‘Sunset’ on HD 209458 b by Professor Frédéric Pont.

What do sunsets look like from other planets?

A University of Exeter astrophysicist has shown what sunsets look like on planets outside our solar system.

He has worked out the colour of sunsets on two planets: HD 209458 b and HD 189733 b, known as ‘extrasolar planets’ because they are outside our solar system.

Extrasolar planets orbit stars, in a similar way to the Earth orbiting the Sun. Professor Frédéric Pont of the University of Exeter has used the extrasolar planets’ ‘transmission spectrum’, taken by the Hubble Space Telescope, to work out the colour of the ‘sunsets’ created by these stars.

Writing on the website ExoClimes.com, where he has posted the two sunset images he has produced, Professor Pont said:  “Unlike its sister planet HD ’189, 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.

“One key difference with a sunset on Earth is that the ‘sun’ is much larger from ‘209, because the planet is very close. Instead of changing colour as it moves near the horizon, the host star spans all colours at once.”
The new website ExoClimes.com is devoted to discussion around the study of planetary atmospheres outside the Solar System. Maintained and animated by astrophysicists at the Universities of Exeter and Oxford, it brings the latest discoveries about extrasolar planets to the attention of other researchers, students and keen amateur astronomers and allows communities to share their knowledge on this fast-growing area of science.

The website has been launched just before the Exoclimes 2012 conference, being held in Aspen, Colorado from 16 to 20 January 2012. This follows the inaugural Exoclimes conference, held at the University of Exeter in September 2010, which was the first conference to focus on the atmosphere of extrasolar planets.

The University of Exeter has one of the UK’s largest astrophysics groups working in the fields of star formation and exoplanet research. The Astrophysics group focuses on some of the most fundamental problems in modern astronomy – when do stars and planets form and how does it happen? They conduct observations with the world’s leading telescopes and carry out numerical simulations to study young stars, their planet-forming discs, and exoplanets. This research helps to put our Sun and the solar system into context and understand the variety of stars and planetary systems that exist in our Galaxy. Through its science strategy, the University is seeking to invest £230 million of internal and external income in five key themes of activity, one of which is Extrasolar Planets.

Date: 17 January 2012

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