The planet probed by the Hubble, designated HD 209458b, is unlike any in our solar system. It orbits so close to its star and gets so hot that its gas is streaming into space, making the planet appear to have a comet-like tail. This new research reveals the layer in the planet’s upper atmosphere where the gas becomes so heated it escapes, like steam rising from a boiler.
HD 209458b is 150 light-years from Earth in the constellation Pegasus and is bigger than Jupiter and much closer to its parent star. The planet completes an orbit around its star every 3.5 days. It orbits 4.7 million miles from its host, 20 times closer than the Earth is to the Sun. By comparison, Mercury, the closest planet to our Sun, is 10 times farther away from the Sun than HD 209458b is from its star. Unlike HD 209458b, Mercury is a small ball of iron with a rocky crust.
The Hubble data show how intense ultraviolet radiation from the host star heats the gas in the upper atmosphere, inflating the atmosphere like a balloon. The gas is so hot that it moves very fast and escapes the planet’s gravitational pull at a rate of 10,000 tons a second, more than three times the rate of water flowing over Niagara Falls. The planet, however, will not wither away any time soon. Astronomers estimate its lifetime is more than 5 billion years.
TFOT reported several related discoveries by NASA’s space based telescopes in recent years. On December 2005 Scientists using NASA’s SWIFT satellite have reported a stellar flare on a nearby star so powerful that, had it been from our sun, it would have triggered a mass extinction on Earth. In 2006 NASA’s Hubble Space Telescope discovered 16 extrasolar planet candidates orbiting a variety of distant stars in the central region of our Milky Way galaxy. Later on that year NASA’s Spitzer Space Telescope made the first measurements of the day and night temperatures of a planet outside our solar system.
The image above shows an artist’s illustration of HD 209458b orbiting very close to its host star. More information on the new findings can be found on HubbleSite.