TOI-791 b
TOI-791 b — a planet as wide as Jupiter that weighs about nine and a half Earths, which makes it about a twentieth as dense as water: a tenth of Saturn, and far thinner than cork. It circles its star every 139 days, at about two-thirds the Earth-Sun distance, where the sunlight leaves it near 434 K. Both its size and its weight are measured, so the density is a real number rather than an inference — which is what makes it hard to explain.
Key facts
Science
Physical
- Mass
- 5.67 × 10²⁵ kg · 9.5 M⊕
- NASA Exoplanet Archive (Dransfield et al. 2026, MNRAS) — adopted transit + mass solution, read 2026-09-23
- Radius
- 70,913 km · 11.1× wider than Earth
- NASA Exoplanet Archive (Dransfield et al. 2026, MNRAS) — adopted transit + mass solution, read 2026-09-23
- Density
- 38 kg/m³ · 0.038× water
- Computed from mass and radius
- Surface gravity
- 0.753 m/s² · 8% of Earth's gravity
- Computed from mass and radius
- Escape velocity
- 10.3 km/s · 92% of Earth's escape velocity
- Computed from mass and radius
Orbital
- Semi-major axis
- 0.602 AU
- From the NASA Exoplanet Archive (transit + orbit fit)
- Eccentricity
- 0.032
- From the NASA Exoplanet Archive (transit + orbit fit)
- Inclination
- 0.00 °
- From the NASA Exoplanet Archive (transit + orbit fit)
- Orbital period
- 139.3 days
- From the NASA Exoplanet Archive (transit + orbit fit)
- Perihelion
- 0.583 AU
- Computed from the orbital elements
- Aphelion
- 0.621 AU
- Computed from the orbital elements
- Mean orbital velocity
- 47 km/s
- Computed from the orbital elements
Composition
- Bulk composition
- A transiting giant of 11.13 ± 0.37 Earth radii and 9.50 (+1.64/−1.81) Earth masses (Dransfield et al. 2026) — a measured density roughly a tenth of Saturn's and a twentieth of water's. Nothing is known of its composition beyond that: no atmospheric spectrum has been taken. A density this low normally implies an enormous, hot, extended envelope, which is straightforward for the young super-puffs that orbit close in and is not yet explained for a world this cool.
- 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 Visual mode; Realism shows a plain spectral colour.
Radiation & temperature
- Insolation
- 8.48 S⊕ · 8.48× the sunlight Earth receives
- Computed from TOI-791's luminosity and the orbital distance (inverse-square; Earth's sunlight = 1).
- Equilibrium temperature
- 434 K (161 °C)
- The archive's computed equilibrium value — not measured.
Discovery & history
- Method
- Transit detection
- Name
- Designated 'b' under the IAU convention that letters a star's planets 'b', 'c', 'd'… in the order found (the star is the unwritten 'a'); no IAU proper name has been assigned.
Flagged by TESS in 2019 as candidate 791.01 and confirmed in 2026, when follow-up measurements weighed it and found a Jupiter-sized world with barely a Neptune's mass.
Images
No telescope has imaged this world — it is known only from the tiny dip it makes crossing its star. Only its host system has been pictured (see the system view); its true surface appearance is unknown.