Artist’s impression of the planet Osiris, which lies 150 light-years from Earth © NASA, ESA, and G. Bacon (STScI)

Hubble’s greatest discoveries: exoplanet atmospheres

The Hubble Space Telescope was first to detect the atmosphere of one of these alien worlds.

As of April 2020, there are more than 4,000 known planets orbiting stars other than our Sun. An impressive photo of one of these ‘exoplanets’ has yet to be taken, but the Hubble Space Telescope was first to detect the atmosphere of one of these alien worlds.

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HD 209458-b, also known as Osiris, is a planet 150 light-years from Earth. Temperatures reach a scorching 1,100°C as it orbits just 6.4 million kilometres from its parent star. As the orbiting planet moves in front of the star, some of the light passes through the planet’s atmosphere.

This is analysed by a spectrograph, which is an instrument that splits the light into constituent wavelengths, explains Prof David Charbonneau, leader of the team behind the discovery.

Discover more about the Hubble Space Telescope:

“The idea was to gather spectra when the planet was in front of the star and when it moved away. By comparing them, we would search for the appearance of new features when the planet was in transit. This required an extremely stable platform that was free from the absorption effects of our atmosphere. Only Hubble could do it!”

In 2001, the procedure revealed signs of sodium – the first atmospheric element detected on a planet outside of our Solar System.

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“This same method has become the standard means to examine exoplanet atmospheres, and Hubble has now gathered similar data on dozens of worlds,” says Charbonneau.