Atmospheric composition of hot Jupiter KELT-2A b reveals chemical history

Observations of the hot Jupiter KELT-2A b using the Gemini-North telescope confirm the presence of water vapor in its atmosphere. By analyzing its chemical makeup, researchers found that the carbon-to-oxygen ratio matches its host star, while overall metallic elements appear depleted. This unique chemical profile suggests the planet likely formed far from its star before migrating inward through a high-eccentricity path.

Star system: KELT-2 A | Distance from the Sun: 439 light years

The hot Jupiter KELT-2A b features an atmosphere containing measurable amounts of water vapor. By observing the planet across five nights, investigators detected water with high certainty. This finding builds on previous research, providing a clearer picture of the chemical elements present in the gas giant's outer layers. While researchers searched for carbon monoxide, the current data remains inconclusive, though hints of its presence appeared near the expected planetary velocity.

To determine the chemical composition, the team used the IGRINS-2 instrument on the Gemini-North telescope. They analyzed light emitted by the planet in the near-infrared range, specifically the H and K bands. By processing this light and combining it with previous data from the Spitzer Space Telescope, the team modeled how temperature and pressure change at different depths in the atmosphere. This modeling allowed them to estimate the abundance of water, carbon, and oxygen, and compare these levels to those found in the host star.

The findings reveal that the carbon-to-oxygen ratio of the planet matches its host star. However, the overall concentration of heavy elements, known as metallicity, is lower than that of the star. These results challenge standard formation theories which typically predict different chemical signatures for planets forming at specific distances from their host stars. The observed combination suggests the planet likely formed in a region far from the star, accreting gas that was low in metals, followed by a migration history involving a highly elongated orbit.

Key findings
Water vapor is confirmed in the atmosphere of KELT-2A b with high significance.
The carbon-to-oxygen ratio of the planet matches the ratio of its host star.
Overall carbon and oxygen abundances are lower than those measured in the host star.
The chemical profile suggests the planet formed far from its star and moved inward through a high-eccentricity migration.

Scientific study

Emission spectroscopy of KELT-2A b with Gemini-North/IGRINS-2: hints of a stellar C/O ratio and sub-stellar metallicity (external link)