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Hottest exoplanet ever discovered has metallic skies, rain like lava

Kelt-9b is so hot because it's 30 times closer to its star than we are to the sun.
Kelt-9b is more than 600 light-years away and is the hottest planet ever observed.Denis Bajram / Geneva University

One day in the distant future, a team of intrepid humans might board a starship and set out for a world beyond our solar system — maybe one of the exoplanets of Alpha Centauri, the nearby star system.

One place we'll never set foot on is Kelt-9b. In addition to being a gas giant without a solid surface, Kelt-9b lies hundreds of light-years away and is the hottest planet ever observed. Temperatures on its outer layer can exceed 4,000 degrees Celsius (7,000 degrees Fahrenheit) — hotter than some stars — and a new study shows that its superheated atmosphere contains vaporized heavy metals.

"Metals have been thought to be an important ingredient in forming exoplanets, but they were never directly directed," study co-author Kevin Heng, a professor of astrophysics at the University of Bern in Switzerland, told NBC News MACH in an email. "Our result is the first robust, direct detection."

For the research, published Aug. 15 in the journal Nature, an international team of scientists used computer simulations to predict the presence of vaporized iron and titanium in Kelt-9b's atmosphere. Then they compared their prediction to observational data already collected by the Galileo National Telescope in the Canary Islands — and saw that the prediction matched the data.

The match was "spot on," Heng said. The same research methods might be used "to detect molecules that hint at biology (biosignatures) in future, yet-to-be-detected exoplanets," he added. "In this way, these hot exoplanets are simply training grounds for us to battle-test our techniques for future detections of biosignatures" — that is, for finding evidence of extraterrestrial life.

Kelt-9b is more than 600 light-years from Earth in the constellation Cygnus. Discovered last year by American astronomers, it's about twice the size of Jupiter and about three times as massive.

The planet's orbit around its host star is about 30 times tighter than Earth's orbit around the sun; as a consequence, Kelt-9b completes one orbit of its star every 36 hours. (Earth, of course, takes 365 days to orbit the sun.) And the star looms large in Kelt-9b's sky. In fact, it covers 35 percent of the exoplanet's sky, which is about 70 times the apparent size of our sun.

Sara Seager, an astrophysicist and planetary scientist at MIT, said in an email that it was "fantastic to know about a planet that is so hot, hotter than many stars. The takeaway ... is that so far, we haven't found any solar system copies, but we have found a growing list of bizarre and enigmatic planets."

The planet's peculiarities don't end with high temperatures and metallic vapors. Like our moon in its orbit around Earth, Kelt-9b is tidally locked. One side always faces the host star and is perpetually illuminated while the other always faces away, locked in perpetual night.

And if the temperatures fall low enough, the metal atoms in the atmosphere might link to form molecules and then coalesce into particles that would sink into the planet's interior. "Just like clouds on Earth drop liquid water raindrops, the rain on the night side would be iron drops," Seager said, adding that the drops would be very dense.

But what would iron rain be like? Jens Hoeijmakers, a postdoctoral student at the Universities of Geneva and Bern and the study's lead author, offered an analogy: "If you want to imagine it, raining lava might get you close!"

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