The hottest planet acts like a mirror

Besides the Moon, the brightest object in our night sky is the planet Venus, whose thick cloud cover reflects about 75% of the Sun’s light. In comparison, the Earth just reflects 30% of sunlight.

Now, for the first time, astronomers have found a exoplanet which can correspond to shine of venus: the planet LTT9779 b.

New detailed measurements from the European Space Agency’s (ESA) Cheops mission reveal that this planet reflects up to 80% of the light radiated by its host star.

The high-precision measurements made by Cheops were a specific follow-up to the initial discovery and characterization of the planet in 2020 by NASA’s TESS mission and ground-based instruments such as the HARPS instrument at the European Southern Observatory (ESO) in Chile.

The exoplanet is roughly the size of Neptune, making it the largest ‘mirror’ of the universe we know of today. Its high reflective capacity is due to the fact that it is covered by metallic clouds. These are mostly made of silicate – the same stuff sand and glass are made of – mixed with metals like titanium.

“Let’s imagine a world on fire, close to its star, with heavy clouds of metals floating in the air, raining drops of titanium”, illustrates James Jenkins, astronomer at the Universidad Diego Portales and CATA (Santiago, Chile). Jenkins is co-author of the scientific paper describing the new research, published today in the journal Astronomy & Astrophysics.

A sky full of metal clouds

O light fraction that reflects an object is called ‘albedo’. Most planets have a low albedo, either because they have an atmosphere that absorbs a lot of light, or because their the surface is dark or rough. Exceptions are usually icy worlds or planets like Venus that have reflective cloud cover.

He high albedo of LTT9779 b was a surprise, as the side of the planet facing its star is estimated to have a temperature of around 2000 °C. Anything above 100°C is too hot for water clouds to form, but the temperature of this planet’s atmosphere must be too hot even for clouds made of metal or glass.

“It was really a puzzle, until we realized that we had to think of this cloud formation in the same way as the condensation that forms in a bathroom after a hot shower,” says Vivien Parmentier, a researcher at the Côte d’Azur Observatory. (France) and co-author of the work.

Cheops proves that a scorching exoplanet acts like a mirror.

The scientist explains: “To fog a bathroom, you can cool the air until the water vapor condenses, or you can run hot water until clouds form, because the air is so saturated with vapor that it simply cannot hold any more. Likewise, LTT9779 b can form metallic clouds despite being so hot because the atmosphere is supersaturated with silicate and metallic vapors.”

The Planet That Shouldn’t Exist

Brightness isn’t the only amazing thing about the LTT9779 b. From him size and temperature make it a ‘Ultrahot Neptune’, although no other planet of this size and mass has orbited this close to its star. This means that it lives in what is known as the ‘hot desert of Neptune’ (the simile refers to a new class of planets located in a place that is like a scorching ‘desert’, where such objects are rarely found).

The planet has a radius 4.7 times that of Earth, and a year on LTT9779 b lasts just 19 hours. All previously discovered planets that orbit their star in less than a day are ‘hot Jupiters’ – gas giants with a radius at least ten times that of Earth – or rocky planets with less than two Earth radii.

“It’s a planet that shouldn’t exist,” says Parmentier. “We expected planets like this one to have their atmosphere destroyed by their star, leaving behind bare rock,” he said.

First author Sergio Hoyer of the Marseille Astrophysics Laboratory suggests that these metallic clouds could help the planet survive in the hot ‘desert of Neptune’: “The clouds reflect light and prevent the planet from warming too much and evaporates. In addition, he points out, “because it is very metallic, the planet and its atmosphere weigh more and are more difficult to drag”.

Study an exoplanet when it is hidden

To determine LTT9779 b’s properties, ESA’s Cheops Exoplanet Characterization mission observed it as the planet moved behind its host star. Because the planet reflects light, the star and planet together send more light to the space telescope just before the planet disappears from view than shortly after. The difference in visible light received just before and just after the planet sets indicates how much light the planet reflects.

This project was based on accuracy and coverage permanent by Cheops. “To accurately measure the small change in the sign of the eclipsing star to the planet was only possible with Cheops”, says Hoyer.

Maximilian Günther, also a project scientist, adds: “Cheops is the first space mission dedicated to monitoring and characterizing known exoplanets. Unlike large survey missions focused on discovering new exoplanetary systems, Cheops is flexible enough to quickly zero in on interesting targets and can achieve coverage and accuracy that we sometimes don’t get otherwise.”

By observing the same exoplanet with different instruments, you get the full picture. Indeed, as ESA Science Operations Scientist Emily Rickman points out, “LTT9779 b is an ideal target for monitoring thanks to the exceptional capabilities of the telescopes space Hubble and James Webb”. The specialist concludes that these “will allow exploring this exoplanet with a wider range of wavelengthsincluding infrared and ultraviolet light, to better understand the composition of its atmosphere”.

The future of exoplanet research is bright, according to the researchers in the presentation of this study, as Cheops is the first of a trio of missions dedicated to exoplanets. It will join Plato in 2026, which will focus on Earth-like planets that orbit at a distance from their star that could be sufficient for life. Ariel will join the fleet in 2029 and specialize in studying exoplanet atmospheres.