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K2-3d
	Size comparison between the; Earth and K2-3d.
Discovery
Discovery site	Kepler Space Observatory
Discovery date	2015
Detection method	Transit
Orbital characteristics
Semi-major axis	0.2014+0.0034; −0.0033 AU
Eccentricity	0.091+0.120; −0.064
Orbital period (sidereal)	44.55603+0.00013; −0.00012 d
Inclination	89.788°+0.033°; −0.029°
Semi-amplitude	<0.39 m/s
Star	K2-3
Physical characteristics
Mean radius	1.458+0.056; −0.051 R🜨
Mass	<1.6 M🜨
Mean density	<2.1 g/cm3
Temperature	305.2+3.1; −3.2 K (32.1 °C; 89.7 °F, equilibrium)

K2-3d, also known as EPIC 201367065 d, is a confirmed exoplanet of probable mini-Neptune type orbiting the red dwarf star K2-3, and the outermost of three such planets discovered in the system. It is located 143 light-years (44 parsecs) away from Earth in the constellation of Leo.^[4]^[5] The exoplanet was found by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured. It was the first planet in the Kepler "Second Light" mission to receive the letter "d" designation for a planet. Its discovery was announced in January 2015.^[1]

Characteristics

Mass, radius, density and temperature

K2-3d is a super-Earth or a mini-Neptune, meaning it has a mass and radius bigger than Earth's, but smaller than that of the ice giants Uranus and Neptune. It has an equilibrium temperature of 305 K (32 °C; 89 °F) and a radius of 1.5^[2] to 1.6 R_🜨.^[3] The planet is likely to be a mini-Neptune, with no solid surface.^[6] While originally estimated to have a very high density,^[4] later analysis of HARPS data in 2018 constrained the mass to less than 4 M_E to a 1σ confidence,^[7] and by 2023 this upper limit has been reduced to 2 M_E.^[3] This corresponds to a relatively low density, similar to that of Neptune, suggesting a very large volatile layer and significantly reducing the potential habitability of the world.

Host star

The planet orbits a (M-type) red dwarf star named K2-3, orbited by a total of three known planets, of which K2-3d has the longest orbital period.^[1] The star has a mass of 0.60 M_☉ and a radius of 0.56 R_☉.^[5]^[1] It has a temperature of 3896 K and is about 1 billion years old.^[5] In comparison, the Sun is 4.6 billion years old^[8] and has a surface temperature of 5778 K.^[9]

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 12.168.^[10] Therefore, it is too dim to be seen with the naked eye.

Orbital statistics

K2-3d orbits its host star, which has about 6% of the Sun's luminosity, with an orbital period of 44 days and an orbital radius of about 0.2 times that of Earth (compared to the distance of Mercury from the Sun, which is about 0.38 AU).^[5]

Habitability

The planet orbits on the edge of the inner (empirical) habitable zone, a region where, with the proper atmospheric properties and pressure, liquid water may exist on the surface of the planet. However, it is very likely tidally locked to its star, with one side facing towards its star in scorching heat, and the opposite side in bitter darkness.^[11] Despite this, there is an area – the terminator line – where the surface temperatures may be comfortable enough to support liquid water. However, given that most models of the habitable zone parameters put K2-3d slightly beyond the inner edge of the habitable zone, it is likely to be too hot even at the terminator line and thus not habitable at all. Also, the stellar flux for the planet is an abnormally high 1.4 times that of Earth,^[1] which could result in surface temperatures of up to 400–500 K (127–227 °C; 260–440 °F) because of a runaway greenhouse effect.

Discovery

The planet, along with the other two known planets in the K2-3 system, was announced in early January 2015 as part of the first results from the second mission of the Kepler spacecraft. With this, it was the first multiplanetary system of the mission as well.^[1]

References

^ ^a ^b ^c ^d ^e ^f Crossfield, Ian J. M.; et al. (2015). "A Nearby M Star with Three Transiting Super-Earths Discovered by K2". The Astrophysical Journal. 804 (1). 10. arXiv:1501.03798. Bibcode:2015ApJ...804...10C. doi:10.1088/0004-637X/804/1/10.
^ ^a ^b ^c Diamond-Lowe, Hannah; Kreidberg, Laura; Harman, C. E.; Kempton, Eliza M.-R.; Rogers, Leslie A.; Joyce, Simon R. G.; Eastman, Jason D.; King, George W.; Kopparapu, Ravi; Youngblood, Allison; Kosiarek, Molly R.; Livingston, John H.; Hardegree-Ullman, Kevin K.; Crossfield, Ian J. M. (2022), "The K2-3 System Revisited: Testing Photoevaporation and Core-powered Mass Loss with Three Small Planets Spanning the Radius Valley", The Astronomical Journal, 164 (5): 172, arXiv:2207.12755, Bibcode:2022AJ....164..172D, doi:10.3847/1538-3881/ac7807, S2CID 251067119
^ ^a ^b ^c ^d ^e ^f Bonomo, A. S.; Dumusque, X.; et al. (April 2023). "Cold Jupiters and improved masses in 38 Kepler and K2 small-planet systems from 3661 high-precision HARPS-N radial velocities. No excess of cold Jupiters in small-planet systems". Astronomy & Astrophysics. arXiv:2304.05773. doi:10.1051/0004-6361/202346211. S2CID 258078829.
^ ^a ^b "K2-3 d CONFIRMED PLANET OVERVIEW PAGE". NASA Exoplanet Archive. Retrieved 2015-10-17.
^ ^a ^b ^c ^d "The Extrasolar Planet Encyclopaedia — K2-3 d". Extrasolar Planets Encyclopaedia.
^ Angus, Ruth (31 July 2014). "Most 1.6 Earth-radius planets are not rocky". Astrobites.
^ Damasso, Mario; et al. (2018). "Eyes on K2-3: A system of three likely sub-Neptunes characterized with HARPS-N and HARPS". Astronomy and Astrophysics. 615. A69. arXiv:1802.08320. Bibcode:2018A&A...615A..69D. doi:10.1051/0004-6361/201732459. S2CID 58923147.
^ Fraser Cain (16 September 2008). "How Old is the Sun?". Universe Today. Retrieved 19 February 2011.
^ Fraser Cain (15 September 2008). "Temperature of the Sun". Universe Today. Retrieved 19 February 2011.
^ Henden, A. A.; et al. (2016). "VizieR Online Data Catalog: AAVSO Photometric All Sky Survey (APASS) DR9 (Henden+, 2016)". VizieR On-line Data Catalog: II/336. Originally Published in: 2015AAS...22533616H. 2336. Bibcode:2016yCat.2336....0H.Vizier catalog entry
^ "K2-3 d (M-Warm Terran)". phl.upr.edu. July 2018. Archived from the original on 9 August 2019. Retrieved 30 June 2024.