Aside from the Moon, the brightest object in our night sky is planet Venus, whose thick cloud layer reflects around 75% of the Sun’s light. By comparison, Earth only reflects around 30% of incoming sunlight. A new analysis of data from ESA’s Cheops spacecraft shows that the ultra-short-period Neptune-sized exoplanet LTT 9779b reflects 80% of the light shone on it by its host star, LTT 9779, making it the shiniest exoplanet ever found. The reason for its high reflectivity is that it is covered by reflective clouds of metal and silicate.
An artist’s impression of LTT 9779b orbiting its host star. Image credit: Ricardo Ramírez Reyes, Universidad de Chile.
LTT 9779 is a G8-type star located 260 light-years away in the constellation of Sculptor.
Also known as TOI-193, CD-38 15670 and HIC 117883, the star is around 2 billion years old and is metal-rich, having twice the amount of iron in its atmosphere than the Sun.
LTT 9779b was first discovered in 2020 by astronomers using NASA’s Transiting Exoplanet Survey Satellite (TESS).
The planet is 4.6 times larger than Earth and 29 times as massive. Its mean density is similar to that of Neptune and its atmosphere makes up around 9% of the total planetary mass.
The alien world has an orbital period of 19 hours and is located in the so-called ‘Neptunian desert,’ a region devoid of planets when we look at the population of planetary masses and sizes.
The stellar radiation heats the planet to 1,727 degrees Celsius (3,141 degrees Fahrenheit). At these temperatures, heavy elements like iron can be ionized in the atmosphere and molecules disassociated.
“Imagine a burning world, close to its star, with heavy clouds of metals floating aloft, raining down titanium droplets,” said Dr. James Jenkins, an astronomer at Diego Portales University and CATA.
LTT 9779b’s high albedo (the fraction of light that an object reflects) came as a surprise.
Any temperature above 100 degrees Celsius (212 degrees Fahrenheit) is too hot for clouds of water to form, but the temperature of this planet’s atmosphere should even be too hot for clouds made of metal or 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 Dr. Vivien Parmentier, an astronomer at the Observatory of Côte d’Azur.
“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.”
“Similarly, LTT 9779b can form metallic clouds despite being so hot because the atmosphere is oversaturated with silicate and metal vapors.”
“We believe these metal clouds help the planet to survive in the hot Neptune desert,” said Dr. Sergio Hoyer, an astronomer at 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.”
To determine LTT 9779b’s properties, the astronomers used data gathered by ESA’s Cheops mission.
“LTT 9779b is an ideal target for follow-up with the exceptional capabilities of both the Hubble and James Webb space telescopes,” said ESA science operations scientist Emily Rickman.
“They will allow us to explore this exoplanet with a wider wavelength range including infrared and UV light to better understand the composition of its atmosphere.”
The findings were published in the journal Astronomy & Astrophysics.
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S. Hoyer et al. 2023. The extremely high albedo of LTT 9779 b revealed by CHEOPS: An ultrahot Neptune with a highly metallic atmosphere. A&A 675, A81; doi: 10.1051/0004-6361/202346117
