Sub-Earth

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.^[1] 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.

Space observatory Kepler opened the realm of sub-Earths by discovering them. 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 <1R_ⴲ, with 135 of them being smaller than 0.8 R_ⴲ.^[1][2]

Sub-Earths commonly lack substantial atmospheres because of their low gravity and weak magnetic fields, allowing stellar radiation to wear away their atmospheres.^[1] 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

Mercury
Geography	General Albedo features Atmosphere Features Magnetic field Regions Quadrangles Quadrangles Bach Beethoven Borealis Debussy Derain Discovery Eminescu Hokusai Kuiper Michelangelo Neruda Shakespeare Raditladi Tolstoj Victoria Mountains and volcanoes Caloris Montes Plains and plateaus Borealis Planitia Budh Planitia Caloris Planitia Odin Planitia Sobkou Planitia Suisei Planitia Tir Planitia Canyons and valleys Arecibo Catena Arecibo Vallis Goldstone Catena Goldstone Vallis Haystack Catena Haystack Vallis Simeiz Vallis Ridges and rupes Antoniadi Dorsum Adventure Rupes Beagle Rupes Discovery Rupes Enterprise Rupes Hero Rupes Resolution Rupes Santa María Rupes Victoria Rupes Basins and fossae Caloris Basin Pantheon Fossae Raditladi Basin Rembrandt Basin Skinakas (hypothetical basin) Craters Abedin Abu Nuwas Africanus Horton Ahmad Baba Ailey Aksakov Al-Akhtal Al-Hamadhani Al-Jāhiz Alencar Amaral Amru Al-Qays Andal Apollodorus Aristoxenes Aśvaghosa Atget Bach Balagtas Balanchine Balzac Bartók Barma Bashō Beckett Beethoven Bek Belinskij Bello Benoit Berkel Bernini Bjornson Boccaccio Boethius Botticelli Brahms Bramante Brontë Bruegel Brunelleschi Burns Byron Callicrates Camoes Carducci Calvino Cervantes Cézanne Chaikovskij Chao Meng-Fu Chekov Chiang K'ui Chong Ch'ol Chopin Chu Ta Coleridge Copland Copley Couperin Crater of eternal darkness Cunningham Dali Dario De Graft Debussy Degas Delacroix Derain Derzhavin Desprez Dickens Dominici Donne Dostoevskij Dowland Durer Dvorak Eastman Eitoku Eminescu Enwonwu Equiano Fet Firdousi Flaubert Futabatei Gainsborough Gauguin Geddes Ghiberti Gibran Giotto Glinka Gluck Goethe Gogol Goya Grieg Guido d'Arezzo Hals Han Kan Handel Harunobu Hauptmann Hawthorne Haydn Heine Hemingway Hesiod Hiroshige Hitomaro Hodgkins Hokusai Holbein Holberg Holst Homer Horace Hovnatanian Hugo Hun Kal Ibsen Ictinus Imhotep Ives Izquierdo Janáček Jokai Judah Ha-Levi Kalidasa Keats Kenko Kertész Khansa Kipling Kōshō Kuan Han-ch'ing Kuiper Kunisada Kurosawa Lange Leopardi Lermontov Lessing Li Ch'ing-Chao Li Po Liang K'ai Liszt Lu Hsun Lysippus Ma Chih-Yuan Machaut Mahler Mansart Mansur March Mark Twain Marti Martial Matabei Matisse Melville Mena Mendes Pinto Michelangelo Mickiewicz Milton Mistral Mofolo Molière Monet Monteverdi Moody Mozart Munch Munkácsy Murasaki Mussorgskij Myron Nampeyo Navoi Nawahi Neruda Nervo Neumann Nizami Okyo Oskison Ovid Petrarch Phidias Picasso Poe Polygnotus Praxiteles Prokofiev Qi Baishi Rachmaninoff Raden Saleh Raditladi Rameau Raphael Rembrandt Renoir Rodin Rudaki Sander Scarlatti Schubert Shakespeare Sholem Aleichem Sinan Stravinsky Sullivan Sveinsdóttir Titian To Ngoc Van Tolstoj Verdi Vivaldi Vyasa Xiao Zhao Yeats Zola Other Geology Ghost craters Inter-crater plains
Moons	Mercury's "moon"
Astronomy	Transits Earth Jupiter Mars Neptune Saturn Uranus Venus Asteroids Mercury-crossing asteroids
Exploration	Current and past Mariner 10 (1973–1975) MESSENGER (2004–2015) Planned BepiColombo (2017) Proposed Mercury-P (~2031) See also Colonization of Mercury
Related	Fiction Sub-Earth
Book Category Portal