View source for Astronomy:Mega-Earth

{{short description|Terrestrial planet with ten times the mass of Earth}}
[[File:Exoplanet Comparison Kepler-10 c.png|300px|thumb|Size comparison of Kepler-10c, with Earth and Neptune]]
A '''mega-Earth''' is a proposed neologism for a massive terrestrial [[Astronomy:Exoplanet|exoplanet]] that is at least ten times the mass of [[Astronomy:Earth|Earth]]. Mega-Earths would be substantially more massive than [[Astronomy:Super-Earth|super-Earth]]s (terrestrial and ocean planets with masses around 5–10 Earths). The term "mega-Earth" was coined in 2014, when [[Astronomy:Kepler-10c|Kepler-10c]] was revealed to be a Neptune-mass planet with a density considerably greater than that of Earth,<ref>{{cite web|url=https://www.cfa.harvard.edu/news/2014-14|title=Astronomers Find a New Type of Planet: The "Mega-Earth"2014-14|archive-url=https://web.archive.org/web/20140602204141/https://www.cfa.harvard.edu/news/2014-14|archive-date=2 June 2014}}</ref> though it has since been determined to be a typical volatile-rich planet weighing just under half that mass.<ref name="upping"/>

==Examples==
Kepler-10c was the first exoplanet to be classified as a mega-Earth. At the time of its discovery, it was believed to have a mass around 17 times that of Earth ({{Earth mass|link=yes}}) and a radius around 2.3 times Earth's ({{Earth radius|link=yes}}), giving it a high density that implied a mainly rocky composition. However, several follow-up radial velocity studies produced different results for Kepler-10c's mass, all much below the original {{Earth mass|17}} estimate. In 2017, a more careful analysis using data from multiple different telescopes and spectrographs found that Kepler-10c is more likely around {{Earth mass|7.4}}, making it a typical volatile-rich [[Astronomy:Mini-Neptune|mini-Neptune]] and not a mega-Earth.<ref>[https://arxiv.org/abs/1707.06192 Pinning down the mass of Kepler-10c: the importance of sampling and model comparison], Vinesh Rajpaul, Lars A. Buchhave, Suzanne Aigrain, 19 July 2017</ref><ref name="upping">[https://astrobites.org/2017/08/07/the-mass-of-kepler-10c-revisited-upping-the-radial-velocities-game/ The mass of Kepler-10c revisited: upping the radial velocities game], Leonardo dos Santos, 7 August 2017, Astrobites</ref>

K2-56b, also designated BD+20594b, is a much more likely mega-Earth,<ref>{{cite conference |url=https://www.hou.usra.edu/meetings/lpsc2017/pdf/1078.pdf |title=BD+20594B: A Mega-Earth Detected in the C4 field of the Kepler K2 mission |first=P |last=Futó |year=2017 |conference=48th Lunar and Planetary Science Conference |conference-url=https://www.hou.usra.edu/meetings/lpsc2017/ |language=en |access-date=6 September 2020}}</ref> with about {{Earth mass|16}} and {{Earth radius|2.2}}. At the time of its discovery in 2016, it had the highest chance of being rocky for a planet its size, with a posterior probability that it is dense enough to be terrestrial at about 0.43. For comparison, at the time the corresponding probability for Kepler-10c was calculated as 0.1, and as 0.002 for Kepler-131b.<ref>[https://arxiv.org/abs/1601.07608 Discovery and Validation of a High-Density sub-Neptune from the K2 Mission], Néstor Espinoza, Rafael Brahm, Andrés Jordán, James S. Jenkins, Felipe Rojas, Paula Jofré, Thomas Mädler, Markus Rabus, Julio Chanamé, Blake Pantoja, Maritza G. Soto, Katie M. Morzinski, Jared R. Males, Kimberly Ward-Duong, Laird M. Close, Submitted on 28 January 2016, last revised 14 July 2016</ref>

Kepler-145b is one of the most massive planets classified as mega-Earths, with a mass of {{Earth mass|37.1}} and a radius of {{Earth radius|2.65}}, so large that it could belong to a sub-category of mega-Earths known as supermassive terrestrial planets (SMTP). It likely has an Earth-like composition of rock and iron without any volatiles. A similar mega-Earth, [[Astronomy:K2-66b|K2-66b]], has a mass of about {{Earth mass|21.3}} and a radius of about {{Earth radius|2.49}}, and orbits a subgiant star. Its composition appears to be mainly rock with a small iron core and a relatively thin steam atmosphere.<ref>{{cite conference |url=https://www.hou.usra.edu/meetings/lpsc2018/pdf/1224.pdf |title=Kepler-145b and K2-66b: A Kepler- and a K2-Mega-Earth with Different Compositional Characteristics |first=P |last=Futó |year=2018 |conference=49th Lunar and Planetary Science Conference |conference-url=https://www.hou.usra.edu/meetings/lpsc2018/ |language=en |access-date=6 September 2020}}</ref>

[[Astronomy:Kepler-277b|Kepler-277b]] and [[Astronomy:Kepler-277c|Kepler-277c]] are a pair of planets orbiting the same star, both thought to be mega-Earths with masses of about {{Earth mass|87.4}} and {{Earth mass|64.2}}, and radii of about {{Earth radius|2.92}} and {{Earth radius|3.36}}, respectively.<ref>{{cite conference |url=http://www.hou.usra.edu/meetings/lpsc2020/pdf/1055.pdf |title=Kepler-277 b: A Supermassive Terrestrial Exoplanet in the Kepler-277 Planetary System |first=P |last=Futó |year=2020 |conference=51st Lunar and Planetary Science Conference |conference-url=https://www.hou.usra.edu/meetings/lpsc2020/ |language=en |access-date=6 September 2020}}</ref>

[[Astronomy:PSR J1719−1438 b|PSR J1719−1438 b]] is the most massive mega-Earth ever known with a mass of about {{Earth mass|330}} and a radius less than {{Earth radius|4}}. PSR J1719−1438 b is a [[Astronomy:Pulsar planet|pulsar planet]] which is most likely composed largely of crystalline carbon but with a [[Physics:Density|density]] far greater than [[Chemistry:Diamond|diamond]].<ref name="Reuters">{{cite news |last=Hirschler |first=Ben |date=August 25, 2011 |title=Astronomers discover planet made of diamond |work={{wipe|Reuters}} |url=https://www.reuters.com/article/us-planet-diamond-idUSTRE77O69A20110825 |access-date=August 25, 2011}}</ref><ref>{{cite journal |last1=Bailes |first1=M. |last2=Bates |first2=S. D. |last3=Bhalerao |first3=V. |last4=Bhat |first4=N. D. R. |last5=Burgay |first5=M. |last6=Burke-Spolaor |first6=S. |last7=d'Amico |first7=N. |last8=Johnston |first8=S. |last9=Keith |first9=M. J. |last10=Kramer |first10=M. |last11=Kulkarni |first11=S. R. |display-authors=2 |date=August 25, 2011 |title=Transformation of a Star into a Planet in a Millisecond Pulsar Binary |url=https://www.science.org/doi/10.1126/science.1208890 |journal=Science |volume=333 |issue=6050 |pages=1717–1720 |arxiv=1108.5201 |bibcode=2011Sci...333.1717B |doi=10.1126/science.1208890 |pmid=21868629 |last12=Levin |first12=L. |last13=Lyne |first13=A. G. |last14=Milia |first14=S. |last15=Possenti |first15=A. |last16=Spitler |first16=L. |last17=Stappers |first17=B. |last18=Van Straten |first18=W. |s2cid=206535504}}</ref>

==See also==
* [[Astronomy:Sub-Neptune|Sub-Neptune]]
* [[Astronomy:Super-Neptune|Super-Neptune]]

==References==
<references />

===Works cited===
* {{cite conference |url=https://www.hou.usra.edu/meetings/lpsc2018/pdf/1453.pdf |title=Basic Mineralogical Models for Silicate- and Carbon-Rich Mega-Earths Considering Compositional and Geophysical Constraints |first1=P |last1=Futó |first2=A |last2=Gucsik |year=2018 |conference=49th Lunar and Planetary Science Conference |conference-url=https://www.hou.usra.edu/meetings/lpsc2018/ |language=en |access-date=6 September 2020}}{{rs?|date=July 2021}}

==Further reading==
*[https://www.nationalgeographic.com/adventure/article/140603-massive-rocky-planet-mega-earth-astronomy-science Astronomers Find "Mega-Earth," Most Massive Rocky Planet Yet], BY MARCUS WOO FOR NATIONAL GEOGRAPHIC, JUNE 5, 2014
*[https://www.newscientist.com/article/dn25663-impossibly-heavy-planet-is-the-first-mega-earth/ Impossibly heavy planet is the first 'mega-Earth'], New Scientist, 2 June 2014, By Jacob Aron
*[https://www.washingtonpost.com/national/health-science/kepler-space-telescope-spies-a-mega-earth/2014/06/02/b73cf006-e8e7-11e3-afc6-a1dd9407abcf_story.html Kepler space telescope spies a ‘Mega-Earth’], Washington Post, June 2 2014
*[https://www.space.com/26085-godzilla-mega-earth-kepler-10c-aas224.html 'Godzilla of Earths': Alien Planet 17 Times Heavier Than Our World Discovered], By Miriam Kramer June 02, 2014

{{exoplanet}}


[[Category:Terrestrial planets|*]]
[[Category:Types of planet]]
[[Category:Giant planets]]

{{Sourceattribution|Mega-Earth}}