A European team of astronomers has used a brand new technique to uncover evidence of a magnetic field surrounding the exoplanet HD 209458b. It is hoped the method can be applied to many of the other exoplanetary systems found to date, adding another dimension to our understanding of these distant, alien worlds.

Discovered in 1999, HD 209458b is 30% larger than Jupiter. However, unlike Jupiter, it completes one orbit in just 3.5 days, as the planet sits very close to its parent star – more than 50 times closer than the Earth is to the Sun. Being at such close quarters, its gaseous atmosphere is puffed up by the intense heat that it has to endure. Such planets are known as “hot Jupiters”. Researchers led by Kristina Kislyakova of the Austrian Academy of Sciences in Graz have now confirmed this picture by observing the planet with the Hubble Space Telescope.

Speedy hydrogen

As HD 209458b passes between us and its parent star, the star’s light passes through the planet’s atmosphere before continuing towards Earth. Some wavelengths of light are missing, however, because they are absorbed by the elements in the planet’s atmosphere. This shows up as dark bands in the light’s spectrum known as absorption lines. Of particular interest to Kislyakova were the so-called Lyman-alpha lines, which are created when hydrogen absorbs light. “There was enhanced absorption in Lyman alpha, which means the planet has an extended hydrogen atmosphere,” she told physicsworld.com.

Most of the spectral lines were shifted towards the blue and red ends of the spectrum. This implies that the hydrogen atoms that absorbed the light were moving at very high speeds – those moving towards us resulted in blueshifted spectral lines, those moving away in redshifted ones. Kisylakova and her team explored various mechanisms for how these atoms undergo such high acceleration, but they found the best fit when factoring in the interaction between the stellar winds blowing off the star and a planetary magnetic field. As the star is very similar to the Sun, the team were able to use our star as a proxy in the model.

Windy atmospheres

The researchers found that the data were best matched by a solar wind travelling at about 400 km s^–1 and a planetary magnetic moment about 10% that of Jupiter’s. “This is the first time an exoplanetary magnetic field has been determined from Lyman-alpha lines,” says Kislyakova. As well as providing an insight into other planets’ magnetic fields, the technique could also be used to study the solar winds on stars other than the Sun. However, it can only be deployed for “transiting” planets, i.e. those that move in front of their star from our perspective.

The findings could also provide clues about the likely future of HD 209458b. Closely orbiting hot Jupiters are thought to be gradually spiralling inwards, since their orbits decay as the result of gravitational interactions with their host star. Losing mass in the form of these fast-moving hydrogen atoms affects this process. “This result shows that hot Jupiters can have strong magnetic fields that capture and channel the material driven off by radiation from their star,” says Coel Hellier of Keele University in the UK. “This will affect how much material evaporates from the planet, and will thus be important in understanding what will happen to such a planet and how it will meet its end.”

The research is published in Science.