For the first time, scientists have determined the absolute mass of an exoplanet. And the new technique they used revealed high winds in the atmosphere of Osiris, a Jupiter-like planet orbiting a star 150 light years from Earth.

What's exciting about this discovery, reported today in Nature, is the way the scientists were able to determine the planet's mass. According to a release about their paper:

Most of the known exoplanets have been discovered by observing the wobble induced in the host stars by their orbiting companions. For a planetary orbit viewed edge-on, the amplitude and period of the star's wobble can be used to calculate the planet's mass, as a proportion of the stellar mass - which itself must be estimated from the star's spectral characteristics and distance.


Now Ignas Snellen and colleagues report a new method of obtaining an exoplanet's mass, by measuring its orbital velocity. As the planet HD 209458b [Osiris] passed between its host star and the Earth, the authors measured the absorption of the star's light by the planet's atmosphere. Using a high-resolution spectrograph, the authors measured the changing Doppler shift of molecular absorption lines of carbon monoxide, caused by the planet's orbital motion. From the orbital velocities of both star and planet, the authors derived their masses directly, by applying Newton's law of gravity.

An examination of the particles streaming through Osiris' atmosphere also revealed that strong winds are coursing through its upper atmosphere. One side of Osiris always faces its sun, and one side always faces away. This creates a steady stream of wind from the sunny side to the shadowed side.


So what exactly is Osiris' absolute mass? According to Snellen and his coauthors, the mass of its sun and the planet are, respectively, "1.00+0.22MSun and 0.64+0.09MJup." OK then.

Full scientific paper via Nature