An alien planet orbits so close to its star that its atmosphere is being blasted away, forming a gaseous, comet-like tail, astronomers announced Thursday.Related: “Odd Star Sheds Comet-like Tail.”About 153 light-years from Earth, planet HD 209458b hugs its star so tightly that the planet’s atmosphere is likely a scorching 2,000 degrees Fahrenheit 1,093 degrees Celsius an a year passes in just 3.5 days—making Mercury’s 88-day orbit seem downright leisurely.That tight orbit also means this gas giant—meaning it’s made primarily of gas—is subjected to blistering forces from its host star, which scientists say are the cause of HD 209458b’s comet-like tail.”This planet is getting blasted by the star and that’s blowing the atmosphere off,” said astronomer Cynthia Froning of the University of Colorado, who was involved in the study of the comet-like planet.
Comet-like Tail Streaming Toward Earth
HD 209458b is one of the few known transiting planets—planets that appear to periodically cross the faces of their host stars, as seen from Earth.
This means light from HD 209458b’s star must pass through the planet’s atmosphere before reaching us.
By studying the filtered starlight, astronomers can determine such a planet’s atmospheric composition and also the direction and velocity that any gas lost to space is moving.
Using the Hubble Space Telescope’s Cosmic Origins Spectrograph, astronomers recently detected evidence of a tapered gas tail streaming away from HD 209458b and toward Earth at a speed of about 22,000 miles an hour (35,400 kilometers an hour).