The Milky Way's shiniest known planet has metal clouds that rain down titanium. Astronomers say it 'shouldn't exist.'

• A planet five times the size of Earth was found to be the shiniest planet outside our solar system. 
• Its mirror-like layer of clouds is laced with scorching titanium that rains down on its surface.
• This ultra-hot planet "shouldn't exist" as it is and could be a new class of planet altogether. 

Astronomers have spotted the shiniest known planet in the Milky Way, and it has metal clouds that appear to be raining down titanium droplets. 

Exoplanet LTT9779 b, which orbits a star about 262 light-years from Earth, is so reflective it acts almost like a mirror, the European Space Agency (ESA) said in a press release.

This planet's hot metal atmosphere is defying astronomers' rules for how planets of this size should behave.

"It's a planet that shouldn't exist," said Vivien Parmentier, a researcher at the Observatory of Côte d'Azur.

Hot clouds of molten titanium

Exoplanet LTT9779 b was first spotted in 2020, but it's only recently that ESA discovered how shiny it truly is. 

Thanks to the agency's CHEOPS satellite, which studies exoplanets, scientists found the planet reflects about 80% of the light it receives.

By comparison, Earth, which is fairly reflective, only bounces back 30% of its sunlight. Venus, the solar system's shiniest planet, reflects 75% of the sunlight.

Scientists believe the planet's mirror-like quality is due to the composition of its atmosphere. The metallic clouds that surround the planet are mostly made of silicate – the stuff that makes up sand and glass — and metals like titanium, per ESA. 

"Imagine a burning world, close to its star, with heavy clouds of metals floating aloft, raining down titanium droplets," said study co-author James Jenkins, an astronomer of Diego Portales University and CATA.

And it's precisely this bizarre atmosphere that's fascinating scientists.

A Neptune-like planet with an atmosphere that shouldn't exist

Exoplanet LTT9779 b is really close to its star. It is so close it orbits it in just under a day. It's also very big and very hot, about 3,6oo degrees Fahrenheit, which means it's similar to Neptune. 

And therein lies the conundrum. Neptune is very far away from the sun. Every time a planet has been found orbiting its star that closely before, it has either been a huge gas giant, similar to Jupiter, or a barren tiny rock planet smaller than Earth.

"We expect planets like this to have their atmosphere blown away by their star, leaving behind bare rock," Parmentier said.

The secret may lie in the composition of the clouds themselves. 

"We believe these metal clouds help the planet to survive" in these conditions said study author Sergio Hoyer of the Marseille Astrophysics Laboratory.

"The clouds reflect light and stop the planet from getting too hot and evaporating. Meanwhile, being highly metallic makes the planet and its atmosphere heavy and harder to blow away," he said. 

The findings were published in the journal Astronomy & Astrophysics Monday.