The planet HD 209458b, sometimes known as Osiris, was the first to be found through the transit method back in 1999. From our line of sight, the orbit of the planet takes it between us and the star 209458 every 3.5 days. The planet and its bloated atmosphere then block out about eight percent of the light shining from the star, allowing the passage of a planet 153 light years away to be detected from the ground.
Observational techniques, modelling and instruments have all been quickly improving as the field of exoplanetology expands. Simulations of a Jupiter like planet near to a star in similar conditions to Osiris showed that the atmosphere expanded as far as the roche lobes, suggesting escape of gases was possible. Just last month Osiris was seen to have raging winds shown up by the Doppler shifts of gases light up by the star. Now, using the new Cosmic Origins Spectrograph, the Hubble Space Telescope has used more Doppler shifts to characterise the velocity structure of the atmosphere. And the only thing that fits the results is a comet like tail of gases leaving the planet’s night side.
The researchers believe the proximity of the star heats up the atmosphere to such an extent that the upper layers of the atmosphere expand farther than the gravitational tug of the planet can compensate for. Winds of particles, basically plasma from the star doing the same thing, expanding thermally away from it, then push the gas away, creating a comet like tail. It should however be noted that at the current rate of evaporation, the planet still has a trillion years left in it, which means the star will likely swallow it during its evolution.