On the Emergent Spectra of Hot Protoplanet Collision Afterglows

We explore the appearance of terrestrial planets in formation by studying the emergent spectra of hot molten protoplanets during their collisional formation. While such collisions are rare, the surfaces of these bodies may remain hot at temperatures of 1000-3000 K for up to millions of years during the epoch of their formation. These object are luminous enough in the thermal infrared to be observable with current and next generation optical/IR telescopes, provided that the atmosphere of the forming planet permits astronomers to observe brightness temperatures approaching that of the molten surface. Detectability of a collisional afterglow depends on properties of the planet’s atmosphere – primarily on the mass of the atmosphere. A planet with a thin atmosphere is more readily detected, because there is little atmosphere to obscure the hot surface. Paradoxically, a more massive atmosphere prevents one from easily seeing the hot surface, but also keeps the planet hot for a longer time. In terms of planetary mass, more massive planets are also easier to detect than smaller ones because of their larger emitting surface areas. We present preliminary calculations assuming a range of protoplanet masses (1-10 $M_\earth$), surface pressures (1-1000 bar), and atmospheric compositions, for molten planets with surface temperatures ranging from 1000 to 1800 K, in order to explore the diversity of emergent spectra that are detectable. While current 8- to 10-m class ground-based telescopes may detect hot protoplanets at wide orbital separations beyond 30 AU (if they exist), we will likely have to wait for next-generation extremely large telescopes or improved diffraction suppression techniques to find terrestrial planets in formation within several AU of their host stars.

💡 Research Summary

The paper investigates the observable signatures of terrestrial planets during their formative, highly energetic collisional phase, focusing on the emergent infrared spectra of hot, molten protoplanets—so‑called “collision afterglows.” The authors begin by noting that giant impacts, while statistically rare, can leave a planetary surface at temperatures of 1,000–3,000 K for timescales ranging from 10⁴ to 10⁶ years, depending on the efficiency of cooling through the overlying atmosphere. Such temperatures correspond to thermal emission peaking in the near‑ to mid‑infrared (2–15 µm), a regime accessible to both current 8–10 m class ground‑based telescopes and upcoming facilities such as the Extremely Large Telescope (ELT) and the James Webb Space Telescope (JWST).

To quantify the emergent spectra, the authors construct a suite of one‑dimensional radiative‑convective models that span a broad parameter space: planetary masses of 1, 5, and 10 M⊕; surface pressures from 1 to 1,000 bar; and surface temperatures between 1,000 and 1,800 K. Atmospheric compositions are varied among water‑rich, CO₂‑rich, nitrogen‑dominated, and mixed metal‑oxide scenarios, reflecting plausible post‑impact outgassing products. For each configuration, the radiative transfer equation is solved to obtain the outgoing flux as a function of wavelength, allowing the authors to extract brightness temperatures and identify diagnostic absorption bands.

A central result is the strong dependence of detectability on atmospheric mass. Thin atmospheres (≤10 bar) are largely transparent in the infrared, so the observed brightness temperature approaches the actual molten surface temperature. Consequently, such planets produce high‑contrast signals that can be detected with current instruments, provided the planet lies at a sufficiently wide orbital separation (≥30 AU) where stellar glare is manageable. Conversely, massive atmospheres (≥100 bar) act as insulating blankets: they dramatically increase the cooling timescale, keeping the planet hot for millions of years, but they also introduce strong molecular absorption (especially H₂O and CO₂) that reduces the emergent flux in key windows and masks the surface temperature. In these cases, detection requires higher sensitivity and sophisticated starlight suppression techniques.

Planetary mass further modulates observability. Larger planets have greater surface areas, boosting the total emitted power roughly in proportion to the square of the radius. Moreover, higher gravity tends to retain denser atmospheres, which can either aid detection (by extending the hot phase) or hinder it (by increasing opacity). The authors find that a 10 M⊕ planet with a 10 bar atmosphere can be up to three times brighter in the L‑band (3–4 µm) than a 1 M⊕ counterpart with identical surface temperature, making it a more favorable target for both current and next‑generation telescopes.

The paper also evaluates instrumental capabilities. Using typical sensitivities of 8–10 m telescopes (≈10 µJy in L‑band) and projected ELT/JWST sensitivities (≈1 µJy), the authors map out the region of parameter space where detection is feasible. They conclude that present‑day facilities could plausibly detect hot protoplanets beyond ~30 AU if such objects exist, while ELTs and JWST will be required to probe the more common, closer‑in (≤10 AU) population. The authors stress that high‑performance coronagraphs (e.g., vortex or apodized pupil designs) and advanced post‑processing (angular differential imaging, spectral deconvolution) will be essential to suppress stellar leakage and reveal the faint planetary afterglow.

In summary, the study provides a comprehensive theoretical framework linking impact‑driven heating, atmospheric insulation, and emergent infrared spectra. It demonstrates that, despite the paradoxical role of atmospheric mass—both obscuring and preserving the hot surface—there exists a viable observational window for detecting molten protoplanets, especially those with thin, infrared‑transparent envelopes. The work lays the groundwork for future surveys aimed at directly witnessing terrestrial planet formation, highlighting the need for next‑generation telescopes, refined starlight suppression, and targeted spectroscopic follow‑up to discriminate between atmospheric compositions and infer surface conditions.