Research

Rocky exoplanets with ultra-short orbital period are ideal targets for observations and offer a unique opportunity to study planets in extremely hot environment. Due to atmospheric escape, these planets are more likely to be airless like Mercury or have a secondary atmosphere (non-hydrogen dominated) like Earth. Charaterizing secondary atmospheres of such exoplanets provides crucial insights into atmospheric escape, redox and equilibrium chemistry, and the exchange with the interior.

Orbital periods and radii of confirmed exoplanets as of May 2025. The planets located in the red box are rocky exoplanets with ultra-short orbital periods. The color of each point represents the equilibrium temperature of the planet, ranging from 500 to 3,000 K.

Despite extensive observations, most current interpretations predominantly relied on 1D models with non-self-consistent heat redistribution or oversimplified 3D models with unrealistic radiative transfer. However, few 3D models with realistic radiative transfer have been applied to these hot (non-habitable) rocky exoplanets, mainly due to the incompatibility of standard GCM radiative transfer codes with the extremely high temperatures.

Gaussian and Lorentz profile for absorption line broadening calculations.The coefficients of determination α_G (half-width at half-maximum) and α_L strongly depends on temperature (highlighted in red). More information on HITRAN.

Here we perform non-grey GCM simulations using the custom correlated-k coefficients developed from the ExoMol database. We first use ExoCross to calculated the absorption cross section from the absorption lines at a wide range of temperatures and pressures. Then we develop custom correlated-k coefficients from the absorption cross section (a paper describing the correlated-k method here) and validate them against line-by-line radiative transfer PyRADS. We then perform non-grey GCM simulations, Isca coupled with SOCRATES for planets including 55 Cancri e.

We have some preliminary results for the reinterpretation of the JWST observations (Hu et al. 2024) of 55 Cancri e. Stay tuned for our upcoming paper.

White dwarfs are compact stars with a size comparable to Earth and a mass comparable to Sun. Planets in the habitable zone (read more about the habitable zone edges here) around white dwarfs have ultra-short orbital periods ranging from hours to days. Due to the small size, white dwarfs offer a unique opportunity to search nearby stellar systems for signs of life. The habitable zone around white dwarfs, however, is still poorly understood.

A schematic of transit timing variations of a G dwarf, M dwarf and a white dwarf planet system.

Here we use the ExoCAM Global Climate Model to investigate the inner edge of the habitable zone around white dwarfs. Our simulations show habitable planets with ultrashort orbital periods (P < 1 day) enter a “bat rotation” regime, which differs from typical atmospheric circulation regimes around M dwarfs. Bat rotators feature mean equatorial subrotation and a displacement of the surface’s hottest regions from the equator toward the midlatitudes.

Surface temperature and zonal mean zonal wind as a function of rotation period. From left to right: bat rotator (P = 0.5 days; this work), compared to a rapid rotator (P = 2 days), Rhines rotator (P = 10 days), and slow rotator (P = 20 days). Dynamical regimes for the slower rotating planets are defined as in Haqq Misra et al. (2018). In (a), “SP” corresponds to the substellar point. In (b), red vs. blue shading represent eastward vs. westward winds. The simulations assume a 10,000 K white dwarf and 2.12 times Earth’s instellation.

The “bat rotation” regime expands the white dwarf habitable zone by ∼50% compared to estimates based on 1D models.

Estimated rotation regimes inside the habitable zone of white dwarfs with different stellar temperatures, as a function of relative stellar flux.

The James Webb Space Telescope should be able to quickly characterize bat rotators around nearby white dwarfs thanks to their distinct JWST via thermal phase curves.

We also qualitatively explain the onset of bat rotation using shallow water theory. Read more here: [arxiv]/ [journal].