Atmosphere of hot exoplanet is boiling away

Parent star’s gravitational pull is siphoning planet’s atmospheric hydrogen.

The atmosphere of the hottest known exoplanet is boiling away, and its escaping gases are being captured by its parent star, according to a new study published in the journal Nature Astronomy.

A hot Jupiter, or gas giant planet orbiting very close to its star, KELT-9b was discovered last year by a team of scientists using the Kilodegree Extremely Little Telescope (KELT) at Winer Observatory in southeastern Arizona.

KELT uses the transit method to search for exoplanets, looking for periodic drops in a star’s brightness caused by a planet transiting or passing in front of its parent star.

The extremely hot star, KELT-9, is both hotter and larger than the Sun, with temperatures of 17,540 degrees Fahrenheit (9,726 degrees Celsius). Because the planet is in such a close orbit, circling KELT-9 once every 1.5 days, it is tidally locked to the star, with one hemisphere always facing the star and the other always facing away from it.

Temperatures on KELT-9b’s dayside are hotter than those of most stars and can reach 7,800 degrees Fahrenheit (4,300 degrees Celsius).  The planet is about twice the diameter of Jupiter and has nearly three times its mass.

In spite of being in such a close orbit, KELT-9b will not fall into its star, as some hot Jupiters do.

“This planet reminds me of the mythical Icarus, who came close to the Sun and crashed. Our planet will not crash, but it certainly will lose an essential part of itself, namely its atmosphere,” said Thomas Henning of the Max Planck Institute for Astronomy in a public statement.

When the researchers viewed the planet with the CARMENES instrument on the 3.5-meter telescope at Calar Alto Observatory in Spain, they found it to have an extended hydrogen atmosphere, a discovery that indicated the star’s gravity is not just heating the planet’s atmosphere, but sucking in its hydrogen.

“The large size (of the atmosphere) suggests the planet is losing hydrogen gas at a significant rate of more than 100,000 tons of hydrogen per second. The star is ‘boiling off’ the planet’s atmosphere, and pulling the gas onto itself, in a blatant case of interplanetary theft,” the researchers note in their statement.

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