Extremely Hot Giant Exoplanet KELT-9b Has Boiling Atmosphere

Astronomers have found that the atmosphere of the hottest known exoplanet, the hot Jupiter-like planet KELT-9b, is “boiling off,” due to the central star’s great heat.

The escaping hydrogen atmosphere of the planet was detected by Fei Yan and Thomas Henning of the Max Planck Institute for Astronomy in Heidelberg who used the CARMENES instrument at Calar Alto Observatory.

Their observations indicate a spread-out hydrogen envelope that is being pulled towards the host star, according to a press release.

Artist’s impression of the hot star KELT-9 and its planet KELT-9b, a hot Jupiter. Researchers have now detected the extended hydrogen atmosphere of the planet, which is “boiling off” due to the central star’s great heat. Image: MPIA

KELT-9b is a hellish kind of exoplanet: Due to its proximity to an extremely hot host star, the planet itself is the hottest exoplanet yet discovered.

The planet’s host star KELT-9 is an extremely hot star with a temperature of up to 10,000 K (compare this with the Sun’s much more modest 5800 K, or 5500 degrees Celsius). The planet’s orbit is extremely small – ten times smaller than the orbit of Mercury in our Solar system (corresponding to only about 3% of the diameter of Earth’s orbit around the Sun).

When the planet was discovered in 2017 by a team of astronomers led by B. Scott Gaudi (Ohio State University), the astronomers measured its day-side temperature to be at 4600 K (4300 degrees Celsius), which is hotter than many stars!

Now Fei Yan and Thomas Henning of the Max Planck Institute for Astronomy have detected that planet’s extended atmosphere, showing that the star is not only heating up the planet’s hydrogen atmosphere – it is then using its gravity to pull the hydrogen onto itself.

“This planet reminds me of the mythical Icarus, who came to close to the Sun and crashed. Our planet will not crash, but it will certainly lose an essential part of itself, namely its atmosphere,” says Thomas Henning, director at the MPIA and co-author of the study.

The planet itself is a significantly larger version of our Solar System’s Jupiter, at almost 3 times Jupiter’s mass and almost twice Jupiter’s diameter. These properties combined place KELT-9b firmly in the class of what astronomers call “hot Jupiter”.

The planet’s orbit regularly takes it between the host star and an observer on Earth – during each such transit, the planet blocks some of the starlight, causing the star to dim a little bit as measured by telescope on Earth.

By using the CARMENES spectrograph installed at the 3.5 meter telescope at Calar Alto Observatory, researchers found traces of the planet’s atmosphere.

The extended hydrogen atmosphere surrounding KELT-9b is surprisingly large – more than half as large again as the planet’s radius. Models of how the star’s gravity will pull on the planet’s gas show that this is close to the maximal size of such an atmosphere. The large size suggests that 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.

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