K2-138, also designated EPIC 245950175 or EE-1,[5] is a large early K-type main sequence star with a system of at least 6 planets discovered by citizen scientists. Four were found in the first two days of the Exoplanet Explorers project on Zooniverse in early April 2017, while two more were revealed in further analysis. The system is about 660 light-years (200 parsecs) away in the constellation Aquarius, within K2 Campaign 12.
K2-138 is notable for its large number of planets, all found through the efforts of citizen scientists. They are designated K2-138b, c, d, e, f, and g in order from their host star. The first five were validated by Christiansen et al., while K2-138g was noted as being a likely candidate. However, since there were only two transits of it, K2-138g could not be validated. There was a possibility that the two transits for this candidate were from two individual long-period planets.[2] K2-138g was confirmed by follow-up studies in 2019 and 2021.[4][6]
All six planets are within the super-Earth and mini-Neptune categories, with radii between about 1.6 REarth to 3.3 REarth. The outer five, including K2-138g, are likely small gaseous worlds with no solid surface. However, the smaller K2-138b could be rocky. The masses of the planets were initially unknown, as the data for K2-138 did not have a high enough signal-to-noise ratio for transit-timing variation (TTV) analysis. However, the Spitzer Space Telescope could be able to accurately detect TTVs and lead to the masses of the planets being calculated. Planets b through f are predicted to cause TTVs on the order of 2.5 to 7.1 minutes, for predicted masses between 4 MEarth and 7 MEarth.[2]
The five validated planets of K2-138 are very close to the parent star and form an unbroken chain of near-3:2 resonances. Their orbital periods range from 2.35 to 12.76 days, with the sixth planet orbiting much further out with a period of about 41 days. K2-138b, c, d, e, and f are locked in several chains of three-body resonances, a feat shared by only a handful of systems, including TRAPPIST-1 and Kepler-80. Like the former, K2-138 could show the end result of slow, inward disk migration.[2]
Spitzer observations of K2-138g were announced on the AAS Meeting #233. The iPoster shows an updated radius of K2-138g of 3.7 REarth, making it the largest planet in the system. This result was preliminary[7][8] until being confirmed in February 2021.[6]
A team of astronomers collected 215 spectra over 79 nights with the instrument HARPS mounted on the ESO 3.6 m Telescope. With a Bayesian analysis of the K2 photometry and HARPS radial-velocities (RVs) the team were able to constrain the mass of planet b to e. The bulk densities of the planets range from Earth-like density for planet b to Neptune-like density for planet e. The masses and densities constrain the composition of the planets. They have likely rocky cores and a substantial atmospheric layer, composed of volatiles. For planets f and g this team was able to constrain the upper limit of the mass to 8.7 and 25.5 earth masses.[4]
A paper by Acuña et al. studied the water content of the K2-138 system, assuming a volatile layer constituted of water in steam and supercritical phases.[9] They find that the planet b has an upper water-mass-fraction of 0.7% and is a volatile-poor planet. Planet b could have formed with a thick water atmosphere that was blown away by XUV-radiation coming from the host star. A process called photoevaporation.
Planet f is possibly the most water-rich planet in the system, with an upper water-mass-fraction of 66%. The radius of planet g is larger than a planet with a water-rich composition and the researchers conclude that planet g has an atmosphere rich in hydrogen and helium and in this case the upper volatile-mass fraction would be only 5%. All planets of the system likely have a less massive core compared to earth.[9]
K2-138 was selected as a target by ESA in the first Announcement of Opportunity (AO-1) Program of the CHEOPS mission, which was launched in December 2019. For 87.6 orbits the spacecraft will record the transits to measure TTVs of the planets.[10] K2-138 could become a benchmark system to compare RV and TTV masses. The system is also a good candidate to search for co-orbital bodies, which are predicted to exist and to be stable in resonant chain systems like K2-138.[4]
55 Cancri e is an exoplanet in the orbit of its Sun-like host star 55 Cancri A. The mass of the exoplanet is about 8.63 Earth masses and its diameter is about twice that of the Earth, thus making it the first super-Earth discovered around a main sequence star, predating Gliese 876 d by a year. It takes fewer than 18 hours to complete an orbit and is the innermost-known planet in its planetary system. 55 Cancri e was discovered on 30 August 2004. However, until the 2010 observations and recalculations, this planet had been thought to take about 2.8 days to orbit the star. In October 2012, it was announced that 55 Cancri e could be a carbon planet.
Gliese 436 b is a Neptune-sized exoplanet orbiting the red dwarf Gliese 436. It was the first hot Neptune discovered with certainty and was among the smallest-known transiting planets in mass and radius, until the much smaller Kepler exoplanet discoveries began circa 2010.
A super-Earth is an exoplanet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to the mass of the planet, and so does not imply anything about the surface conditions or habitability. The alternative term "gas dwarfs" may be more accurate for those at the higher end of the mass scale, although "mini-Neptunes" is a more common term.
A Mini-Neptune 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.
K2-3, also known as EPIC 201367065, is a red dwarf with three known planets. All are Super-Earths, and the outermost is in the habitable zone. It is on the borderline of being a late orange dwarf/K-type star, but because of its temperature, it is classified as a red dwarf.
HD 219134 b is one of at least five exoplanets orbiting HD 219134, a main-sequence star in the constellation of Cassiopeia. HD 219134 b has a size of about 1.6 REarth, and a density of 6.4 g/cm3 and orbits at 21.25 light-years away. The exoplanet was initially detected by the instrument HARPS-N of the Italian Telescopio Nazionale Galileo via the radial velocity method and subsequently observed by the Spitzer telescope as transiting in front of its star. The exoplanet has a mass of about 4.5 times that of Earth and orbits its host star every three days. In 2017, it was found that the planet likely hosts an atmosphere.
GJ 9827 is a star in the constellation of Pisces. It is a K-type main-sequence star with an apparent magnitude of 10.250. It is 97 light-years away, based on parallax.
K2-138b is a potentially rocky Super-Earth exoplanet orbiting every 2 days around a K1V star. The planet, along with the four others in the system, was found by citizen scientists of the Exoplanet Explorers project on Zooniverse. It was the final planet found in the system and was officially announced on January 8, 2018.
K2-38b, also designated EPIC 204221263 b, is a massive rocky exoplanet closely orbiting a Sun-like star and is one of the densest planets ever found. Discovered in 2016 by Crossfield et al. and later characterized by Sinukoff et al., K2-38b is a rocky super-Earth about 55% larger than Earth but about 12 times more massive indicating a composition rich in iron and an extremely high surface gravity. The planet is within K2 Campaign 2, in the constellation Scorpius.
K2-187, also known as EPIC 212157262, is a Sun-like star in K2 Campaign 5. It is very close in size and temperature to the Sun, and has a system of four confirmed exoplanets ranging between 1.4 REarth and 3.2 REarth. The innermost planet takes just 18 hours to orbit its star, while the outermost planet orbits every 2 weeks.
HD 3167 is a single, orange-hued star in the zodiac constellation of Pisces that hosts a system with three exoplanets. The star is too faint to be seen with the naked eye, having an apparent visual magnitude of 8.97. The distance to HD 3167 can be determined from its annual parallax shift of 21.1363 mas as measured by the Gaia space observatory, yielding a range of 154 light years. It has a relatively high proper motion, traversing the celestial sphere at the rate of 0.204″ per year. Since it was first photographed during the Palomar observatory sky survey in 1953, it had moved over 12.5″ by 2017. The star is moving away from the Earth with an average heliocentric radial velocity of +19.5 km/s.
TOI-1227 b is one of the youngest transiting exoplanets discovered, alongside K2-33b and HIP 67522 b. The exoplanet TOI-1227 b is about 11±2 Myrs old and currently 9.5 REarth large. It will become a 3-5 REarth planet in about 1 billion years, because the planet is still contracting. TOI-1227 b orbits a very low-mass star every 27.36 days.
1 2 3 4 5 Lopez, T. A.; Barros, S. C. C.; Santerne, A.; Deleuil, M.; Adibekyan, V.; Almenara, J.-M.; Armstrong, D. J.; Brugger, B.; Barrado, D.; Bayliss, D.; Boisse, I.; Bonomo, A. S.; Bouchy, F.; Brown, D. J. A.; Carli, E.; Demangeon, O.; Dumusque, X.; Díaz, R. F.; Faria, J. P.; Figueira, P.; Foxell, E.; Giles, H.; Hébrard, G.; Hojjatpanah, S.; Kirk, J.; Lillo-Box, J.; Lovis, C.; Mousis, O.; da Nóbrega, H. J.; Nielsen, L. D.; Neal, J. J.; Osborn, H. P.; Pepe, F.; Pollacco, D.; Santos, N. C.; Sousa, S. G.; Udry, S.; Vigan, A.; Wheatley, P. J. (2019-11-01). "Exoplanet characterisation in the longest known resonant chain: the K2-138 system seen by HARPS". Astronomy & Astrophysics. 631: A90. arXiv:1909.13527. Bibcode:2019A&A...631A..90L. doi:10.1051/0004-6361/201936267. ISSN0004-6361. S2CID203593804.
1 2 Hardegree-Ullman, Kevin K.; Christiansen, Jessie L.; Ciardi, David R.; Crossfield, Ian J. M.; Dressing, Courtney D.; Livingston, John H.; Volk, Kathryn; Agol, Eric; Barclay, Thomas; Barentsen, Geert; Benneke, Björn; Gorjian, Varoujan; Kristiansen, Martti H. (2021), "K2-138 g: Spitzer Spots a Sixth Planet for the Citizen Science System", The Astronomical Journal, 161 (5): 219, arXiv:2102.08978, Bibcode:2021AJ....161..219H, doi:10.3847/1538-3881/abeab0, S2CID231951815
↑ Hardegree-Ullman, Kevin; Christiansen, Jessie (January 2019). "K2-138 g: Spitzer Spots a Sixth Sub-Neptune for the Citizen Science System". American Astronomical Society Meeting Abstracts #233. 233: 164.07. Bibcode:2019AAS...23316407H.
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