Astronomers solve mystery of how a mirror-like planet formed so close to its star

Enlarge / An artist impression of exoplanet LTT9779b orbiting its host star. The planet is around the size of Neptune and reflects 80 percent of the light shone on it.

It has been about three decades since the first confirmed exoplanet was discovered. In the following 30 years, using a variety of telescopes and instruments on the ground and in space, astronomers have cataloged more than 5,000 planets around other stars.

As part of this process of scientific discovery, astronomers have confirmed that our Milky Way galaxy teems with billions of planets. They exist around many (if not most) stars, and they come in all sizes and flavors. There are very large and very small planets and very hot and very cold ones. There are more than a few that could harbor life as we know it on Earth.

After this initial wave of discovery, powered by such NASA survey missions as the Kepler Space Telescope and the Transiting Exoplanet Survey Satellite, second-generation instruments like Europe’s small Cheops satellite have sought to characterize the nature of these exoplanets. Launched less than three years ago on a Soyuz rocket, the Cheops instrument has delivered some valuable insights on planets orbiting other stars.

A very bright planet

On Monday, in the journal Astronomy & Astrophysics, European scientists describe one of these worlds that has an extremely high albedo, planet LTT9779 b. An albedo is simply the amount of light reflected by a planet back into space. Earth reflects about 30 percent of the Sun’s light into space, whereas Venus, with its thick clouds, reflects 75 percent of its light.

The planet LTT9779 b, which is located around a Sun-sized star about 260 light-years from Earth, has a higher albedo than that of Venus, about 80 percent. One big question for scientists is how the planet could reflect so much light, as it’s hot enough that it should not have any clouds. This is because it is located extremely close to its star, orbiting once every 19 hours.

This is a pretty hellish planet with a radius slightly larger than that of Neptune (and 4.7 times that of Earth) and a surface temperature on the order of about 2,000° Celsius. Based upon their observations of other exoplanets, astronomers were surprised to find a Neptune-sized world so close to its star. Before, only large worlds (similarly sized to Jupiter) or much smaller worlds have been found so close to stars. Accordingly, the environment near stars has been characterized as a “hot Neptune desert.”

But with the discovery of planet LTT9779 b in 2020, astronomers had themselves a mystery. Here was a ‘hot Neptune’ in a hot Neptune desert. And it was extremely bright, the shiniest exoplanet ever observed.

Mystery explained

These were the kinds of mysteries that missions such as Cheops were designed to solve with their ability not to survey the entire night sky but to focus in on a single world for an extended period of time.

In this case, Cheops observed the planet as it transited behind its star on 10 different occasions. During these transits, the telescope measured the combined light coming from both the star and planet to observe the difference and infer the intrinsic brightness of LTT9779 b.

From this data, scientists have developed a theory that the planet started out as a gas giant but has been losing mass over time. It must have an atmosphere composed of silica material, like glass, along with titanium. Effectively, then, the atmosphere has a mirror-like composition.

No clouds should be able to exist at the surface temperatures on a world like this, estimated at about 2,000° Celsius—even those made of metal and glass.

“It was really a puzzle until we realized we should think about this cloud formation in the same way as condensation forming in a bathroom after a hot shower,” said Vivien Parmentier, a researcher at the Observatory of Côte d’Azur (France) and co-author of the study. “To steam up a bathroom, you can either cool the air until water vapor condenses, or you can keep the hot water running until clouds form because the air is so saturated with vapor that it simply can’t hold any more.”

In the case of LTT9779 b, he said, the planet can form metallic clouds because the atmosphere is super-saturated with silicate and metal vapors. This means that, quite literally, it rains titanium on this weird world.

Additional information could be gleaned about this shiny new object if other space-based telescopes, such as Hubble and James Webb, make similar observations about the planet and its atmosphere.

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