HD 118203 b
HD 118203 b is a hot Jupiter orbiting HD 118203 in the constellation Ursa Major. It lies about 301 light-years from Earth and was discovered in 2005 using the radial velocity method.
How Big Is HD 118203 b?
HD 118203 b has a radius of 12.78 times that of Earth, or 1.14 times the radius of Jupiter. Its mass is 680 times that of Earth, giving it a density of 1.79 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is HD 118203 b in the Habitable Zone?
HD 118203 b orbits inside the inner edge of the habitable zone of HD 118203. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of HD 118203: 1.515–3.573 AU (conservative: 1.919–3.388 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
See the full interactive habitable-zone view in the Exoplanet Explorer app ›
Temperature on HD 118203 b
The equilibrium temperature of HD 118203 b is about 1,361 K (1,088 °C) — hot enough to melt many metals. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C.
Orbit and Year Length
A year on HD 118203 b — one full orbit around HD 118203 — lasts 6.13 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.071 AU — closer to its star than Mercury is to the Sun. Its orbit is highly elliptical (eccentricity 0.31), swinging dramatically closer to and farther from its star.
How Was HD 118203 b Discovered?
HD 118203 b was discovered in 2005 using the radial velocity method, with observations from Haute-Provence Observatory.
The radial velocity method measures the subtle wobble of a star caused by the gravitational pull of an orbiting planet, visible as periodic shifts in the star's light spectrum. The size of the wobble reveals the planet's minimum mass.
How Far Away Is HD 118203 b?
HD 118203 b is 300.9 light-years (92.3 parsecs) from Earth. Light arriving here tonight left the planet around the year 1726. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 5,295,840 years to make the journey.
Earth Similarity Index
The Earth Similarity Index (ESI) scores how physically similar a planet is to Earth, from 0 to 1, based on radius, density, escape velocity and surface temperature. HD 118203 b scores 0.07, ranking #4,914 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: HD 118203
HD 118203
- Surface temperature
- 5,742 K (Sun: 5,772 K)
- Mass
- 1.27 M☉
- Radius
- 2.04 R☉
- Age
- 4.7 billion years (Sun: 4.6)
The HD 118203 Planetary System
HD 118203 b is one of 2 known planets in the HD 118203 system. Its siblings:
- HD 118203 c (Cold Gas Giant)
HD 118203 b — Complete Data
| Radius | 12.778 Earth radii (1.140 Jupiter radii) |
|---|---|
| Mass | 680.15 Earth masses (2.140 Jupiter masses) |
| Density | 1.79 g/cm³ (Earth: 5.51) |
| Orbital period | 6.13 days |
| Orbital distance | 0.071 AU |
| Eccentricity | 0.314 |
| Equilibrium temperature | 1,361 K (1,088 °C) |
| Earth Similarity Index | 0.07 |
| Distance from Earth | 300.9 light-years (92.3 parsecs) |
| Constellation | Ursa Major |
| Discovery method | Radial Velocity |
| Discovery facility | Haute-Provence Observatory |
| Discovery year | 2005 |
Data: NASA Exoplanet Archive, last updated 2025-05-27. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HD 118203 b
Is HD 118203 b habitable?
No — HD 118203 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is HD 118203 b?
HD 118203 b is about 301 light-years from Earth in the constellation Ursa Major. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 5,295,840 years to get there.
How big is HD 118203 b compared to Earth?
HD 118203 b has 12.78 times the radius of Earth and about 680 times its mass.
How long is a year on HD 118203 b?
One orbit around HD 118203 takes 6.1 Earth days — short enough that 60 of its years would fit into one Earth year.
Explore HD 118203 b in the app
Browse, filter and compare 6,000+ exoplanets on iPhone, iPad and Apple Watch — with habitable-zone views, widgets and offline data.
