Why does this Neptune-like exoplanet have a tail?

Chuck Bednar for redOrbit.com – @BednarChuck

A red dwarf star is causing a giant cloud of hydrogen gas to escape from a warm, Neptune-mass world, causing the exoplanet to have a massive comet-like tail, astronomers at the University of Warwick in the UK report in the latest edition of the journal Nature.

According to the university, the phenomenon was depicted in an image by Dr. Mark Garlick, and the discovery seems to indicate that low mass exoplanets orbiting close to their host stars may have had a percentage of their atmospheres burned off due to extreme irradiation from the star.

The planet in question is Gliese 436b (also known as GJ 436b), which was discovered in August 2004 by R. Paul Butler from the Carnegie Institute of Washington and Geoffrey Marcy from the University of California, Berkeley using the radial velocity method. At the time, it was one of the smallest known transiting planets in terms of mass and radius.

First confident detection of atmosphere loss in Neptune-sizes exoplanets

According to co-author Dr. Peter Wheatley of the Warwick Astronomy and Astrophysics Group, a large cloud of hydrogen gas was seen absorbing light from its parent red dwarf star. The cloud is created as a result of x-ray emissions from the star burning off the planet’s upper atmosphere.

“We knew that some Jupiter-mass planets are gradually evaporating due to irradiation by their parent stars, and we set out to use the Hubble Space Telescope to try to detect absorption by hydrogen gas escaping from the Neptune-sized planet GJ 436b,” Dr. Wheatley told redOrbit via email. “We were amazed by the strength of the absorption we found.”

“Usually signals of planetary atmospheres are very subtle, but here we see a very extended comet-like tail from the planet that covers more than half of the star!” he added. “This is the first time anyone has made a confident detection of the atmosphere escaping from a Neptune-sized exoplanet. With such a strong signal, future measurements can probe the composition of the planetary atmosphere – and for other small planets as well.”

In fact, Dr. Wheatley and his colleagues believe that not only are such processes occurring on other exoplanets, but that they could be strong enough to result in the evaporation of the planet’s entire atmosphere.


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