Something weird is going on in the neighbourhood of HD 189733b.
Sixty-three light years away, this huge gas planet circles an orange dwarf star, known as HD 189733A, the larger partner in a mysterious binary system, deep within the constellation of Vulpecula, the Fox. Discovered by French astronomers in October 2005, the planet has attracted much scientific curiosity in recent years: it was the first ‘exoplanet’ to be crudely mapped and has shown tantalising clues of carbon dioxide, methane and water vapour in its atmosphere. But don’t expect life here. HD 189733b is a so-called ‘hot Jupiter’: around the same size as the largest planet in our Solar System, it orbits so close to its parent star that atmospheric temperatures probably skyrocket to more than a thousand degrees Celsius.
Since April 2010, the Hubble Space Telescope and, more recently, NASA’s Swift observatory have revealed that this atmosphere is changing markedly due to dynamic interactions with HD 189733A itself. Powerful eruptions from the star’s surface are evaporating upwards of a thousand tons of hydrogen every second, causing the gas to escape into space at velocities greater than 300,000 miles per hour. This phenomenon of atmospheric evaporation has been debated and studied for years, but the new data has offered a real insight into its progress.
“The multi-wavelength coverage by Hubble and Swift has given us an unprecedented view of the interaction between a flare on an active star and the atmosphere of a giant planet,” said lead researcher Alain Lecavelier des Etangs of the Paris Institute of Astrophysics, based at the French capital’s Pierre and Marie Curie University. When Swift made its observations in September 2011, it revealed a massive X-ray ‘spike’ from the star’s surface. Unfortunately for HD 189733b – which circles its host at a distance of only 3.3 million miles, taking 2.2 Earth-days to complete a single orbit – the planet was subjected to an X-ray blast tens of thousands of times worse than our world suffers even during an X-Class burst. For us, the X-Class is the strongest category of solar flare and has the capacity to create massive geomagnetic storms and knock out power systems for extended periods of time.
Previous Hubble data had already shown that HD 189733b’s atmosphere reaches a peak temperature of around 1,030 degrees Celsius. Thirteen percent more massive than Jupiter, it was mapped in 2007 by NASA’s Spitzer Space Telescope and is thought to be tidally locked to HD 189733A, meaning that it has a permanent ‘day’ and ‘night’ side. Assuming that this tidal lock is accurate, astronomers suspect that powerful easterly winds, moving at several thousand miles per hour, redistribute heat throughout its atmosphere and as a result the temperature difference between the star-facing side and the space-facing side is relatively low.
‘Hot Jupiters’ typically orbit their parent stars at distances of between 1.4 million miles and 46.5 million miles (or 0.015 and 0.5 astronomical units). One of the most well-known is 51 Pegasi b, which was the first exoplanet ever found around a Sun-like star at the time of its discovery in 1995. As for HD 189733b, it circles an orange dwarf, a little smaller than our Sun and probably only around 600 million years old. The other star in the binary system, a dim red dwarf known as HD 189733B, lies at a separation distance of approximately 20 billion miles (or 216 astronomical units) and was first detected in 2006.