Earth is famous for its blue skies. It gets it blue skies through an effect known as Rayleigh scattering. When light from the Sun collides with an air molecule, it is scattered. The closer the wavelength of light is to the size of the air molecule, the more likely it is to be scattered. Because air molecules are smaller than the wavelength of light in the visible spectrum, long wavelengths (more red) tend to scatter less than short wavelengths (blue). Since blue light is scattered more than red, we tend to see the sky as blue from all the scattered sunlight. This is also why sunsets tend to be red. When the Sun is low in the sky, most of the blue light is scattered by the atmosphere, leaving a red sunset.
Not all planets have a blue sky. Mars, for example, with its much thinner atmosphere has more of a brown sky. Rayleigh scattering is a subtle effect, so it’s been difficult to observe beyond our solar system. But recently a team has observed Rayleigh scattering in the atmosphere of a planet about 100 light years away.
The planet, known as GJ 3470b, was discovered in 2012 through the transit method. That is, as the planet passes in front of its star we observe a slight dimming of the star. The amount of dimming tells us about the size and orbit of the planet. From transit observations we know GJ 3470b is a Neptune-sized planet with an orbital period of about 3 days. But if we can only observe the planet by its effect on starlight, how do we know there is Rayleigh scattering in its atmosphere?
To do this the team used a rather ingenious technique. They analyzed the brightness dip of the star during a transit at different wavelengths, and found that the calculated size of the planet increased at shorter wavelengths. In other words the planet appears larger at blue wavelengths than it does at red. This makes sense if the atmosphere is Rayleigh scattering light. Red light isn’t scattered much, so at red wavelengths only the planet itself blocks light from the star. Blue light is strongly scattered, so it doesn’t reach us, so effectively both the planet and its atmosphere block light at blue wavelengths.
Just because Rayleigh scattering occurs in the atmosphere of GJ 3470b, we shouldn’t assume its atmosphere is Earth-like. Given its size the atmosphere is likely hydrogen and helium rather than our nitrogen-oxygen mix. But this is a great example of how we’re starting to study the atmospheres of alien planets.
Paper: Diana Dragomir, et al. Rayleigh Scattering in the Atmosphere of the Warm Exo-Neptune GJ 3470b. The Astrophysical Journal, Volume 814, Number 2 (2015)