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Beyond Earthly Skies

The Faint Emission from Isolated Neptunes

13 Jun 2016, 22:00 UTC
The Faint Emission from Isolated Neptunes
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Figure 1: Artist’s impression of what could be an isolated Neptune-like planet.The orbits of some Kuiper Belt Objects suggest the presence of a planet with ~10 times the mass of Earth on a very distant ~700 AU semimajor axis orbit around the Sun. Assuming the hypothesized planet is Neptune-like (i.e. the planet is modelled as a simple two-layer model that consists of a rocky/icy core and a hydrogen/helium envelope), Ginzburg et al. (2016) show that the planet can potentially be detected via the internal flux it gives off as it cools over time. Because the planet is so far from the Sun, its own internal flux is much greater than the incident irradiation it gets from the Sun. As a result, the thermal evolution of the planet progresses as if the planet were an isolated object. Measuring the temperature of the planet can allow the mass of its hydrogen-helium envelope to be constrained. This is because a thick hydrogen-helium envelope acts as an insulator, slowing down the planet’s rate of cooling. If the planet’s effective temperature is ~50 K, its wavelength at peak emission will be ~60 μm.Figure 2: Effective temperature as a function of age for a planet with ...

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