TOI-1803 c
TOI-1803 c is a Neptune-like planet orbiting TOI-1803 in the constellation Ursa Major. It lies about 389 light-years from Earth and was discovered in 2025 using the transit method.
How Big Is TOI-1803 c?
TOI-1803 c has a radius of 4.29 times that of Earth. Its mass is 8.0 times that of Earth, giving it a density of 0.42 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is TOI-1803 c in the Habitable Zone?
TOI-1803 c orbits inside the inner edge of the habitable zone of TOI-1803. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of TOI-1803: 0.374–0.917 AU (conservative: 0.474–0.869 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on TOI-1803 c
The equilibrium temperature of TOI-1803 c is about 588 K (315 °C) — hotter than anywhere on Earth. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C. It receives 22.00 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on TOI-1803 c — one full orbit around TOI-1803 — lasts 12.9 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.097 AU — closer to its star than Mercury is to the Sun.
How Was TOI-1803 c Discovered?
TOI-1803 c was discovered in 2025 using the transit method, with observations from Transiting Exoplanet Survey Satellite (TESS).
The transit method watches a star for the tiny, regular dip in brightness that occurs when a planet crosses in front of it. The depth and timing of these dips reveal the planet's size and orbital period.
How Far Away Is TOI-1803 c?
TOI-1803 c is 388.9 light-years (119.2 parsecs) from Earth. Light arriving here tonight left the planet around the year 1638. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 6,844,640 years to make the journey.
The Host Star: TOI-1803
TOI-1803
- Surface temperature
- 4,687 K (Sun: 5,772 K)
- Mass
- 0.76 M☉
- Radius
- 0.72 R☉
- Luminosity
- 0.2220 L☉
- Age
- 1.4 billion years (Sun: 4.6)
The TOI-1803 Planetary System
TOI-1803 c is one of 2 known planets in the TOI-1803 system. Its siblings:
- TOI-1803 b (Mini Neptune)
TOI-1803 c — Complete Data
| Radius | 4.290 Earth radii (0.383 Jupiter radii) |
|---|---|
| Mass | 8.00 Earth masses (0.025 Jupiter masses) |
| Density | 0.42 g/cm³ (Earth: 5.51) |
| Orbital period | 12.89 days |
| Orbital distance | 0.097 AU |
| Equilibrium temperature | 588 K (315 °C) |
| Stellar irradiation | 22.00× Earth |
| Distance from Earth | 388.9 light-years (119.2 parsecs) |
| Constellation | Ursa Major |
| Discovery method | Transit |
| Discovery facility | Transiting Exoplanet Survey Satellite (TESS) |
| Discovery year | 2025 |
Data: NASA Exoplanet Archive, last updated 2025-05-13. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About TOI-1803 c
Is TOI-1803 c habitable?
No — TOI-1803 c orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is TOI-1803 c?
TOI-1803 c is about 389 light-years from Earth in the constellation Ursa Major. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 6,844,640 years to get there.
How big is TOI-1803 c compared to Earth?
TOI-1803 c has 4.29 times the radius of Earth and about 8.0 times its mass.
How long is a year on TOI-1803 c?
One orbit around TOI-1803 takes 12.9 Earth days — short enough that 28 of its years would fit into one Earth year.
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