{{short description|Planet smaller than Earth}} [[File:Comparing the size of Earth, Mars, and exoplanets of Kepler-20 and Kepler-42.jpg|thumb|300px|Comparing the size of Earth, Mars, and exoplanets of Kepler-20 and Kepler-42]]

A '''sub-Earth''' is a planet "substantially less massive" than Earth and Venus.<ref name="STEP" /> In the Solar System, this category includes Mercury and Mars.

== Characteristics == Sub-Earth exoplanets are among the most difficult type to detect because their small sizes and masses produce the weakest signals. 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.<ref name="discovery">{{Cite journal|arxiv=1510.06387|last1=Vanderburg|first1=Andrew|title=A disintegrating minor planet transiting a white dwarf|journal=Nature|volume=526|issue=7574|pages=546–549|author2=John Asher Johnson|last3=Rappaport|first3=Saul|last4=Bieryla|first4=Allyson|last5=Irwin|first5=Jonathan|author6=John Arban Lewis|last7=Kipping|first7=David|last8= Brown|first8=Warren R.|last9=Dufour|first9=Patrick|last10= Ciardi|first10=David R.|last11=Angus|first11=Ruth|last12=Schaefer|first12=Laura|author12-link=Laura K. Schaefer|last13= Latham|first13=David W.|last14=Charbonneau|first14=David|last15=Beichman|first15=Charles|last16=Eastman|first16=Jason|last17=McCrady|first17=Nate|last18= Wittenmyer|first18=Robert A.|last19= Wright|first19=Jason T.|display-authors=4|year=2015|doi=10.1038/nature15527|pmid=26490620|bibcode = 2015Natur.526..546V |s2cid=4451207 }}</ref> 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.<ref>{{cite journal |title=Drifting asteroid fragments around WD 1145+017 |first1=S. |last1=Rappaport |first2=B. L. |last2=Gary |first3=T. |last3=Kaye |first4=A. |last4=Vanderburg |first5=B. |last5=Croll |first6=P. |last6=Benni |first7=J. |last7=Foote |journal=Monthly Notices of the Royal Astronomical Society |volume=458 |issue=4 |date=June 2016 |pages=3904–3917 |doi=10.1093/mnras/stw612 |doi-access=free |arxiv=1602.00740 |bibcode=2016MNRAS.458.3904R }}</ref>

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<sub>ⴲ</sub>. In addition, there are over 310 planet candidates with an estimated radius of <1 R<sub>ⴲ</sub>, with 135 of them being smaller than 0.8 R<sub>ⴲ</sub>.<ref name="STEP">{{cite journal |arxiv=1308.6308 |title=Below One Earth Mass: The Detection, Formation, and Properties of Subterrestrial Worlds |date=2013-08-28 |first2=B. |first3=L. |first4=E. |last1=Sinukoff |first1=E. |last2=Fulton |last3=Scuderi |last4=Gaidos |doi=10.1007/s11214-013-0019-1 |volume=180 |journal=Space Science Reviews |issue=1–4 |pages=71–99|bibcode=2013SSRv..180...71S |s2cid=255071638 }}</ref><ref>[http://exoplanetarchive.ipac.caltech.edu/ NASA Exoplanet Archive]</ref>

There is a confirmed sub-Earth orbiting Proxima Centauri, the closest star to the Sun.<ref>[https://exoplanet.eu/catalog/proxima_centauri_d--7409/ Extrasolar Planets Encyclopaedia]</ref> The mass of Proxima d is believed to be between that of Mars and Venus, over 0.26 Earth masses.<ref name="FariaSuárezMascareño2022">{{cite journal|last1=Faria|first1=J. P.|last2=Suárez Mascareño|first2=A.|last3=Figueira|first3=P.|last4=Silva|first4=A.M.|last5=Damasso|first5=M.|last6=Demangeon|first6=O.|last7=Pepe|first7=F.|last8=Santos|first8=N.C.|last9=Rebolo|first9=R.|last10=Cristiani|first10=S.|last11=Adibekyan|first11=V.|last12=Alibert|first12=Y.|last13=Allart|first13=R.|last14=Barros|first14=S.C.C.|last15=Cabral|first15=A.|last16=D’Odorico|first16=V.|last17=Di Marcantonio|first17=P.|last18=Dumusque|first18=X.|last19=Ehrenreich|first19=D.|last20=González Hernández|first20=J. I.|last21=Hara|first21=N.|last22=Lillo-Box|first22=J.|last23=Lo Curto|first23=G.|last24=Lovis|first24=C.|last25=Martins|first25=C.J.A.P.|last26=Mégevand|first26=D.|last27=Mehner|first27=A.|last28=Micela|first28=G.|last29=Molaro|first29=P.|last30=Nunes|first30=N. J.|last31=Pallé|first31=E.|last32=Poretti|first32=E.|last33=Sousa|first33=S. G.|last34=Sozzetti|first34=A.|last35=Tabernero|first35=H.|last36=Udry|first36=S.|last37=Zapatero Osorio|first37=M. R.|display-authors=4|title=A candidate short-period sub-Earth orbiting Proxima Centauri|journal=Astronomy and Astrophysics|date=February 2022|volume=658|page=A115|arxiv=2202.05188|doi=10.1051/0004-6361/202142337|doi-access=free|bibcode=2022A&A...658A.115F }}</ref>

== Deficiency == Sub-Earths commonly lack substantial atmospheres because of their low gravity and weak magnetic fields, allowing stellar radiation to wear away their atmospheres.<ref name="STEP" /> 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 == {{reflist}} {{Exoplanet}} {{Mercury (planet)}} {{Mars}}

Category:Types of planet * *