Water vapor has been found in the atmosphere of a very hot Saturn-like extrasolar planet 250 light years away

To discover atmospheric signatures from the planet, the team used a technique called transmission spectroscopy. When a planet passes in front of its host star relative to an observer on Earth, some of the star's light passes through the planet's atmosphere, is absorbed into it, and its spectrum changes

In a new study, researchers have detected water vapor in the atmosphere of HD 149026 b, a hot Saturn-like exoplanet located 250 light-years away in the Hercules group. This hot gas giant, which is similar in size to Saturn, orbits its host star at a very close distance.

It orbits an advanced, metal-rich star, HD 149026, which is nearly ten times closer than Saturn's orbit around the Sun, resulting in a year of only about 2.9 days. This proximity causes the temperatures of such hot gas giants to skyrocket beyond 1500 Kelvin. Specifically, the equilibrium temperature of HD 149026 b is about 1700 Kelvin, hot enough to melt even the strongest steel.

To discover atmospheric signatures from the planet, the team used a technique called transmission spectroscopy. When a planet passes in front of its host star relative to an observer on Earth, some of the star's light passes through the planet's atmosphere.

The light from this star is absorbed by various gases in the atmosphere, creating an absorption spectrum of the planet that is superimposed on the spectrum of the star. By separating the two spectra, for example by subtracting the spectrum observed outside the transit (where there is no atmospheric absorption of the planet), the atmospheric signatures of the planet can be identified.

One of the biggest difficulties in observing the atmospheres of extrasolar planets is the very high contrast between the bright star and the dim planet. This makes it difficult to detect the planet's atmospheric signatures, which are often buried beneath the star's photon noise.

The strength of the planet's signatures will be stronger if we observe planets with higher temperatures (the atmospheres are more extensive and easier to detect), closer distances to their host star (the spectra of the planet and the star are easier to separate) or a combination of both. Hot gas giants have both of these properties, making them ideal targets for transmission spectroscopic observations, although their atmospheric signatures are still difficult to detect.

for the scientific article

More of the topic in Hayadan:

• First measurement of isotopes in the atmosphere of an extrasolar planet
• The Webb Space Telescope offers the first glimpse into the interior of an extrasolar planet
• Was a moon discovered outside the solar system for the first time?
• Earth number 2 forms around a star 424 light years away
• 12 new planets in a week