LTT 9779 b, officially named Cuancoá, is a Neptune-sized planet orbiting the sunlike star LTT 9779, or Uúba. As of 2023, it has the highest-known albedo of any planet. It was discovered in 2020 using data from the Transiting Exoplanet Survey Satellite (TESS), and is also called TOI-193 b.
LTT 9779 b is one of the few known planets in the Neptunian desert. It is highly reflective, with an albedo of (reflects 73% of light +/- 11%) . This makes it the most reflective exoplanet discovered so far. It completes an orbit around LTT 9779 in less than a day, making temperatures on the day side soar to over 2,000 degrees Celsius. Global climate models of the planet indicate it has a very metal-rich atmosphere, with clouds made of silicate likely being present.
Being in the Neptunian desert, LTT 9779 b is a very rare class of planet, with few like it being known. It is estimated that only 1 in 200 Sun-like stars possess a planet with an orbital period of less than a day, and most of those are Hot Jupiters or rocky planets, with ultra-hot Neptune planets being rare. Because of this, LTT 9779 b has been extensively studied by many space telescopes including Hubble and James Webb (JWST).
A study using JWST NIRISS found highly reflective white clouds on the western dayside of LTT 9779b. The researchers suggest an eastwards equatorial jet, leading to a colder western dayside, allowing the formation of silicate clouds. These clouds can be best described as composed of MgSiO<sub>3</sub> (enstatite) and Mg<sub>2</sub>SiO<sub>4</sub> (forsterite). The study also detected water vapor in the atmosphere in the form of a dip at 1.4 üm. The dayside temperature was measured to be and the nightside temperature was measured to be <1,330 K. However, a pure cloudy atmosphere is inconsistent with the results of a 2025 study, which indicates that some mixing processes might have affected the atmospheric composition of LTT 9779 b.
Another study in 2025 confirms the planet has a metal-rich atmosphere and also suggest it likely has silicate (MgSiO<sub>3</sub>) clouds.
LTT 9779 b was officially named Cuancoá in 2022 by the International Astronomical Union, as part of the NameExoWorlds competition. Cuancoá is a word that refers to the morning star in the Uwa language (Tunebo). Cuancoá's star was named Uúba after the word for "star," "seed," and "eye" in the same language.