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K2-18b

Artist's impression of K2-18b (right) orbiting red dwarf K2-18 (left). The exoplanet K2-18c is shown between them.
Discovery
Discovery site	Kepler space telescope
Discovery date	2015
Detection method	Transit
Orbital characteristics
Semi-major axis	0.15910+0.00046 −0.00047 au 21,380,000 km
Eccentricity	0.09+0.12 −0.09
Orbital period (sidereal)	32.940045±0.000100 d
Star	K2-18
Physical characteristics
Mean radius	2.610±0.087 R🜨
Mass	8.63±1.35 M🜨
Mean density	2.67+0.52 −0.47 g/cm
Surface gravity	12.43+2.17 −2.07 m/s
Temperature	265 ± 5 K (−8 ± 5 °C)

K2-18b, also known as EPIC 201912552 b, is an exoplanet orbiting the red dwarf K2-18, located 124 light-years (38 pc) away from Earth. The planet, initially discovered with the Kepler space telescope, is about eight times the mass of Earth, and is thus classified as Mini-Neptune. It has a 33-day orbit within the star's habitable zone, meaning that it receives about the same amount of starlight as the Earth receives from the Sun and could have similar conditions, which allow the existence of liquid water.

In 2019 the presence of water vapour in K2-18b's atmosphere was discovered, drawing attention to this system. Its atmosphere may have an unexpectedly small concentration of methane, although observation uncertainties preclude a definitive determination. K2-18b has been studied as a potential habitable world that, temperature aside, resembles more a gas planet like Uranus or Neptune than Earth.

Host star

K2-18 is a M dwarf of the spectral class M3V in the constellation Leo, 38.025 ± 0.079 parsecs (124.02 ± 0.26 ly) away from the Sun. The star is colder and smaller than the Sun, having a temperature of 3,457 K (3,184 °C; 5,763 °F) and a radius 45% of the Sun's, and is not visible to the naked eye. The star displays moderate stellar activity but whether it has star spots, which would tend to create false signals when a planet crosses them, is unclear. K2-18 has an additional planet inside of K2-18b's orbit, K2-18c, which may interact with K2-18b through tides.

It is estimated that 80% of all M dwarf stars have planets in their habitable zones, including the stars LHS 1140, Proxima Centauri and TRAPPIST-1. The small mass, size and low temperatures of these stars and frequent orbits of the planets make it easier to characterize the planets. On the other hand, the low luminosity of the stars can make spectroscopic analysis of planets difficult, and the stars are frequently active with flares and inhomogeneous stellar surfaces (faculae and starspots), which can produce erroneous spectral signals when investigating a planet.

Physical properties

K2-18b has a mass of 8.63±1.35 M_E. It orbits its star in 33 days, from Earth it can be seen passing in front of the star. The planet is most likely tidally locked to the star, although a spin-orbit resonance like Mercury is also possible.

The density of K2-18b is about 2.67+0.52
−0.47 g/cm, intermediate between Earth and Neptune and implying that the planet has a hydrogen-rich envelope. The planet may either be rocky with a thick envelope or have a Neptune-like composition, while a pure water planet with a thin atmosphere is less likely.

The star is 2400±600 million years old and the planet may have taken a few million years to assemble. If an ocean exists, it is probably underlaid by a high-pressure ice layer on top of a rocky core, which might destabilize the planet's climate by preventing material flows between the core and the ocean.

Possible ocean

At temperatures exceeding the critical point, liquids and gases stop being different phases and there is no longer a separation between an ocean and the atmosphere. It is unclear whether observations imply that a separate liquid ocean exists on K2-18b, and detecting such an ocean is difficult from the outside; its existence cannot be inferred or ruled out solely from the mass and radius of a planet.

The existence of a liquid ocean is improbable at K2-18b; the water under the envelope is more likely in a supercritical state. Trace gases such as hydrocarbons and ammonia can be lost from an atmosphere to an ocean if it exists; their presence may thus imply the absence of an ocean-atmosphere separation.

Atmosphere and climate

Observations with the Hubble Space Telescope have found that K2-18b has an atmosphere consisting of hydrogen. Water vapour makes up between 0.7 and 1.6% of the atmosphere, while ammonia concentrations appear to be unmeasurably low, and methane may be either present at standard quantities for this type of planet, or strongly depleted. Carbon oxides were not reported, only an upper limit to their concentrations (a few percent) has been established. The atmosphere makes up at most 6.2% of the planet's mass and its composition probably resembles that of Uranus and Neptune.

There is little evidence of hazes in the atmosphere, while evidence for water clouds, the only kind of clouds likely to form at K2-18b, is conflicting. If they exist, the clouds are most likely icy but liquid water is possible. Apart from water, ammonium chloride, sodium sulfide, potassium chloride and zinc sulfide can form clouds in the atmosphere of K2-18b, depending on the planet's properties. Most computer models expect that a temperature inversion will form at high elevation, yielding a stratosphere.

Evolution

High-energy radiation from the star, such as hard UV radiation and X-rays, is expected to heat the upper atmosphere and fill it with hydrogen formed through the photodissociation of water, thus forming an extended hydrogen-rich exosphere that can escape from the planet. The X-ray and UV fluxes that K2-18b receives from K2-18 are considerably higher than the equivalent fluxes from the Sun; the hard UV radiation flux provides enough energy to drive this exosphere to escape at a rate of about 350+400
−290 tons per second, too slow to remove the planet's atmosphere during its lifespan. Observations of decreases of Lyman alpha radiation emissions during transits of the planet may show the presence of such an exosphere; this discovery requires confirmation.

Alternative scenarios

Detecting atmospheres around planets is difficult, and several reported findings are controversial. Barclay et al. 2021 suggested that the water vapour signal may be due to stellar activity, rather than water in K2-18b's atmosphere. Bézard et al. 2020 proposed that methane may be a more significant component, making up about 3-10% while water may constitute about 5-11% of the atmosphere, and Bézard, Charnay and Blain 2022 proposed that the evidence of water is actually due to methane, although such a scenario is less probable.

Models

Climate models have been used to simulate the climate that K2-18b might have, and an intercomparison of their results for K2-18b is part of the CAMEMBERT project to simulate the climates of sub-Neptune planets. Among the climate modelling efforts made on K2-18b are:

• Charnay et al. 2021, assuming that the planet is tidally locked, found an atmosphere with weak temperature gradients and a wind system with descending air on the night side and ascending air on the day side. In the upper atmosphere, radiation absorption by methane produced an inversion layer. Clouds could only form if the atmosphere had a high metallicity; their properties strongly depended on the size of cloud particles and the composition and circulation of the atmosphere. They formed mainly at the substellar point and the terminator. If there was rainfall, it could not reach the surface; instead it evaporated to form virga. Simulations with a spin-orbit resonance did not substantially alter the cloud distribution. They also simulated the appearance of the atmosphere during stellar transits.
• Innes and Pierrehumbert 2022 conduced simulations assuming different rotation rates and concluded that except for high rotation rates, there is not a substantial temperature gradient between poles and equator. They found the existence of jet streams above the equator and at high latitudes, with weaker equatorial jets at the surface.
• Hu 2021 conducted simulations of the planet's chemistry. They concluded that the photochemistry should not be able to completely remove ammonia from the outer atmosphere and that carbon oxides and cyanide would form in the middle atmosphere, where they could be detectable. The model predicts that a sulfur haze layer could form, extending through and above the water clouds. Such a haze layer would make investigations of the planet's atmosphere much more difficult.

Habitability

Incoming stellar radiation amounts to 1368+114
−107 W/m, similar to the insolation Earth receives. K2-18b is located within or just slightly inside the habitable zone of its star, – it may be close to but fall short the runaway greenhouse threshold – and its equilibrium temperature is about 250–300 K (−23–27 °C; −10–80 °F). Whether the planet is actually habitable depends on the nature of the envelope; the deeper layers of the atmosphere may be too hot, while the water-containing layers might have temperatures and pressures suitable for the development of life.

Microorganisms from Earth can survive in hydrogen-rich atmospheres, illustrating that hydrogen is no impediment to life. However, a number of biosignature gases used to identify the presence of life are not reliable indicators when found in a hydrogen-rich atmosphere, thus different markers would be needed to identify biological activity at K2-18b. Several of these markers could be detected by the James Webb Space Telescope after a reasonable number of observations.

Discovery and research history

The planet was discovered in 2015 by the Kepler Space Telescope, and its existence was later confirmed with the Spitzer Space Telescope and through Doppler velocity techniques. Analyses of the transits ruled out that they were caused by unseen companion stars, by multiple planets or systematic errors of the observations. Early estimates of the star's radius had substantial errors, which led to incorrect planet radius estimates and the density of the planet being overestimated. The discovery of the spectroscopic signature of water vapour on K2-18b in 2019 was the first discovery of water vapour on an exoplanet that is not a Hot Jupiter and drew a lot of discussion. In the future, the planet will be observed by the James Webb Space Telescope.

K2-18b has been used as a test case for exoplanet studies. The properties of K2-18b have led to the definition of a "Hycean planet", a type of planet that has both abundant liquid water and a hydrogen envelope. Planets with such compositions were previously thought to be too hot to be habitable; findings at K2-18b instead suggest that they might be cold enough to harbour liquid water oceans conducive to life. The strong greenhouse effect of the hydrogen envelope might allow them to remain habitable even at low instellation rates.

See also

• Extraterrestrial liquid water – Liquid water naturally occurring outside Earth
• Habitability of natural satellites#In the Solar System – Measure of the potential of natural satellites to have environments hospitable to life
• Habitability of red dwarf systems – Possible factors for life around red dwarf stars
• List of potentially habitable exoplanets
• Planetary habitability – Known extent to which a planet is suitable for life

Notes

1. Observations of transiting planets rely on comparing the appearance of the planet with the appearance of the star's surface that is not covered with the planet, so variations in the star's appearance can be confused with the effects of the planet.
2. Tidal interactions are mutual interactions, mediated by gravity, between astronomical bodies that are in motion with respect of each other.
3. An envelope is an atmosphere that originated together with the planet itself from a protoplanetary disk. In gas giants, atmospheres make up the bulk of the planet's mass.
4. A Neptune-like composition implies that apart from water and rock the planet contains substantial amounts of hydrogen and helium.
5. The lack of ammonia and methane in Neptune-like exoplanet atmospheres is known as the "missing methane problem", and is an unresolved mystery as of 2021. The unusually low ammonia and methane concentrations could be due to life, photochemical processes or the freezing-out of methane.
6. Hard UV radiation means UV radiation with short wavelengths; shorter wavelengths imply a higher frequency and higher energy per photon.

References

1. ^ Benneke et al. 2019, p. 4.
2. ^ Blain, Charnay & Bézard 2021, p. 2.
3. ^ Barclay et al. 2021, p. 12.
4. Adams & Engel 2021, p. 163.
5. ^ Benneke et al. 2019, p. 1.
6. Mendex 2016, p. 5-18.
7. ^ Benneke et al. 2019, p. 5.
8. ^ Barclay et al. 2021, p. 2.
9. ^ Barclay et al. 2021, p. 10.
10. Blain, Charnay & Bézard 2021, p. 15.
11. Spohn 2015, p. 2499.
12. Ferraz-Mello & Gomes 2020, p. 9.
13. ^ Madhusudhan et al. 2020, p. 1.
14. Madhusudhan, Piette & Constantinou 2021, p. 13.
15. Charnay et al. 2021, p. 3.
16. Raymond 2011, p. 120.
17. ^ Madhusudhan et al. 2020, p. 4.
18. Madhusudhan et al. 2020, p. 5.
19. ^ Guinan & Engle 2019, p. 189.
20. Blain, Charnay & Bézard 2021, p. 5.
21. Nixon & Madhusudhan 2021, p. 3420.
22. Nixon & Madhusudhan 2021, pp. 3425–3426.
23. ^ Nixon & Madhusudhan 2021, p. 3429.
24. Pierrehumbert 2023, p. 2.
25. ^ May & Rauscher 2020, p. 9.
26. Changeat et al. 2022, p. 399.
27. Pierrehumbert 2023, p. 6.
28. ^ Madhusudhan et al. 2020, p. 6.
29. Yu et al. 2021, p. 10.
30. ^ Madhusudhan et al. 2020, p. 2.
31. Madhusudhan et al. 2021.
32. ^ Scheucher et al. 2020, p. 16.
33. ^ Blain, Charnay & Bézard 2021, p. 18.
34. Bézard, Charnay & Blain 2022, p. 537.
35. Cubillos & Blecic 2021, p. 2696.
36. Madhusudhan et al. 2020, p. 3.
37. ^ Hu 2021, p. 5.
38. ^ Blain, Charnay & Bézard 2021, p. 1.
39. Charnay et al. 2021, p. 2.
40. Blain, Charnay & Bézard 2021, p. 9.
41. Hu 2021, p. 20.
42. Bark et al. 2000, p. 859.
43. Quintanilla 2015, p. 2651.
44. ^ Santos et al. 2020, p. 1.
45. Santos et al. 2020, p. 4.
46. Santos et al. 2020, p. 3.
47. Changeat et al. 2022, p. 392.
48. Bézard, Charnay & Blain 2022, p. 538.
49. Changeat et al. 2022, p. 393.
50. Christie et al. 2022, p. 6.
51. Charnay et al. 2021, p. 4.
52. Charnay et al. 2021, pp. 4–7.
53. Charnay et al. 2021, p. 8.
54. Charnay et al. 2021, p. 12.
55. Innes & Pierrehumbert 2022, p. 5.
56. Innes & Pierrehumbert 2022, p. 20.
57. Hu 2021, p. 9.
58. Hu 2021, p. 16.
59. Hu 2021, p. 12.
60. Charnay et al. 2021, p. 1.
61. Pierrehumbert 2023, p. 1.
62. Pierrehumbert 2023, p. 7.
63. Madhusudhan, Piette & Constantinou 2021, p. 2.
64. Madhusudhan, Piette & Constantinou 2021, p. 17.
65. ^ Benneke et al. 2017, p. 1.
66. Benneke et al. 2017, p. 8.
67. Benneke et al. 2019, p. 3.
68. Christie et al. 2022, p. 3.
69. James 2021, p. 7.

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External links

• K2-18 b Confirmed Planet Overview Page, NASA Exoplanet Archive

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Discoveries	2015 BP519 US 708 EGS-zs8-1 (announced) WISE J224607.57−052635.0 1SWASP J093010.78+533859.5 ASASSN-15lh EGSY8p7 Tabby's Star 541132 Leleākūhonua VFTS 352 contact binary GW150914 (first observation of gravitational waves) GW151226
Comets	15P/Finlay C/2014 Q2 (Lovejoy) 88P/Howell C/2014 Q1 (PanSTARRS) C/2015 G2 (MASTER) C/2013 US10 (Catalina) 67P/Churyumov–Gerasimenko 10P/Tempel
Space exploration	Rosetta (orbiting comet 67P; Nov 2014 / Mar 2015) Dawn (Ceres orbit insertion; Mar 2015) MESSENGER (end of mission; Apr 2015) New Horizons (flyby of Pluto; Jul 2015) Akatsuki (Venus orbit insertion; Dec 2015)
Outer space portal Category:2014 in outer space — Category:2015 in outer space — Category:2016 in outer space

v t e 2019 in space
« 2018 2020 »
Space probe launches	Beresheet (lunar lander; Feb 2019) LightSail 2 (solar sail demonstration; Jun 2019) Chandrayaan-2 / Vikram / Pragyan (lunar obiter, lander and rover; Jul 2019)
Impact events	Kamchatka meteor (announced) 2019 MO
Selected NEOs	Asteroid close approaches 2018 XB4 2019 AS5 2016 AZ8 66391 Moshup 2019 OK 1620 Geographos 2006 QV89 2100 Ra-Shalom 2019 SU3 2019 TA7 2019 UN13
Exoplanets	AD Leonis b Beta Pictoris c DS Tucanae b Gliese 251 c Gliese 357 d Gliese 588 b c Gliese 687 c Gliese 555 b Gliese 754 b Gliese 784 b HR 5183 b Kepler-47d confirmed K2-288Bb L 1159-16 b c d LP 816-60 b LTT 1445 Ab Luyten's Star d e PDS 70c Proxima Centauri c Struve 2398 Bb Bc Tau Ceti i (hypothesized) Teegarden's Star b c V1298 Tauri b c d e Wolf 359 b c
Discoveries	2019 AQ3 ASASSN-19bt AT2019qiz GRB 190114C EPIC 204376071 GW190412 GW190521g (first-ever light from bh-bh merger) GW190814 (first-ever "mass-gap" collision) J043947.08+163415.7 K2-18b water vapor M87* imaged PSR J0030+0451 mapped PSR J0740+6620 S5-HVS1 2I/Borisov 20 moons of Saturn WD 0145+234 detection of exoasteroid disruption WD J0914+1914
Comets	C/2018 Y1 (Iwamoto) 78P/Gehrels 168P/Hergenrother 163P/NEAT 138P/Shoemaker–Levy 171P/Spahr 289P/Blanpain
Space exploration	New Horizons (encounter with 486958 Arrokoth; Dec 2018 / Jan 2019) Chang'e 4 / Yutu-2 (landing on the far side of the Moon; Jan 2019) Opportunity (end of mission; Aug 2018 / Feb 2019) Hayabusa2 (departure from 162173 Ryugu; Dec 2019)
Outer space portal Category:2018 in outer space — Category:2019 in outer space — Category:2020 in outer space

• Astronomy
• Stars
• Spaceflight
• Solar System

• Exoplanets discovered in 2015
• Transiting exoplanets
• Super-Earths in the habitable zone
• Exoplanets in the habitable zone
• Exoplanets discovered by K2
• Leo (constellation)
• Extraterrestrial water