Water Vapor Found in Atmosphere of Ultra-Hot “Saturn” Exoplanet 250 Light Years Away

Water vapor has been detected in the atmosphere of a hot Saturn.

New research has identified water vapor in the atmosphere of HD 149026 b, a hot Saturn-like exoplanet located approximately 250 light years away in the constellation of Hercules. Similar in size to Saturn, this hot gas giant orbits its host star at an extremely close distance.

It orbits a metal-rich evolved star, HD 149026, nearly 10 times closer than Mercury’s orbit around the Sun, resulting in a year that lasts only about 2.9 days. This proximity causes the temperatures of such hot gas giants to soar above 1500 Kelvin. Specifically, HD 149026 b has an equilibrium temperature of approximately 1700 Kelvin, hot enough to melt even the strongest steel.

Transmission Spectroscopy and Atmospheric Detection

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

This starlight is absorbed by various gases in the atmosphere, creating a planetary absorption spectrum that is imprinted on the stellar spectrum. By separating the stellar spectrum from the planetary spectrum, such as by subtracting the spectrum observed outside of transit (where there’s no atmospheric absorption from the planet), the atmospheric signatures of the planet can be identified.

Challenges in Observing Exoplanetary Atmospheres

One major challenge in observing exoplanetary atmospheres is the extremely high contrast between the bright star and the dim planet. This makes the planet’s atmospheric signatures difficult to detect, often buried below the stellar photon noise.

The strength of the planet’s signatures would be stronger if we observed planets with either higher temperatures (resulting in more extended atmospheres and easier detection), closer distances to their host stars (making it easier to separate the stellar and planet spectra), or a combination of both. Hot gas giants possess both these properties, making them ideal targets for transmission spectroscopy observations, though their atmospheric signatures remain challenging to detect.

Reference: “Evidence of Water Vapor in the Atmosphere of a Metal-rich Hot Saturn with High-resolution Transmission Spectroscopy” by Sayyed A. Rafi, Stevanus K. Nugroho, Motohide Tamura, Lisa Nortmann and Alejandro Sánchez-López, 5 August 2024, The Astronomical Journal.
DOI: 10.3847/1538-3881/ad5be9

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