Planet Has 'Truly' Alien Sunset
There's nothing like a glorious sunset to make you feel like you are in another world - especially if it's green.
Watching the sun go down on the planet Osiris would be a "truly alien" experience, according to British astrophysicist Professor Frederic Pont.
Instead of the familiar orange, scarlet and crimson hues of a sunset on Earth, one would be treated to a palette of blues, greens and browns.
Osiris, the popular name given to "exoplanet" HD 209458b, is one of more than 700 worlds known to exist outside the Solar System.
Prof Pont, a University of Exeter astrophysicist, has calculated what a sunset would look like from Osiris as well as a planet orbiting another star, HD 189733.
Of the two, Osiris has the strangest view.
The planet's parent star lies in the constellation Pegasus, 150 light years from Earth.
Writing on the website exoclimes.com, where he has posted the sunset images, Prof Pont says: "The planet HD '209' (Osiris) has a sunset that looks truly alien.
"The star is white outside the atmosphere, since its temperature is close to that of the Sun. It then acquires a bluish tinge as 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."
Rayleigh scattering is the scattering of light by small particles in the atmosphere. On Earth, it is responsible for the sky being blue.
Prof Pont adds: "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."
Sunset on the other planet, HD 189733b, looks much more like one on Earth, said Prof Pont, who studied light wavelength data from the Hubble Space Telescope.
He wrote: "Basically, the sunset looks like a glorious sunset on Earth, on a very clear day with some dust in the air. This is because in both cases, Rayleigh scattering is the dominant mechanism. On Earth the scattering is caused by molecules and airborne dust in the air. On '189' the Rayleigh scattering is thought to be caused by silicate dust."
The planet, described as a "hot Jupiter", is 63 light years away in the constellation of Vulpecula, the Fox.
Like Osiris, it orbits close to its parent star which is 25 times larger in the sky than the Sun appears from Earth.
The sunset colours seen from HD 189733b range from pale orange to deep burgundy red.