Mini-Neptune

Last updated
Artist's conception of a mini-Neptune or "gas dwarf" Gasdwarf.jpg
Artist's conception of a mini-Neptune or "gas dwarf"

A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen-helium atmosphere, probably with deep layers of ice, rock or liquid oceans (made of water, ammonia, a mixture of both, or heavier volatiles). [1]

Contents

A gas dwarf is a gas planet with a rocky core that has accumulated a thick envelope of hydrogen, helium, and other volatiles, having, as a result, a total radius between 1.7 and 3.9 Earth radii (1.7–3.9 R🜨). The term is used in a three-tier, metallicity-based classification regime for short-period exoplanets, which also includes the rocky, terrestrial-like planets with less than 1.7 R🜨 and planets greater than 3.9 R🜨, namely ice giants and gas giants. [2]

Properties

Theoretical studies of such planets are loosely based on knowledge about Uranus and Neptune. Without a thick atmosphere, they would be classified as an ocean planet instead. [3] An estimated dividing line between a rocky planet and a gaseous planet is around 1.6–2.0 Earth radii. [4] [5] Planets with larger radii and measured masses are mostly low-density and require an extended atmosphere to simultaneously explain their masses and radii, and observations show that planets larger than approximately 1.6 Earth-radius (and more massive than approximately 6 Earth-masses) contain significant amounts of volatiles or H–He gas, likely acquired during formation. [6] [1] Such planets appear to have a diversity of compositions that is not well-explained by a single mass–radius relation as that found for denser, rocky planets. [7] [8] [9] [10] [11] [12]

The lower limit for mass can vary widely for different planets depending on their compositions; the dividing mass can vary from as low as one to as high as 20 ME. Smaller gas planets and planets closer to their star will lose atmospheric mass more quickly via hydrodynamic escape than larger planets and planets farther out. [13] [14] [15] A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature. [16]

Neptune-like planets are considerably rarer than sub-Neptunes, despite being only slightly bigger. [17] [18] This "radius cliff" separates sub-Neptunes (radius < 3 Earth radii) from Neptunes (radius > 3 Earth radii). [17] This is thought to arise because, during formation when gas is accreting, the atmospheres of planets of that size reach the pressures required to force the hydrogen into the magma ocean, stalling radius growth. Then, once the magma ocean saturates, radius growth can continue. However, planets that have enough gas to reach saturation are much rarer, because they require much more gas. [17]

Examples

The smallest known extrasolar planet that might be a gas dwarf is Kepler-138d, which is less massive than Earth but has a 60% larger volume and therefore has a density 2.1+2.2
−1.2 g/cm3 that indicates either a substantial water content [19] or possibly a thick gas envelope. [20] However, more recent evidence suggests that it may be more dense than previously thought, and could be an ocean planet instead. [21]

See also

Related Research Articles

<span class="mw-page-title-main">Giant planet</span> Planet much larger than the Earth

A giant planet, sometimes referred to as a jovian planet, is a diverse type of planet much larger than Earth. Giant planets are usually primarily composed of low-boiling point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. There are four such planets in the Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extrasolar giant planets have been identified.

<span class="mw-page-title-main">Hot Jupiter</span> Class of high mass planets orbiting close to a star

Hot Jupiters are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods. The close proximity to their stars and high surface-atmosphere temperatures resulted in their informal name "hot Jupiters".

<span class="mw-page-title-main">Super-Earth</span> Type of exoplanet

Super earths

This page describes exoplanet orbital and physical parameters.

<span class="mw-page-title-main">Kepler-11</span> Sun-like star in the constellation Cygnus

Kepler-11, also designated as 2MASS J19482762+4154328, is a Sun-like star slightly larger than the Sun in the constellation Cygnus, located some 2,110 light years from Earth. It is located within the field of vision of the Kepler space telescope, the satellite that NASA's Kepler Mission uses to detect planets that may be transiting their stars. Announced on February 2, 2011, the star system is among the most compact and flattest systems yet discovered. It is the first discovered case of a star system with six transiting planets. All discovered planets are larger than Earth, with the larger ones being about Neptune's size.

Kepler-33 is a star about 4,000 light-years in the constellation of Cygnus, with a system of five known planets. Having just begun to evolve off from the main sequence, its radius and mass are difficult to ascertain, although data available in 2020 shows its best-fit mass of 1.3M☉ and radius of 1.6R☉ are compatible with a model of a subgiant star.

Kepler-80, also known as KOI-500, is a red dwarf star of the spectral type M0V. This stellar classification places Kepler-80 among the very common, cool, class M stars that are still within their main evolutionary stage, known as the main sequence. Kepler-80, like other red dwarf stars, is smaller than the Sun, and it has both radius, mass, temperatures, and luminosity lower than that of our own star. Kepler-80 is found approximately 1,223 light years from the Solar System, in the stellar constellation Cygnus, also known as the Swan.

Kepler-68 is a Sun-like main sequence star located 471 light-years away in the constellation Cygnus. It is known to have at least four planets orbiting around it. The third planet has a mass similar to Jupiter but orbits within the habitable zone.

Kepler-68b is an exoplanet orbiting the Sun-like star Kepler-68 in the constellation of Cygnus. Discovered by planetary-transit methods by the Kepler space telescope in February 2013, it has a radius of 2.31 ± 0.07 that of Earth and a density of 2.46–4.3 g/cm3. It has an orbital period of 5.398763 days at a distance of about 0.0617 AU from its star. Doppler measurements were made to determine its mass to be 5.79 times that of Earth.

<span class="mw-page-title-main">Kepler-138</span> Red dwarf in the constellation Lyra

Kepler-138, also known as KOI-314, is a red dwarf located in the constellation Lyra, 219 light years from Earth. It is located within the field of vision of the Kepler spacecraft, the satellite that NASA's Kepler Mission used to detect planets transiting their stars.

<span class="mw-page-title-main">Gas giant</span> Giant planet mainly composed of light elements

A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranus and Neptune are really a distinct class of giant planets, being composed mainly of heavier volatile substances. For this reason, Uranus and Neptune are now often classified in the separate category of ice giants.

Kepler-89e, also known as KOI-94e, is an exoplanet in the constellation of Cygnus. It orbits Kepler-89.

BD+ 20° 594b is a massive exoplanet discovered by the Kepler spacecraft in collaboration with the HARPS spectrometer at La Silla in Chile.

The small planet radius gap is an observed scarcity of planets with radii between 1.5 and 2 times Earth's radius, likely due to photoevaporation-driven mass loss. A bimodality in the Kepler exoplanet population was first observed in 2011 and attributed to the absence of significant gas atmospheres on close-in, low-mass planets. This feature was noted as possibly confirming an emerging hypothesis that photoevaporation could drive atmospheric mass loss This would lead to a population of bare, rocky cores with smaller radii at small separations from their parent stars, and planets with thick hydrogen- and helium-dominated envelopes with larger radii at larger separations. The bimodality in the distribution was confirmed with higher-precision data in the California-Kepler Survey in 2017, which was shown to match the predictions of the photoevaporative mass-loss hypothesis later that year.

Kepler-160 is a main-sequence star approximately the width of our Galactic arm away in the constellation Lyra, first studied in detail by the Kepler Mission, a NASA-led operation tasked with discovering terrestrial planets. The star, which is very similar to the Sun in mass and radius, has three confirmed planets and one unconfirmed planet orbiting it.

<span class="mw-page-title-main">Kepler-93b</span> Super-Earth exoplanet in constellation Lyra

Kepler-93b (KOI-69b) is a hot, dense transiting Super-Earth exoplanet located approximately 313 light-years away in the constellation of Lyra, orbiting the G-type star Kepler-93. Its discovery was announced in February 2014 by American astronomer Geoffrey Marcy and his team. In July 2014, its radius was determined with a mere 1.3% margin of error, the most precise measurement ever made for an exoplanet's radius at the time.

References

  1. 1 2 D'Angelo, G.; Bodenheimer, P. (2016). "In Situ and Ex Situ Formation Models of Kepler 11 Planets". The Astrophysical Journal. 828 (1): id. 33. arXiv: 1606.08088 . Bibcode:2016ApJ...828...33D. doi: 10.3847/0004-637X/828/1/33 . S2CID   119203398.
  2. ↑ Three regimes of extrasolar planets inferred from host star metallicities, Buchhave et al.
  3. ↑ De Mooij, E. J. W.; Brogi, M.; De Kok, R. J.; Koppenhoefer, J.; Nefs, S. V.; Snellen, I. A. G.; Greiner, J.; Hanse, J.; Heinsbroek, R. C.; Lee, C. H.; Van Der Werf, P. P. (2012). "Optical to near-infrared transit observations of super-Earth GJ 1214b: Water-world or mini-Neptune?". Astronomy & Astrophysics. 538: A46. arXiv: 1111.2628 . Bibcode:2012A&A...538A..46D. doi:10.1051/0004-6361/201117205.
  4. ↑ Fabrycky, Daniel C.; Lissauer, Jack J.; Ragozzine, Darin; Rowe, Jason F.; Steffen, Jason H.; Agol, Eric; Barclay, Thomas; Batalha, Natalie; Borucki, William; Ciardi, David R.; Ford, Eric B.; Gautier, Thomas N.; Geary, John C.; Holman, Matthew J.; Jenkins, Jon M.; Li, Jie; Morehead, Robert C.; Morris, Robert L.; Shporer, Avi; Smith, Jeffrey C.; Still, Martin; Van Cleve, Jeffrey (2014). "ARCHITECTURE OF KEPLER 'S MULTI-TRANSITING SYSTEMS. II. NEW INVESTIGATIONS WITH TWICE AS MANY CANDIDATES". The Astrophysical Journal. 790 (2): 146. arXiv: 1202.6328 . Bibcode:2014ApJ...790..146F. doi:10.1088/0004-637X/790/2/146.
  5. ↑ When Does an Exoplanet's Surface Become Earth-Like?, blogs.scientificamerican.com, 20 June 2012
  6. ↑ D'Angelo, G.; Bodenheimer, P. (2013). "Three-Dimensional Radiation-Hydrodynamics Calculations of the Envelopes of Young Planets Embedded in Protoplanetary Disks". The Astrophysical Journal . 778 (1): 77 (29 pp.). arXiv: 1310.2211 . Bibcode:2013ApJ...778...77D. doi:10.1088/0004-637X/778/1/77. S2CID   118522228.
  7. ↑ Fulton, Benjamin J.; Petigura, Erik A.; Howard, Andrew W.; Isaacson, Howard; Marcy, Geoffrey W.; Cargile, Phillip A.; Hebb, Leslie; Weiss, Lauren M.; Johnson, John Asher; Morton, Timothy D.; Sinukoff, Evan; Crossfield, Ian J. M.; Hirsch, Lea A. (2017). "The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets". The Astronomical Journal. 154 (3): 109. arXiv: 1703.10375 . Bibcode:2017AJ....154..109F. doi: 10.3847/1538-3881/aa80eb .
  8. ↑ Dressing, Courtney D.; et al. (2015). "THE MASS OF Kepler-93b AND THE COMPOSITION OF TERRESTRIAL PLANETS". The Astrophysical Journal. 800 (2): 135. arXiv: 1412.8687 . Bibcode:2015ApJ...800..135D. doi:10.1088/0004-637X/800/2/135.
  9. ↑ Rogers, Leslie A. (2015). "MOST 1.6 EARTH-RADIUS PLANETS ARE NOT ROCKY". The Astrophysical Journal. 801 (1): 41. arXiv: 1407.4457 . Bibcode:2015ApJ...801...41R. doi:10.1088/0004-637X/801/1/41.
  10. ↑ Weiss, Lauren M.; Marcy, Geoffrey W. (2014). "The Mass-Radius Relation for 65 Exoplanets Smaller Than 4 Earth Radii". The Astrophysical Journal. 783 (1): L6. arXiv: 1312.0936 . Bibcode:2014ApJ...783L...6W. doi:10.1088/2041-8205/783/1/L6.
  11. ↑ Marcy, Geoffrey W.; Weiss, Lauren M.; Petigura, Erik A.; Isaacson, Howard; Howard, Andrew W.; Buchhave, Lars A. (2014). "Occurrence and core-envelope structure of 1–4× Earth-size planets around Sun-like stars". Proceedings of the National Academy of Sciences. 111 (35): 12655–12660. arXiv: 1404.2960 . Bibcode:2014PNAS..11112655M. doi: 10.1073/pnas.1304197111 . PMID   24912169.
  12. ↑ Marcy, Geoffrey W.; et al. (2014). "MASSES, RADII, AND ORBITS OF SMALL KEPLER PLANETS: THE TRANSITION FROM GASEOUS TO ROCKY PLANETS". The Astrophysical Journal Supplement Series. 210 (2): 20. arXiv: 1401.4195 . Bibcode:2014ApJS..210...20M. doi:10.1088/0067-0049/210/2/20.
  13. ↑ Feng Tian; Toon, Owen B.; Pavlov, Alexander A.; De Sterck, H. (March 10, 2005). "Transonic hydrodynamic escape of hydrogen from extrasolar planetary atmospheres". The Astrophysical Journal. 621 (2): 1049–1060. Bibcode:2005ApJ...621.1049T. CiteSeerX   10.1.1.122.9085 . doi:10.1086/427204. S2CID   6475341.
  14. ↑ Swift, D. C.; Eggert, J. H.; Hicks, D. G.; Hamel, S.; Caspersen, K.; Schwegler, E.; Collins, G. W.; Nettelmann, N.; Ackland, G. J. (2012). "Mass-Radius Relationships for Exoplanets". The Astrophysical Journal. 744 (1): 59. arXiv: 1001.4851 . Bibcode:2012ApJ...744...59S. doi:10.1088/0004-637X/744/1/59.
  15. ↑ Martinez, Cintia F.; Cunha, Katia; Ghezzi, Luan; Smith, Verne V. (2019-04-10). "A Spectroscopic Analysis of the California-Kepler Survey Sample. I. Stellar Parameters, Planetary Radii, and a Slope in the Radius Gap". The Astrophysical Journal. 875 (1). American Astronomical Society: 29. arXiv: 1903.00174 . Bibcode:2019ApJ...875...29M. doi: 10.3847/1538-4357/ab0d93 . hdl: 10150/633733 .
  16. ↑ Batygin, Konstantin; Stevenson, David J. (2013). "Mass-Radius Relationships for Very Low Mass Gaseous Planets". The Astrophysical Journal. 769 (1): L9. arXiv: 1304.5157 . Bibcode:2013ApJ...769L...9B. doi:10.1088/2041-8205/769/1/L9.
  17. 1 2 3 "Why are there so many sub-Neptune exoplanets?". 17 December 2019.
  18. ↑ Kite, Edwin S.; Bruce Fegley Jr.; Schaefer, Laura; Ford, Eric B. (2019). "Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis". The Astrophysical Journal Letters. 887 (2): L33. arXiv: 1912.02701 . Bibcode:2019ApJ...887L..33K. doi: 10.3847/2041-8213/ab59d9 .
  19. ↑ Jontof-Hutter, D; Rowe, J; et al. (18 June 2015). "Mass of the Mars-sized Exoplanet Kepler-138b from Transit Timing". Nature. 522 (7556): 321–323. arXiv: 1506.07067 . Bibcode:2015Natur.522..321J. doi:10.1038/nature14494. PMID   26085271. S2CID   205243944.
  20. ↑ Earth-mass exoplanet is no Earth twin – Gaseous planet challenges assumption that Earth-mass planets should be rocky
  21. ↑ Timmer, John (15 December 2022). "Scientists may have found the first water worlds". Ars Technica . Retrieved 17 December 2022.

Further reading

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.