View ofartist (Is commonly called an artist any person exercising one of the trades or activities…) of the’exoplanet (An exoplanet, or extrasolar planet, is a planet orbiting around a…) K2-18 b, its host star, and a second (Second is the feminine of the adjective second, which comes immediately after the first or which…) planet (A planet is a celestial body orbiting around the Sun or another star in…) of planetary system (A planetary system (sometimes incorrectly called a star system) is…).
ESA/Hubble, M. Kornmesser
By his mass (The term mass is used to designate two quantities attached to one…) (between 7 and 10 terrestrial masses) and its radius (about 2.5 terrestrial radii), the exoplanet K2-18 b, discovered in 2015 by the satellite (Satellite may refer to:) Kepler, is considered a « super-earth (A super-Earth is an extrasolar planet more massive than Earth, but…) » or a « mini-Neptune ». Moreover, it receives about the same quantity (The quantity is a generic term of metrology (account, amount); a scalar,…) ofenergy (In common sense, energy designates everything that allows you to do work, make energy, etc.) of his star (A star is an autonomously light-emitting celestial object, similar to a…)a red dwarf (In astronomy, red dwarfs are the least massive stars; below, they are the…)that the Earth (Earth is the third planet in the Solar System in order of distance…) receives some Sun (The Sun (Sol in Latin, Helios or Ήλιος in Greek) is the star…).
By analyzing data (In information technology (IT), data is an elementary description, often…) obtained by the Hubble Space Telescope (The Hubble Space Telescope (in English, Hubble Space Telescope or HST) is a telescope in…) recorded during the passage of K2-18 b in front of its star, two teams had announced in 2019 the detection of steam () ofwater (Water is a ubiquitous chemical compound on Earth, essential for all…) in his atmosphere (The word atmosphere can have several meanings:): a first for a planet orbiting in the « living area (In astronomy, a habitable zone (ZH) is a region of space where the conditions are…) » of its star. This detection was based on a absorption (In optics, absorption refers to the process by which the energy of a photon is taken up…) observed towards a length (The length of an object is the distance between its two furthest extremities…) ofwave (A wave is the propagation of a disturbance producing a variation in its path…) of 1.4 micrometers. These teams had also concluded that there were significant quantities ofhydrogen (Hydrogen is a chemical element with symbol H and atomic number 1.) and D’helium (Helium is a noble gas or rare gas, practically inert. With atomic number 2, it…) and probably water clouds.
Numerical simulations in support
A team from LESIA has produced numerical simulations of the atmosphere of K2-18b using a model physical (Physics (from the Greek φυσις, nature) is etymologically the…) and coherent chemical (based on the computer program (A computer program is a list of commands telling a computer what to do. It…) Exo-REM) and assuming that the exoplanet had a chemical composition close to that of Neptune. They thus succeeded in reproducing the K2-18 b data recorded by Hubble (The Hubble Space Telescope (in English, Hubble Space Telescope or HST) is a telescope in…)the best adjustment being obtained for a proportion of heavy elements (oxygen, carbon (Carbon is a chemical element of the family of crystallogens, symbol C,…), nitrogen (Nitrogen is a chemical element of the family of pnictogens, symbol N and…)etc.) compared to hydrogen by about 200 times the solar proportion.
Flow Absorption stellar (Stellaria is a genus of herbaceous annual or perennial plants, the chickweed, of the…) by planet K2-18b as it passes in front of its star The transit spectrum calculated with the Exo-REM atmosphere model for a Neptune-like chemical composition reproduces the absorption observed by HST. In this model, this is mainly due to the methane (Methane is a hydrocarbon with the chemical formula CH4. It is the simplest compound of the…) and not steam as previously announced.
B. Bézard et al. 2022
The spectra of the passage of K2-18 b in front of its star, as calculated from this model, show that the absorption observed by Hubble is in fact mainly due to methane, even if the latter is about half as abundant as water vapor. Methane is a gas (A gas is a set of atoms or molecules very weakly bound and…) which one expects to find in abundance in the atmosphere of temperate « mini-Neptunes ».
More generally, these simulations show that the absorption at 1.4 micrometers of wavelength is not a diagnostic (The diagnosis (from the Greek δι?γνωση, diágnosi, from…) of the presence of water vapor for giant exoplanets whose temperature (Temperature is a physical quantity measured using a thermometer and…) equilibrium is less than 600 K.
To differentiate water vapor from methane, it is therefore necessary to observe parallel to other wavelengths, as the Webb Telescope launched at the end of 2021 or the European space mission Ariel planned for 2029 will be able to do.
Reference:
B. Bézard, B. Charnay, D. Blain, Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b, 2022, Nature Astronomy.
Did you like this article? Do you wish to support us ? Share it on social networks with your friends and/or comment on it, this will encourage us to publish more similar topics!
#Methane #temperate #miniNeptune
