Blue Skies on the Warm Neptune-Size Planet GJ 3470b

GJ 3470b is a warm Neptune-size planet in orbit around a red dwarf star. The orbit of the planet around its host star is oriented such that the planet periodically passes in front of its host star every 3.34 days. GJ 3470b has 3.9 times the size and 13.7 times the mass of Earth, and an estimated equilibrium temperature of roughly 700 K. Observations of how light from the planet's host star filters through the planet's atmosphere when the planet passes in front of its host star indicate the presence of Rayleigh scattering in the atmosphere of GJ 3470b.

Figure 1: Artist’s impression of a warm Neptune-size planet.

The size of GJ 3470b is determined by measuring how much light the planet obscures each time it passes in front of its host star. At certain wavelengths of light, a planet's atmosphere can appear more opaque. This means that if the planet is observed to pass in front of its host star at these wavelengths, the planet can obscure more light from its host star and appear larger.

For the case of GJ 3470b, the effect of Rayleigh scattering makes the planet appear larger when observed at shorter (blue) wavelengths of light than at longer (red) wavelengths of light. This is because the planet's atmosphere scatters shorter (blue) wavelengths of light more strongly than longer (red) wavelengths of light. Such an effect also occurs in Earth's atmosphere and is the reason why the sky can appear blue on Earth.

Observations of the Rayleigh scattering in the atmosphere of GJ 3470b also reveal that the planet's atmosphere has a low mean molecular weight. The atmosphere of GJ 3470b is most likely a hydrogen-helium atmosphere covered by high-altitude clouds and hazes. GJ 3470b is currently the smallest known exoplanet with a detection of Rayleigh scattering.

Figure 2: The transmission spectrum of GJ 3470b. The effect of Rayleigh scattering makes the planet appear larger when observed at shorter (blue) wavelengths of light than at longer (red) wavelengths of light. Dragomir et al. (2015)

Reference:
Dragomir et al. (2015), “Rayleigh Scattering in the Atmosphere of the Warm Exo-Neptune GJ 3470b”, arXiv:1511.05601 [astro-ph.EP]