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An ultra-hot exoplanet orbiting its host star every 19 hours is the shiniest exoplanet ever discovered.
The torrid world, named planet LTT9779b, has reflective metallic clouds made of silicates and metals such as titanium.
The exoplanet is the largest “mirror” in the universe that astronomers have observed to date. The planet is located 262 light-years from Earth.
The exoplanet was first spotted by NASA’s TESS planet-hunting mission in 2020 and ground-based observations from the European Southern Observatory in Chile. The exoplanet was selected for follow-up observations by the European Space Agency’s Cheops mission.
Cheops, or the characterizing exoplanet satellite, collected measurements that revealed that LTT9779b reflects 80% of its parent star’s light, which is faster than the high glare of Venus in our own solar system. After the moon, Venus is the brightest object in our night sky and its thick clouds reflect about 75% of the sunlight. Meanwhile, the Earth only reflects about 30% of sunlight.
A study describing the findings was published Monday in the journal Astronomy & Astrophysics.
“Imagine a burning world, close to its star, with heavy clouds of metals floating up and raining titanium droplets,” study co-author James Jenkins, an astronomer at Diego Portales University in Santiago, Chile, said in a statement.
A planetary puzzle
The amount of light reflected from objects is known as albedo, and most planets have low albedo due to dark and rough surfaces or atmospheres that absorb light. Icy worlds, such as Saturn’s moon Enceladus and Jupiter’s moon Europa, and the reflective clouds of Venus are known exceptions.
But the shine of LTT9779b is a surprise. The side of the exoplanet facing its host star likely reaches 3,632 degrees Fahrenheit (2,000 degrees Celsius). Temperatures above 212 degrees Fahrenheit (100 degrees Celsius) are too hot for water clouds to form. And LTT9779b is so blazing hot there should be no clouds, not even metal or glass.
“It was really a puzzle until we realized that we had to think about this cloud formation in the same way that condensation forms in a bathroom after a hot shower,” says study co-author Vivien Parmentier, a researcher at the Observatory of the Côte d’Azur in Nice. , France, in a statement.
“To humidify a bathroom, you can either cool the air until the water vapor condenses, or you can run the hot water until clouds form, because the air is so saturated with vapor that it just can’t hold any more. Similarly, LTT9779b can form metal clouds despite being so hot because the atmosphere is supersaturated with silicate and metal fumes,” Parmentier explains.
There are other confounding factors about LTT9779b, including size. With a size similar to Neptune and scorching temperatures, the planet is what astronomers call an “ultra-hot Neptune” – but it’s the first time a planet like this has been found so close to its star.
“It’s a planet that shouldn’t exist,” Vivien said. “We expect the atmospheres of such planets to be blown away by their star and bare rocks left behind.”
Instead, the researchers think the planet’s metallic clouds help it survive in such an unlikely location.
“The clouds reflect light and prevent the planet from getting too hot and evaporating,” first study author Sergio Hoyer, a postdoctoral researcher at France’s Marseille Astrophysics Laboratory working on the Cheops mission, said in a statement. “Meanwhile, the planet and its atmosphere are heavy and harder to blow away because they are very metallic.”
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