's architecture—five planets all fitting inside Mercury's orbit—illustrates the diversity of compact planetary systems. The planet's mass remains uncertain, with a wide range depending on composition, and radial velocity data only provide an upper limit. The host star's age, estimated at 8.8 billion years with large uncertainty, and its solar-like metallicity provide context for planet formation. Kepler-20f's legacy lies in its role as a benchmark for Earth-sized exoplanets, showing that such planets can exist in extreme environments and that radius alone does not guarantee habitability.

Did You Know?

• Kepler-20f has a radius very close to Earth's, making it one of the most Earth-sized exoplanets known.
• Its equilibrium temperature is about 705 K, hot enough to melt some types of metal.
• Kepler-20f is the fourth closest planet from its star, with three planets closer and at least one farther.