LTT 9779 b

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.^[3] It was discovered in 2020 using data from the Transiting Exoplanet Survey Satellite (TESS),^[4] and is also called TOI-193 b.^[5]

LTT 9779 b / Cuancoá

Discovery
Discovered by	Jenkins et al.
Discovery site	Transiting Exoplanet Survey Satellite
Discovery date	2020
Detection method	Transit
Designations
Alternative names	Cuancoá, TOI-193 b
Orbital characteristics[1]
Semi-major axis	0.01734+0.00136 −0.00160 AU
Eccentricity	<0.01[citation needed]
Orbital period (sidereal)	0.79206418(12) days
Inclination	76.55°+1.27° −1.75°
Star	LTT 9779
Physical characteristics[1]
Mean radius	4.501+0.135 −0.157 R🜨
Mass	29.65+0.78 −0.77 M🜨
Albedo	0.73±0.11
Temperature	Dayside: 2,260+40 −50 K[2] Nightside: 1,330 K[2]

Characteristics

LTT 9779 b is one of the few known planets in the Neptunian desert.^[6] It is highly reflective, with an albedo of 0.73±0.11.^[1] 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.^[7] Global climate models of the planet indicate it has a very metal-rich atmosphere, with clouds made of silicate likely being present.^[3]

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,^[6] and most of those are Hot Jupiters or rocky planets, with ultra-hot Neptune planets being rare.^[6] Because of this, LTT 9779 b has been extensively studied by many space telescopes including Hubble and James Webb (JWST).^[8]

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 MgSiO₃ (enstatite) and Mg₂SiO₄ (forsterite). The study also detected water vapor in the atmosphere in the form of a dip at 1.4 μm. The dayside temperature was measured to be 2,260+40
−50 K and the nightside temperature was measured to be <1,330 K.^[2][9] 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.^[8]

Another study in 2025 confirms the planet has a metal-rich atmosphere and also suggest it likely has silicate (MgSiO₃) clouds.^[1]

Name

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).^[10] Cuancoá's star was named Uúba after the word for "star," "seed," and "eye" in the same language.

See also

• List of proper names of exoplanets

• Neptunian desert

• NGTS-4b, another planet in the Neptunian desert