Escaping atmosphere of the ultrahot Jupiter KELT-18 b reveals metastable helium

Observations of the ultrahot Jupiter KELT-18 b confirm that the planet is losing its atmosphere. By using high-resolution spectroscopy, researchers detected a signature of metastable helium escaping from the planet. This finding highlights the intense photoevaporation occurring on the massive gas giant as it orbits its host star.

Star system: KELT-18 A | Distance from the Sun: 1032 light years

The ultrahot Jupiter KELT-18 b is currently losing its outer gaseous layers into space. This process, known as photoevaporation, happens when the intense radiation from a host star strips away the atmosphere of a planet. Observations have now confirmed the presence of a specific helium signature in the planet's escaping gas, providing a clear window into how these massive worlds interact with their parent stars over time.

To measure this atmospheric loss, the CARMENES spectrograph was used to analyze the light passing through the planet's atmosphere during a single transit. Because the target light was often obscured by interference from Earth's own atmosphere, the data were processed using two distinct analytical methods. By combining these approaches, the detection of a helium triplet absorption feature was isolated, confirming that the atmosphere of KELT-18 b is actively expanding and thinning.

Further computer modeling of this helium signature allowed for a detailed estimation of the mass-loss rate. By accounting for the planet's rotation and the energy received from the star, the simulations confirm that KELT-18 b is indeed experiencing significant atmospheric escape. This discovery makes KELT-18 b the most massive planet orbiting such a hot host star where this specific type of helium-based atmospheric loss has been observed.

Key findings
A helium absorption feature was detected in the atmosphere of KELT-18 b at a confidence level of 5.1 sigma.
The atmosphere of KELT-18 b is undergoing active photoevaporation due to stellar radiation.
Mass-loss calculations demonstrate a sustained escape of gas from the planet's upper atmosphere.
KELT-18 b is the most massive exoplanet known to orbit its specific type of hot host star with this confirmed helium signal.

Scientific study

High-resolution transmission spectroscopy of KELT-18b Metastable helium detection at 1083 nm (external link)