The exoplanet HD 209458b has a magnetic field which has been measured from Earth for the first time, using a unique method which could be used by astronomers examining other alien worlds.

Planet HD 209458b (Osiris) has a mass slightly less than that of Jupiter, packed into a globe roughly one-third larger than the largest planet in our solar system. This giant world orbits around its parent star once every 3.5 Earth days, as the world races around its star at a distance just one-eigth that between Mercury and the Sun. This was the first exoplanet on which an atmosphere was discovered. The alien world lies 150 light years from Earth. Temperatures on the gas giant can reach 1,800 degrees Fahrenheit.  

The first planet orbiting an alien world was discovered 20 years ago. At first, each new planet discovered made the news, but now over 1,000 are known, and many more are likely to be confirmed with further observations. Today, astronomers are able to detect moons orbiting alien planets, as well as measure the content of atmospheres, and produce general weather reports.

The magnetic field of the Earth helps protect life on the planet, including human beings, from dangerous radiation. Astronomers will be able to gain significant information about the composition of exoplanets by studying their magnetic fields.

Researchers were able to determine the strength and alignment of the alien world's magnet field by studying the atmosphere of the planet. As the world orbited in front of its stellar parent, as seen from Earth, astronomers carefully measured the absorption of light by the distant atmosphere. This allowed them to determine the composition and shape of the air, which revealed data about the world's magnetic makeup. Effects of solar wind, gravity, pressure, and more were removed from computer models of the alien atmosphere.

"We modeled the formation of the cloud of hot hydrogen around the planet and showed that only one configuration, which corresponds to specific values of the magnetic moment and the parameters of the stellar wind, allowed us to reproduce the observations," Kristina Kislyakova from the Space Research Institute at the Austrian Academy of Sciences, said.

Interactions between the upper atmosphere of the alien world and solar wind were studied, to provide further refinements to the model. The magnetic field of Planet HD 209458b was found to be 10 percent that seen at Jupiter. This process could be applied to the study of other worlds orbiting alien stars.

Planet HD 209458b was discovered on November 5, 1999, and has been intensely studied by astronomers over the last 15 years.

Astronomers published their method of estimating magnetic fields of exoplanets in the journal Science.