An article published in the journal “Astronomy & Astrophysics” reports a study on the exoplanet LTT 9779 b, an ultrahot Neptune that was defined as the largest mirror in the universe because it reflects about 80% of the light it receives from its star. A team of researchers used observations conducted with ESA’s CHEOPS space telescope to study LTT 9779 b. They were able to calculate what is technically called albedo, the amount of light it reflects, in this case at exceptional levels. The conclusion is that such an extreme albedo is due to reflective metal clouds.
Just over 260 light-years from Earth, the exoplanet LTT 9779 b is slightly larger in size than Neptune but is very close to its star, which is very similar in size and mass to the Sun. Discovered in 2020 thanks to NASA’s TESS space telescope, it was already the subject of some studies but it took the CHEOPS space telescope, launched on December 18, 2019, precisely to characterize exoplanets that are already known in follow-up studies, to understand its uniqueness.
The albedo of the planet Venus is considered extraordinary because its thick clouds reflect 75% of sunlight. Normally, a planet reflects a much smaller percentage of the light it receives from its star: for example, the Earth reflects about 30% of the Sun’s light.
Typically, very-high temperatures cause compounds in a planet’s atmosphere to evaporate, and at an estimated temperature of around 2000° Celsius on the day side of exoplanet LTT 9779 b, even the metals should evaporate. It’s so close to its star that its year is 19 hours long, so one would expect the atmosphere to have been swept away by stellar radiation. However, an anomalous concentration of heavy elements may have led to condensation phenomena, a sort of sauna in which metals are present in such quantities as to saturate the atmosphere causing them to condense.
Metal-rich clouds that formed under highly abnormal conditions may have retained the atmosphere of exoplanet LTT 9779 b. Data collected by the CHEOPS and TESS space telescopes plus data collected by NASA’s Spitzer Space Telescope indicate that those clouds are mostly made of silicate with significant amounts of titanium and other metals.
The unique composition of those clouds may have resulted in an extraordinary albedo with 80% of the starlight being reflected. To measure it, the CHEOPS space telescope used what is called in jargon a secondary eclipse, when an exoplanet moves behind its star and until it reappears on the opposite side, its brightness is no longer measured.
The exoplanet LTT 9779 b was already considered interesting because it was discovered in what is called the Neptunian desert because it’s an area close to a star where planets with a mass similar to Neptune’s are rarely found. The fact that it hasn’t lost its atmosphere and the curious metallic composition of that atmosphere and its clouds make it what ESA called the largest mirror in the universe, and that makes it all the more interesting. It’s a planet that shouldn’t exist and its characteristics make it an ideal target for other follow-up studies with other instruments capable of observing it in other bands of the electromagnetic spectrum to better understand the reasons for its uniqueness.
