K2-18 b

K2-18 b was identified as part of the Kepler space telescope program, one of more than 1,200 exoplanets discovered during the Second Light mission , K2 ^[8] . The discovery of K2-18b was made in 2015 in the orbit of a red dwarf star (currently known as K2-18) with a stellar spectral type M2.8 at a distance of about 124 light-years from Earth. The planet was discovered as a result of a change in the star’s light curve caused by the passage of the planet in front of the star when viewed from Earth ^{[3] [9] [10]} . The planet was designated “K2-18b,” since it was the eighteenth planet discovered during the K2 mission. The projected relatively low contrast between the planet and its host star will facilitate future observations of K2-18b. ^[9] Spectroscopic studies in 2019 showed the presence of water vapor in the atmosphere of the planet. The observation data most likely correspond to the model of a helium-hydrogen atmosphere with water clouds ^[11] .

In 2017, the Spitzer Space Telescope data confirmed that the K2-18b orbits in the habitable zone around K2-18 with a 33-day period short enough to observe the many K2-18b orbital cycles, and improved the statistical significance of the signal. This led to widespread interest and continued observations of K2-18b ^[12] .

More recent K2-18b studies using high-precision radial planet search (HARPS) and Calar Alto high resolution M-dwarf searches using Exoearths with near-infrared and optical Echelle Spectrographs (CARMENES) also identified a likely second exoplanet, K2-18c, s with a calculated mass of 5.62 ± 0.84 M _⊕ in a closer 9-day orbit ^[3] , but this additional planet has not yet been confirmed, and instead can be caused by stellar activity ^[1] .

Coordinates K2-18 in the International Celestial Reference System - right ascension 11 ^h 30 ^m 14.518 ^s , declination + 07 ° 35 ′ 18.257 ″. This point lies in the constellation Leo , but outside of its lion asterism ^[13] . At the first detection, the distance K2-18 from the Earth was estimated at 110 light years (34 pc) ^[9] . However, more accurate data from Gaia (the star mapping project) showed that K2-18 is at a distance of 124.02 ± 0.26 light years (38.025 ± 0.079 pc). This improved measurement of distance has helped to clarify the properties of the exoplanetary system ^[3] .

K2-18 b revolves around K2-18 at a distance of about 0.1429 a. e. (21.38 million km), which is located in the habitable zone of the red dwarf, 0.12-0.25 a. e. (18-37 million km) ^[7] . The exoplanet has an orbital period of about 33 days ^[12] , which suggests that it is always turned to the star on one side ^[14] . According to estimates, the equilibrium temperature of the planet is about 265 ± 5K (-8 ± 5 ° C) ^[3] , it receives 5% more light than the Earth ^[2] . K2-18b is estimated to have a radius of 2.71 ± 0.07 R _⊕ and a mass of 8.63 ± 1.35 M _⊕ , based on analysis using the HARPS and CARMENES tools ^[3] . The researchers found that the average density of the planet is 2.4 ± 0.4 g / cm³. Most likely, it consists mainly of water and has a thick hydrogen-helium atmosphere ^[2] , which classifies it as mini-neptunes . According to the spectra obtained by two groups of researchers, the planet is surrounded by an atmosphere consisting mainly of hydrogen and helium ^{[4] [15]} .

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  The artist's idea of ​​the star system K2-18

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  A diagram of the planetary system K2-18, showing the orbits of K2-18 b and the unconfirmed candidate K2-18 c, as well as the star’s habitat

Water Discovery

In further studies using the Hubble Space Telescope , the results of observations by Kepler and Spitzer were confirmed and an additional study of the planet's atmosphere was carried out. Two separate Hubble data analyzes were published in 2019 under the supervision of researchers from the University of Montreal and University College London (UCL). Both studied the spectra of starlight passing through the planet’s atmosphere during transit, and found that K2-18 b has a helium-hydrogen atmosphere with a high concentration of water vapor from 20% to 50%, high enough to form clouds ^{[6] [7] [16]} . The UCL-led study was published on September 11, 2019 in the journal Nature Astronomy ; a study conducted by the University of Montreal and not yet reviewed, was published a day earlier on the arXiv.org preprint server ^[14] . An analysis of the group led by UCL found water with a statistical significance of 3.6 standard deviations, which is equivalent to a confidence level of 99.97% ^[7] .

This was the first super-earth in the habitable zone with a detected atmosphere ^[7] and the first discovery of water on an exoplanet in the habitable zone ^{[5] [6]} . Water was previously detected in the atmospheres of non-residential exoplanets, such as HD 209458 b , XO-1 b , WASP-12 b , WASP-17 b and WASP-19 b ^{[17] [18] [19]} .

Astronomers emphasized that the discovery of water in the atmosphere of K2-18 b does not mean that the planet can sustain life or even be habitable, because it probably lacks a solid surface or atmosphere that can support life. Nevertheless, the presence of water in the habitable zone of the exoplanet helps to understand how the planets form ^[5] . K2-18 b is expected to be observed with the James Webb Space Telescope , which is due to be launched in 2021, and the ARIEL Space Telescope, which is due to be launched in 2028. Both will carry instruments designed to determine the composition of the atmospheres of exoplanets ^[6] .

• Water vapor - and maybe even rain - found on distant world twice the size of Earth