Watching Alien Sunsets on Exoplanets


If you’ve watched the movies “Star Wars” or “Avatar,” you know what an alien sunset looks like: Kinda like the sunsets here on Earth, with some alien landscape thrown into the foreground and supersaturated skies. But with the diverse array of star types and weird exoplanetary atmospheres, shouldn’t alien sunsets be more… alien?

Now, University of Exeter exoplanetary scientist Frédéric Pont has thrown some astronomical data into the mix to produce what you can expect to see as the sun sets over a real alien world. You can keep your CGI, Hollywood, this is science!

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Shown above is the scene you’d see if you were orbiting 10,000 km above the exoplanet HD 209458 b, a world 150 light-years from Earth. HD 209458 b — unofficially known as “Osiris” — is a large gas giant world orbiting very close to its star (it is therefore a “hot-Jupiter”).

As Osiris orbits so close to its star, atmospheric temperatures are high (around 1,000 degrees Celsius or 1,800 degrees Fahrenheit). This means its outer atmospheric layers become “puffed up.

The exoplanet’s atmospheric “puffiness” makes it easier to analyze, and using data from the STIS spectrograph on the Hubble Space Telescope, Pont was able to calculate how the star would look as seen at the exoplanet’s horizon.

In the case of Osiris, as its star drops through the atmosphere from an observer’s perspective, it will turn from predominantly white to blue. The blueness is caused by atmospheric sodium absorbing the orange/red light being emitted by the star, causing its light to shift toward the blue part of the spectrum.

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As the star drops further, Rayleigh scattering by atmospheric molecules will scatter even the blue light from the star. (Rayleigh scattering is the same mechanism that gives Earth’s sky a blue hue.) The only light able to make it through the atmosphere will be green and eventually murky brown. The atmosphere will then glow due to emission from atmospheric molecules and the Rayleigh scattering will cause a blue hue to remain.


And that’s not all, here’s a rendering of another sunset on a different alien world.

For more on the fascinating science of exoplanetary atmospheres, visit Pont’s website Also, for an in-depth analysis, read Phil Plait’s take on Bad Astronomy.

Image credit: Prof. Frédéric Pont/University of Exeter/

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