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A sub-Earth is a planet "substantially less massive" than Earth and Venus. In the Solar System, this category includes Mercury and Mars. 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 PSR B1257+12. The smallest known is WD 1145+017 b with a size of 0.15 Earth radii, or somewhat smaller than Pluto. However, WD 1145+017 b is not massive enough to qualify as a sub-Earth classical planet and is instead defined as a minor, or dwarf, 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 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. 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. 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 forces when orbiting close to the parent star, sub-Earths also have short periods of geologic activity.

References

1. ^ Sinukoff, E.; Fulton, B.; Scuderi, L.; Gaidos, E. (2013-08-28). "Below One Earth Mass: The Detection, Formation, and Properties of Subterrestrial Worlds". Space Science Reviews. 180 (1–4): 71–99. arXiv:1308.6308. Bibcode:2013SSRv..180...71S. doi:10.1007/s11214-013-0019-1. S2CID 255071638.
2. Vanderburg, Andrew; John Asher Johnson; Rappaport, Saul; Bieryla, Allyson; et al. (2015). "A disintegrating minor planet transiting a white dwarf". Nature. 526 (7574): 546–549. arXiv:1510.06387. Bibcode:2015Natur.526..546V. doi:10.1038/nature15527. PMID 26490620. S2CID 4451207.
3. Rappaport, S.; Gary, B. L.; Kaye, T.; Vanderburg, A.; Croll, B.; Benni, P.; Foote, J. (June 2016). "Drifting asteroid fragments around WD 1145+017". Monthly Notices of the Royal Astronomical Society. 458 (4): 3904–3917. arXiv:1602.00740. Bibcode:2016MNRAS.458.3904R. doi:10.1093/mnras/stw612.
4. NASA Exoplanet Archive
5. Extrasolar Planets Encyclopaedia
6. Faria, J. P.; Suárez Mascareño, A.; Figueira, P.; Silva, A.M.; et al. (February 2022). "A candidate short-period sub-Earth orbiting Proxima Centauri". Astronomy and Astrophysics. 658: A115. arXiv:2202.05188. Bibcode:2022A&A...658A.115F. doi:10.1051/0004-6361/202142337.

v t e Exoplanets
Planet Definition IAU Planetary science
Main topics	Exoplanet Exoplanet orbital and physical parameters Methods of detecting exoplanets Planetary system Planet-hosting stars
Sizes and types	Terrestrial Carbon planet Coreless planet Desert planet Dwarf planet Hycean planet Ice planet Iron planet (Super-Mercury) Lava planet Ocean world Mega-Earth Sub-Earth Super-Earth Gaseous Eccentric Jupiter Mini-Neptune (Gas dwarf) Helium planet Hot Jupiter Hot Neptune Gas giant Ice giant Super-Jupiter Super-Neptune Super-puff Ultra-hot Jupiter Ultra-hot Neptune Other types Blanet Brown dwarf Chthonian planet Circumbinary planet Circumtriple planet Disrupted planet Double planet Ecumenopolis Eyeball planet Giant planet Mesoplanet Planemo Planet/Brown dwarf boundary Planetesimal Protoplanet Pulsar planet Sub-brown dwarf Sub-Neptune Toroidal planet Ultra-cool dwarf Ultra-short period planet (USP)
Formation and evolution	Accretion Accretion disk Asteroid belt Circumplanetary disk Circumstellar disc Circumstellar envelope Cosmic dust Debris disk Detached object Disrupted planet Excretion disk Exozodiacal dust Extraterrestrial materials Extraterrestrial sample curation Giant-impact hypothesis Gravitational collapse Hills cloud Internal structure Interplanetary dust cloud Interplanetary medium Interplanetary space Interstellar cloud Interstellar dust Interstellar medium Interstellar space Kuiper belt List of interstellar and circumstellar molecules Merging stars Molecular cloud Nebular hypothesis Oort cloud Outer space Planetary migration Planetary system Planetesimal Planet formation Protoplanetary disk Ring system Rubble pile Sample-return mission Scattered disc Star formation
Systems	Exocomet Interstellar Exomoon Tidally detached Rogue planet Orbits Retrograde Trojan Mean-motion resonances Titius–Bode law
Host stars	A B Binary star Brown dwarfs F/Yellow-white dwarfs G/Yellow dwarfs Herbig Ae/Be K/Orange dwarfs M/Red dwarfs Pulsar Red giant Subdwarf B Subgiant T Tauri White dwarfs Yellow giants
Detection	Astrometry Direct imaging list Microlensing list Polarimetry Timing list Radial velocity list Transit method list Transit-timing variation
Habitability	Astrobiology Astrooceanography Circumstellar habitable zone Earth analog Extraterrestrial liquid water Galactic habitable zone Habitability of binary star systems Habitability of F-type main-sequence star systems Habitability of K-type main-sequence star systems Habitability of natural satellites Habitability of neutron star systems Habitability of red dwarf systems Habitability of yellow dwarf systems Habitable zone for complex life List of potentially habitable exoplanets Tholin Superhabitable planet
Catalogues	Nearby Habitable Systems Exoplanet Data Explorer Extrasolar Planets Encyclopaedia NASA Exoplanet Archive NASA Star and Exoplanet Database Open Exoplanet Catalogue
Lists	Exoplanetary systems Host stars Multiplanetary systems Stars with proto-planetary discs Exoplanets Discoveries Extremes Firsts Nearest Largest Heaviest Terrestrial candidates Kepler 1–500 501–1000 1001–1500 1501–2000 K2 Potentially habitable Proper names Discovered exoplanets by year before 2000 2000–2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024
Other	Carl Sagan Institute Exoplanet naming convention Exoplanet phase curves Exoplanetary Circumstellar Environments and Disk Explorer Extragalactic planet Extrasolar planets in fiction Geodynamics of terrestrial exoplanets Neptunian desert Nexus for Exoplanet System Science Planets in globular clusters Small planet radius gap Sudarsky's gas giant classification
Discoveries of exoplanets Search projects

• Types of planet
• Terrestrial planets
• Sub-Earth exoplanets