Hints of a stratosphere in the atmosphere of warm Neptune Gliese 436 b

Star system: Gliese 436 | Distance from the Sun: 31.9 light years

Gliese 436 b is a warm Neptune — a planet roughly the size and mass of Neptune — orbiting the red dwarf star Gliese 436, located about 33 light-years from Earth. Astronomers used the Keck II telescope on Mauna Kea, Hawaii, fitted with an instrument called KPIC (Keck Planet Imager and Characterizer). KPIC routes starlight through a hair-thin single-mode optical fiber before feeding it into a high-resolution spectrograph — a technique that suppresses the bright infrared glow of Earth's own sky by a factor of 100 compared with conventional instruments. This dramatically improved sensitivity allowed the team to search for direct thermal emission from the planet's atmosphere in the L-band of the infrared spectrum, covering wavelengths from 2.91 to 3.85 micrometers.

The researchers performed a cross-correlation analysis — a method that shifts model spectra and compares them with observations to find the best match — and obtained a tentative detection of Gliese 436 b best matched by a model in which the planet's upper atmosphere is warmer than the layers beneath. This condition is known as a thermal inversion, analogous to Earth's stratosphere where temperatures rise with altitude above the cloud tops. Complementary retrieval analysis — a statistical method for inferring atmospheric composition from spectral data — favors the presence of water vapor (H2O) and possibly methane (CH4), with these molecules producing emission features, meaning they radiate infrared light outward rather than absorbing it. This differs from earlier Spitzer space telescope results and high-resolution transmission spectroscopy observations, which did not support such an inverted temperature profile for this planet.

The best-fitting atmospheric model appeared substantially brighter than expected from the planet's equilibrium temperature and from previous low-resolution spectroscopy. The team demonstrate that this apparent excess is not real, but an artifact of high-resolution spectral processing, which removes broad continuum brightness information and retains only relative spectral shapes. When the model is rescaled to match the absolute flux from low-resolution observations of Gliese 436 b, the detection remains statistically comparable. The most plausible mechanism driving the thermal inversion is a haze or aerosol layer in the upper atmosphere, above which temperatures rise — similar in principle to how ozone drives warming in Earth's stratosphere. Follow-up observations taken after the planet passes behind its host star would help resolve remaining ambiguities and confirm this possible detection. The study also demonstrates that L-band high-resolution spectroscopy can be applied to planets significantly smaller and cooler than the hot Jupiters that have traditionally been the targets of such campaigns.

Scientific study

Possible stratospheric emission in the warm Neptune GJ 436 b from high-resolution spectroscopy (external link)