Image of comet-like planet

Artist's impression of HD 209458b seen from a hypothetical moon (Image credit: NASA, ESA and G. Bacon, STScI )

Some 150 light years from Earth in the constellation of Pegasus lies the star HD 209458 and its planetary system. The star is almost a clone of our own Sun, but one its planets is completely unlike anything in our Solar System.

The exoplanet HD 209458b has a mass more than 200 times Earth’s and has a diameter greater than Jupiter’s. It orbits its star at about an eighth of the distance Mercury orbits the sun, so the exoplanet takes only 3.5 days to complete an orbit. This proximity to a star means that the planet is hot, perhaps 1000°C, hence it is classed as a “hot Jupiter”. Note that it will not be as hot as this everywhere; the planet is tidally locked to its star, having one side in permanent blazing day and one in an eternal cooler night.

The temperature differential of this planet was expected to mean that it has a violently seething atmosphere (which we know contains carbon dioxide, water vapour and methane). This has been proven: wind speeds of 5000 to 10 000 km per hour have been observed. Perhaps even more dramatic is the fact that the atmosphere is leaking into space, with stellar winds pushing the escaping material into a long stream behind the planet. HD 209458b must look like a gigantic comet! This was long expected but now has been proven thanks to observations with the HST. (The artist’s impression shows it viewed from a hypothetical moon, we have no images of this planet.)

In billions of years, HD 209458b will entirely lose its atmosphere, and be a barren sphere of rock and metal (a so-called cthonian planet), arid on one side, frigid on the other.


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