Image: Absorption spectrum corresponding to water vapor in the exoplanet's atmosphere (blue graph) and red dwarf star spots (orange graph) compared to measurements from the James Webb telescope. Source: NASA, ESA, CSA, Joseph Olmsted (STScI).
The James Webb Space Telescope, through spectroscopic measurements, has detected water vapor in an exoplanetary system, which includes the planet GJ 486 b, located 26 light-years away in the constellation Virgo. According to NASA, astronomers have yet to figure out exactly where the water was found – on an exoplanet or on the star it orbits.
GJ 486 b refers to Earth-like exoplanets, that is, it consists of stone and metal. Its star is a red dwarf, the planet revolves around it in one and a half Earth days, being always turned to it by the same side. The average surface temperature of exoplanet GJ 486 b is 430°C; This planet is three times heavier than Earth. Relative to the earthly observer, it is oriented so that we can see how it passes through the stellar disk. Using the James Webb telescope, astronomers measured the absorption spectrum of stellar radiation observed at the moments of the planet's passage across the disk. This spectrum most of all corresponded to the absorption of stellar radiation by water vapor in the exoplanet's atmosphere.
The data obtained can be most easily interpreted as evidence of the presence of atmospheric water on exoplanet GJ 486 b. At such a temperature and proximity to the star, the atmosphere would have to evaporate due to the stellar wind and the loss of fast molecules. However, the disappearance of the atmosphere does not occur – apparently, due to the fact that it is constantly replenished due to volcanic eruptions.
Water vapor has previously been observed on gas giant exoplanets, but not on terrestrial exoplanets, so James Webb's current observation is the first of its kind.
Astronomers are interested in the presence of water on terrestrial exoplanets, since such exoplanets are theoretically habitable. (However, this does not apply to GJ 486 b, as it is too hot.) Another reason to be interested in atmospheric water is the question of whether exoplanets so close to their star can maintain a constant atmosphere, despite the strong X-ray and ultraviolet radiation.
An alternative explanation for observing the absorption spectrum of water vapor would be the detection of water on the surface of the star itself. This sounds strange, since it is believed that at the temperatures that are available on the surface of a star, no molecules can exist. However, the temperature of the red dwarf itself is quite low, and, like on the Sun, spots can occur on it, that is, areas where the surface temperature of the star is much lower than average. In the region of star spots, water vapor could exist.
To find out where exactly the signal related to water came from – from a star or from an exoplanet, one should measure the spectrum in its shorter wavelength (less than 1 micrometer) part of it. The study of radiation absorption models shows that the absorption spectra corresponding to the exoplanetary atmosphere and star spots differ significantly in the short wavelength region, as shown in the figure, but are very similar in the longer wavelength region. The James Webb team plans to do this with a different instrument, the Near Infrared Slitless Spectrometer and Camera (NIRISS).
