Blackbody Radiation from Isolated Neptunes

Abstract

Recent analyses of the orbits of some Kuiper Belt objects hypothesize the presence of an undiscovered Neptune-size planet at a very large separation from the Sun. The energy budget of Neptunes on such distant orbits is dominated by the internal heat released by their cooling rather than solar irradiation (making them effectively "isolated"). The blackbody radiation that these planets emit as they cool may provide the means for their detection. Here we use an analytical toy model to study the cooling and radiation of isolated Neptunes. This model can translate a detection (or a null detection) to a constraint on the size and composition of the hypothesized "Planet Nine". Specifically, the thick gas atmosphere of Neptune-like planets serves as an insulating blanket which slows down their cooling. Therefore, a measurement of the blackbody temperature, $T_{\mathrm{eff}}\sim 50\text{K}$, at which a Neptune emits can be used to estimate the mass of its atmosphere, $M_{\mathrm{atm}}$. Explicitly, we find the relation $T_{\mathrm{eff}}\propto M_{\mathrm{atm}}^{1/12}$. Despite this weak relation, a measurement of the flux at the Wien tail can constrain the atmospheric mass, at least to within a factor of a few, and provide useful limits to possible formation scenarios of these planets. Finally, we constrain the size and composition of Planet Nine by combining our model with the null results of recent all-sky surveys.

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Cited by in corpus (12)

• The Planet Nine Hypothesis
• What if Planet 9 is a Primordial Black Hole?
• Is there an exoplanet in the Solar System?
• Consequences of a Distant Massive Planet on the Large Semi-major Axis Trans-Neptunian Objects
• The fates of Solar system analogues with one additional distant planet
• The Hunt for Planet Nine: Atmosphere, Spectra, Evolution, and Detectability
• Making Planet Nine: A Scattered Giant in the Outer Solar System
• Preliminary constraints on the location of the recently hypothesized new planet of the Solar System from planetary orbital dynamics
• Finding Planet Nine: a Monte Carlo approach
• Finding Planet Nine: apsidal anti-alignment Monte Carlo results
• H-Atmospheres of Icy Super-Earths Formed in situ in the Outer Solar System: An Application to a Possible Planet Nine
• On the origin of extreme trans-Neptunian objects within Modified Newtonian Dynamics