Sunsets on Exoplanets

Came across a series of interesting pieces of concept art, detailing what sunsets might look like on exoplanets.

An artist's concept of what a sunset would look like on HD 209458b

Whatever you do, don't look directly into the star...

The first is an imagining of what a sunset behind exoplanet HD 209458b, or “Osiris”, would look like to the naked eye. It’s a stunning piece of computer generated imagery that uses serious science to determine how things would look.

To wit:

The astronomers observed the stars when the planets were hidden from view, so they got spectra of just the stars. Then they repeated the observations when the planets were next to their stars, yielding spectra of the two together (from Earth, the planet and star appear as a single point of light because they are so far away from us). Once they have both spectra, they can carefully subtract the light from the star form the combined star and planet, leaving just the planet: star + planet – star = planet. Bang! Done

As the article goes on to say, it’s not quite that easy but it is facinating how much we can tell about the make up of worlds so far away just by the way light reacts with them. Truly amazing.

An artists concept of what a sunset would look like if one were on the exoplanet HD 209458 b.

That's not smog making it all pretty... but you probably shouldn't breathe it either.

The second image was created by Professor Frédéric Pont of the University of Exeter, attempting to show what the sunset of the same world would look like if one were actually standing on it.

(Not that that’s possible, given that it’s a gas giant, so it was positioned so the viewer was 10,000 km above the planet’s “surface”. Still. Cool.)

Again, like the “Osiris” image above, Professor Pont used science to determine how this should look, again using the chemical make up of the world as told to us through light:

The star is white outside the atmosphere, since its temperature is close to that of [our] Sun. It then acquires a bluish tinge at it sinks deeper, because the absorption by the broad wings of the neutral sodium lines (the spectral lines responsible for the gloomy orange of sodium street lighting) remove the red and orange from the star light. Deeper down, Rayleigh scattering by the molecules in the atmosphere starts scattering the blue part of the spectrum as well, so that the only frequencies that are able to squeeze past are green, then murky brown. Outside the star’s disc, the atmosphere has a faint glow in its upper parts, due to re-emission in the sodium line, then it become bluer because of the Rayleigh scattering.

One key difference with a sunset on Earth is that the “sun” is much larger from ‘209, because the planet is very close. As a result, there is no perspective from which the star would fit in only one layer of the atmosphere, as it does in an Earthly sunset. Instead of changing colour as it moves near the horizon, the host star spans all colours at once.

An artists concept of what a sunset would look like if one were on the exoplanet HD 189733 b

Looks vaguely... familiar...

And, of course, the good man did the same thing with another exoplanet, HD 189733b.

Now, none of these planets are habitable, in fact, the Osiris sunset image is really the only way we’d ever see that particular view given the intense temperatures and hostile atmosphere of the planet even if we had the ability to get there, but it’s an interesting thought exercise to engage in since all we know is the sunset of our own world and that of Mars (because we’re cool enough to send shit there).

One wonders then, given human needs for a oxygen/nitrogen rich atmosphere whether any habitable world we tried to settle, if the sunset of a different star might not look familiar to us since so much of the way it looks is determined by the material the light must pass through.

It’s less exotic than imagining exoplanets but, with this technique, it should also be possible to determine what the sunsets on the other planets in our own solar system look like.