Blackbody radiation from isolated Neptunes

Sivan Ginzburg, Re'em Sari, Abraham Loeb

Research output: Contribution to journalArticlepeer-review

15 Scopus citations


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 that slows down their cooling. Therefore, a measurement of the blackbody temperature, Teff ∼ 50 K, at which a Neptune emits, can be used to estimate the mass of its atmosphere, Matm. Explicitly, we find the relation Teff μ Matm 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.

Original languageAmerican English
Article numberL11
JournalAstrophysical Journal Letters
Issue number1
StatePublished - 2016

Bibliographical note

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© 2016. The American Astronomical Society. All rights reserved.


  • Planets and satellites: composition
  • Planets and satellites: physical evolution


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