Research

Rocky Exoplanet Interiors: Thermal Emission Excess

Planets are expected to emit at a blackbody temperature set by the irradiation they receive. However, JWST secondary eclipse and phase curve observations revealed that some rocky exoplanets emit at super-blackbody temperatures. This puzzling thermal emission excess implies that some unknown processes, possibly interior to the planet, are generating extra heat. In Lin & Daylan (2026), I investigated three possible processes, namely residual heat from formation, tidal heating, and induction heating, using models. However, the mystery is not fully resolved. I found that internal processes fail to explain the thermal emission excesses at a population level. Stellar contamination, surface processes, geometric effects, or other heating mechanisms may reconcile the remaining gap.

Rocky Exoplanet Interiors: The Origin of High-Density Planets

Among all rocky exoplanets known to date, there is a population of super-Mercuries with unexpectedly high densities. If their high densities are due to large iron cores - just like our own Mercury - their formation history must diverge from that of Earth-like planets. Preferential accretion of iron or mantle-stripping giant impacts could occur in the early history of these planets to increase their core mass fractions (CMFs). In Lin, Cambioni & Seager (2025), I investigate another possibility: these planets are dense not because of high CMFs, but because they are remnant cores of giant planets that remain in a fossil-compressed state. Using planetary interior and thermal models, I found that high-density planets are unlikely to be naked cores of giant planets, because they are largely molten and will expand as the envelope is stripped away by photoevaporation, and hence cannot remain in a fossil-compressed state.

Planetary Interiors: The Compositions of Uranus and Extrasolar Sub-Neptunes

The interior compositions of planets with intermediate masses and radii, such as Uranus, Neptune, and extrasolar sub-Neptunes and Neptune-like planets, suffer from the most severe degeneracy. Many possible compositions - including predominantly rocky composition with thick H/He envelope, a roughly even mixture of rock and ice, and the water world composition - can all explain the observed masses and radii of these planets. To make the degeneracy even worse, both layered structure with sharp density jumps and smooth structure with gradual density gradients are possible. In Lin, Seager & Weiss (2025), I used my planetary interior model CORGI to study different possible interior compositions and structures of Uranus, and predict the respective gravity field, which will be measureable by the Uranus Orbiter and Probe. In Lin & Seager (2025), I studied a novel carbon-rich composition for sub-Neptunes. I found that substantial carbon layers are allowed for K2-18 b and TOI-270 d - both are well-studied archetypical sub-Neptunes - and their atmospheric compositions are consistent with high C/O ratios, which is a possible consequence of carbon-rich interiors.

Atmospheres of White Dwarf Exoplanets

Hypothetical Earth-sized exoplanets orbiting white dwarfs (WDs) are ideal targets for atmospheric characterzation. This is because WDs - the highly compressed remnants of stellar evolution - are much smaller than main sequence stars, thereby enhancing the planet-to-star contrast ratio. In Lin et al. (2022), I simulated the atmospheric chemistry and transmission spectra of H₂-, CO₂-, and N₂-dominated atmospheres on Earth-sized exoplanets orbiting cool (less than or equal to 6000 K) WDs. I studied the atmospheric loss of Earth-sized planets throughout the post-main-sequence evolution and concluded that any first-generation H₂-dominated atmosphere cannot survive. Thus, H₂-dominated atmosphere is a signature of second-generation planets accreting water-rich materials from WD debris disks and reviving a reduced H₂-rich atmosphere.

Atmospheres of Potentially Habitable Exoplanets

One of the foremost goals of exoplanetary science is to find the evidence for life elsewhere. Spectral biosignatures - features of a molecule or a combination of molecules with biogenic origin - are the key for remote characterization of potentially habitable exoplanets. In a series of works, I simulated the climate, photochemistry, and high-resolution transmission spectra of potentially habitable exoplanets such as TRAPPIST-1e, and estimated the prospect for JWST to detect an atmosphere - and even evidence for life - on such exoplanets. I also simulated the transmission spectra of Earth through geological time - from the very beginning of life to present day - around various types of host stars (F, G, K, and M).