LTT 9779 b

LTT 9779 b

Planet· orbits the Sun

LTT 9779 b — an ultra-hot Neptune (4.7 Earth radii, ~29 Earth masses) skimming its star every 19 hours at ~2,000 K. It is the MOST REFLECTIVE planet known: inferred to reflect ~80% of the light that strikes it, because clouds of metal and silicate condense on its glaring dayside and glint like a mirror. Because it transits with a measured mass, its density is real. Its cloud tops have never been imaged.

Open in 3D viewSee LTT 9779 b in the live, interactive 3D map

Key facts

Mass
1.75 × 10²⁶ kg
29.3 M⊕
Radius
30,071 km
4.72× wider than Earth
Surface gravity
12.9 m/s²
1.32× Earth's
Year
19.0 hours
0.79 Earth days
Distance from LTT 9779
2,511,748 km

Science

Physical

Mass
1.75 × 10²⁶ kg · 29.3 M⊕
Radius
30,071 km · 4.72× wider than Earth
Density
1,537 kg/m³ · 1.5× water
Surface gravity
12.9 m/s² · 1.32× Earth's
Escape velocity
27.9 km/s · 2.49× Earth's

Orbital

Semi-major axis
2,511,748 km
Eccentricity
0
Inclination
0.00 °
Orbital period
19.01 hours
Perihelion
2,511,748 km
Aphelion
2,511,748 km
Mean orbital velocity
231 km/s

Composition

Bulk composition
An ultra-hot Neptune (4.7 Earth radii, ~29 Earth masses, real density from transit + mass). It is the MOST REFLECTIVE planet known: Cheops and JWST (2023) found it reflects ~80% of its starlight, inferred to be clouds of metal and silicate condensing on its glaring ~2,000 K dayside — glinting like a mirror. It survives inside the 'hot-Neptune desert' where such worlds were thought to evaporate. Never imaged.
Cloud appearance
Appearance based on NASA/JPL-Caltech's artist's concept. No spacecraft has imaged this world, so its real appearance is unknown — The banded clouds shown are a representation of its class, shown only in Vision mode; Realism shows a plain spectral colour.

Radiation & temperature

Insolation
2,526 S⊕ · 2,526× the sunlight Earth receives
Equilibrium temperature
1,978 K (1705 °C)

Discovery & history

Method
Transit detection
Name
Designated 'b' under the IAU convention that letters a star's planets 'b', 'c'… in the order found (the star is the unwritten 'a'); no IAU proper name is assigned.

Found transiting by TESS in 2020 inside the 'hot-Neptune desert'; its record albedo was measured by Cheops and JWST in 2023.

Images

Artist's concept of the 'mirror planet' LTT 9779 b, its reflective metal-and-silicate clouds glinting on the dayside (JWST) — an artist's impression, not a photograph.
NASA / ESA / CSA / R. Crawford (STScI) (CC BY 2.0) · Wikimedia Commons

Images from open archives · credit shown per image