Exoplanet magnetic field estimated for the first time 02.12.2014

In the two decades since the discovery of the first planet outside the solar system, astronomers have found more than 350 exoplanets and taken a big step in their study. Twenty years ago, the big event was simply the discovery of a new exoplanet, and now astrophysicists are studying the structure and chemical composition of their atmospheres, climate and other characteristics. And in some cases, their moons can even be observed.

One of the important characteristics of the planets is their magnetic field. It affects the behavior of the planet's atmosphere, since it protects it from the destructive effects of the stellar wind and interacts with its ionized part - the ionosphere and magnetosphere. In addition, it can affect the evolution of the planet itself.

Direct observations of the magnetic field of exoplanets are currently not possible. So far, attempts to detect their radio emission, which would also make it possible to estimate the magnetic field, have also been unsuccessful. However, when studying the planet HD 209458b, informally named Osiris, a cloud of hot atomic hydrogen was discovered outside its magnetosphere, "evaporating" from the planet's atmosphere under the influence of a star.

The size and shape of the hydrogen shell is determined by the interaction between the outflow of gas from the planet and the incoming stellar wind of protons, which, as it were, blow off this cloud. Knowing the parameters of the hydrogen cloud, using a certain model, one can estimate the parameters of the magnetosphere and, as a consequence, the parameters of the magnetic field.

The method of such an assessment was proposed by an international team of scientists, which includes Russian physicists, currently representing the Space Research Institute of the Austrian Academy of Sciences, Kristina Kislyakova (formerly an employee of the Nizhny Novgorod State University named after N.I. Lobachevsky) and Maxim Khodachenko (also an employee of the SINP named after D.V. Skobeltsyn, Moscow State University). With its help, they were able to estimate the magnitude of the magnetic moment of the planet HD 209458b. The research results are published in the journal Science.

Scientists simulated the formation of a cloud of hot hydrogen around the planet and showed that the observed configuration of the cloud corresponds to only one specific value of the magnetic moment and stellar wind parameters.
To make the model more accurate, astrophysicists took into account a large number of factors that determine the interaction between the stellar wind and the planet's atmosphere: the so-called recharging between the stellar wind and neutral atmospheric particles and their ionization, gravitational effects, pressure, radiative acceleration, spectral line broadening.

The planet HD 209458b, discovered in 1999 150 light-years from Earth, is among several of the most studied and intensively studied exoplanets, as it is one of the few known objects that can be seen as they pass across the star's disk. At the same time, the light of a star comes to Earth, having passed through the atmosphere of the planet, which makes it possible to study its structure and chemical composition by spectral methods. For observations, scientists used the Hubble Space Telescope.

Simulations have shown that the planet's magnetosphere is relatively small, about 2,9 planetary radii, which corresponds to a magnetic moment of only 10% of Jupiter's. This is consistent with preliminary estimates of the effectiveness of the planetary dynamo for this planet.

Such a small magnetic field is due to the fact that the exoplanet HD 209458b is a typical hot Jupiter, that is, a gas giant with a mass of the order of Jupiter, but located much closer to the star. Thus, the exoplanet under consideration is located at a distance of less than 5 million km from the star, which is 100 times closer than Jupiter in the solar system, and even 10 times closer than the closest planet to the Sun, Mercury. Its orbital period is only 3,5 days, its mass is 0,7 Jupiter masses, and its radius is 1,4 Jupiter masses.

Since the orbit of such a planet is very close to the star, it experiences a strong gravitational attraction that slows down the rotation of the planet. Since, according to the planetary dynamo theory, the generation of a magnetic field is associated with the rotation of the cores of the planets, the slow rotation of the planet leads to weak magnetic fields.

Scientists believe that the method they proposed for estimating the magnetic field can be used for all planets, including those similar to the Earth, if there is a high-energy hydrogen shell around them. It is worth noting that about 15% of exoplanets are hot Jupiters.