6 February 2007

Hubble sights gas giant’s escaping atmosphere

Cosmos Online
A planet 150 light years from Earth is so close to its sun that its atmosphere is literally boiled away, streaming behind it like a comet's tail.
Hubble sights gas giant's escaping atmosphere

An artist's impression of HD 209458b orbiting its star. the planet orbits so close that its atmosphere is literally boiling away into space, say astrnomers. Credit: NASA, ESA, and G. Bacon (STScI)

SYDNEY: A planet 150 light years from Earth is so close to its sun that its atmosphere is literally boiled away, streaming behind it like a comet’s tail.

According to a group of U.S. astronomers, data from the Hubble Space Telescope show how intense ultraviolet radiation from the host star heats the gas in the planet’s upper atmosphere, inflating it like a balloon. The gas becomes so hot and energetic that it escapes the planet’s gravitational pull, blasting out into space at a rate of nearly 10,000 tonnes per second – more than three times the rate of water flowing over Niagara Falls.

The escaping gas gives the planet a tail, said the team, but is only a tiny fraction of the amount of gas in the planet’s atmosphere; at the current loss rate, the planet has enough for a lifespan of five billion years.

The scorched planet – named only HD 209458b – is unlike any world in our solar system. Although more than half the size of Jupiter, the planet rockets around its star – HD 209458 – every 3.5 days. Its orbits is only 7.5 million kilometres from HD 209458 – 20 times closer than the Earth is to our Sun. By comparison, Mercury, a small ball of iron with a rocky crust and the closest planet to our Sun, is 10 times farther away.

HD 209458b is one of the most intensely studied extrasolar planets because it is one of the few known alien worlds that can be seen passing in front of, or transiting, its star, causing the star to dim slightly. In fact, the gas giant was the first such alien world found to transit its star.

The Hubble observations revealed oxygen, carbon, and sodium in HD 209458b’s atmosphere, as well as a huge amount of hydrogen in its upper atmosphere. The escaping gas comes from only this top-most layer, said research team leader Gilda Ballester, of the University of Arizona in Tucson. “The layer we studied is actually a transition zone where the temperature skyrockets from about 727° C to 14,727° C – which is hotter than the Sun,” she said.

Ten to fifteen per cent of the more than 200 known extrasolar planets are ‘hot Jupiters’. A recent Hubble survey netted 16 hot Jupiter candidates in the central region of our Milky Way Galaxy, suggesting to astronomers that there may be billions of these gas-giant star huggers in our galaxy.”With this detection we see the details of how a planet loses its atmosphere,” said Ballestar. “This planet’s extreme atmosphere could yield insights into the atmospheres of other ‘hot Jupiters’.”

Studying distant planets like HD 209458b can help develop the techniques and technology to search for terrestrial planets around distant stars, according to team. Additionally, understanding the properties of distant planetary systems can also help estimate the potential number of habitable environments in the universe.

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