A sub-Earth is a

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...

"substantially less massive" than

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

and

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...

. In the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

, this category includes Mercury and

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

. Sub-Earth exoplanets are among the most difficult type to detect because their small sizes and masses produce the weakest signal. Despite the difficulty, one of the first exoplanets found was a sub-Earth around a

millisecond pulsar A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio, X-ray, and gamma ray portions of the electromagnetic spectrum. The leading theory for the origin of ...

PSR B1257+12 PSR B1257+12, previously designated PSR 1257+12, alternatively designated PSR J1300+1240, is a millisecond pulsar located 2,300 light-years from the Sun in the constellation of Virgo, rotating at about 161 times per second (faster than ...

. The smallest known is

WD 1145+017 b WD may refer to: Arts and entertainment * ''The Walking Dead'' (TV series) * ''White Dwarf'' (magazine) Businesses and organizations Government agencies * Royal Canadian Air Force Women's Division * War Department (United Kingdom) Ot ...

with a size of 0.15 Earth radii, or somewhat smaller than

Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...

. However,

is not massive enough to qualify as a sub-Earth

classical planet In classical antiquity, the seven classical planets or seven luminaries are the seven moving astronomical objects in the sky visible to the naked eye: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. The word ''planet'' comes fro ...

and is instead defined as a minor, or

dwarf Dwarf or dwarves may refer to: Common uses *Dwarf (folklore), a being from Germanic mythology and folklore * Dwarf, a person or animal with dwarfism Arts, entertainment, and media Fictional entities * Dwarf (''Dungeons & Dragons''), a humanoid ...

, planet. It is orbiting within a thick cloud of dust and gas as chunks of itself continually break off to then spiral in towards the star, and within around 5,000 years it will have more-or-less disintegrated. The

Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...

space telescope opened up a new realm of sub-Earth discoveries. On January 10, 2012, Kepler discovered the first three sub-Earths around an ordinary star,

Kepler-42 Kepler-42, formerly known as KOI-961, is a red dwarf located in the constellation Cygnus and approximately 131 light years from the Sun. It has three known extrasolar planets, all of which are smaller than Earth in radius, and likely ...

. As of June 2014, Kepler has 45 confirmed planets that are smaller than Earth, with 17 of them being smaller than 0.8 R_ⴲ. In addition, there are over 310 planet candidates with an estimated radius of <1 R_ⴲ, with 135 of them being smaller than 0.8 R_ⴲ. There is suspected to be a sub-Earth orbiting Proxima Centauri, the closest star to the Sun.Extrasolar Planets Encyclopaedia
/ref> The mass of Proxima d is believed to be between that of Mars and Venus. Sub-Earths commonly lack substantial atmospheres because of their low gravity and weak magnetic fields, allowing stellar radiation to wear away their atmospheres. Due to their small sizes, and unless there are significant

tidal force The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...

s when orbiting close to the parent star, sub-Earths also have short periods of geologic activity.

References

{{Mars Types of planet *