This Mirror-Like Exoplanet Is the Most Reflective Ever Discovered

The ultra-hot world is wreathed in metallic clouds that prevent the planet from evaporating and keep its atmosphere intact

An artist's rendition of a brightly shining star against the darkness of space, with an exoplanet seen orbiting it to its right
An artist's rendition of the highly reflective exoplanet LTT 9779 b orbiting its host star. Ricardo Ramírez Reyes (University of Chile)

Astronomers have identified the most reflective exoplanet ever found, an ultra-hot, Neptune-like world gleaming in distant space.

The planet, called LTT9779 b, reflects around 80 percent of the light that hits it from the star it orbits, according to a statement from the European Space Agency (ESA). By comparison, only 30 percent of our sun’s light bounces off Earth. And Venus, our solar system’s shiniest planet, reflects 75 percent of sunlight.

Researchers determined that metallic clouds, made mostly of silicate (which makes up sand and glass) as well as titanium, blanket the exoplanet, reflecting light and giving the world its unusual luster. They published their findings Monday in the journal Astronomy & Astrophysics.

“It’s a giant mirror in space,” James Jenkins, a co-author of the study and an astronomer at Diego Portales University in Chile, tells Reuters Will Dunham.

Located 262 light-years from Earth, LTT9779 b was first discovered in 2020. Its mass is almost 30 times that of Earth, and its radius is about half of Jupiter’s. But the distance between the exoplanet and its sun is only a tiny fraction of the space between Earth and our own. As a result, the exoplanet completes one orbit very quickly—a year on this mirror world lasts only about 19 hours.

But this type of planet is extremely uncommon. All known planets with such short orbits are either gas giants that measure more than ten times larger than Earth, or rocky planets roughly Earth’s size, per the 2020 paper. Adding to its rarity, the mirror planet has managed to have an atmosphere, despite being a scorching 3,650 degrees Fahrenheit.

“It’s a planet that shouldn’t exist,” Vivien Parmentier, a co-author of the new study and a physicist at the Observatory of Côte d’Azur in France, says in the statement. “We expect planets like this to have their atmosphere blown away by their star, leaving behind bare rock.”

However, in the case of LTT9779 b, its clouds formed in such hot temperatures because “the atmosphere is oversaturated with silicate and metal vapors,” Parmentier says in the statement. It’s like steaming up a bathroom by continuously running a hot shower, he adds—“clouds form because the air is so saturated with vapor that it simply can’t hold any more.”

The mirror-planet’s atmospheric clouds are the key for it to exist in such an unlikely way, astronomers say. “The clouds reflect light and stop the planet from getting too hot and evaporating,” Sergio Hoyer, a co-author of the new study and an astronomer at the Marseille Astrophysics Laboratory in France, says in the statement. “Meanwhile, being highly metallic makes the planet and its atmosphere heavy and harder to blow away.”

Researchers conducted the new study of LTT9779 b using the ESA’s Cheops satellite, which aims to learn more about exoplanets bigger than Earth but no larger than Neptune that are closely orbiting bright stars. The satellite orbits Earth from about 435 miles away.

The team watched the exoplanet as it swung behind its sun to determine its reflectivity. By measuring the difference between how much light the planet and star reflected together and how much light was reflected by just the star once the planet was hidden, the researchers could calculate how much light the exoplanet alone reflected.

At the time of the planet’s discovery, Nicolas Cowan, a co-author of the 2020 study who researches exoplanets at McGill University in Canada, said in a statement, “The planet is much cooler than we expected, which suggests that it is reflecting away much of the incident starlight that hits it, presumably due to dayside clouds.”

With the new research, astronomers have confirmed those initial suspicions. And in the future, they hope to take a closer look at the exoplanet’s reflective atmosphere in ultraviolet and infrared light using the Hubble and James Webb space telescopes, per the ESA statement.

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