HAT-P-18 b
HAT-P-18 b is a hot Jupiter orbiting HAT-P-18 in the constellation Hercules. It lies about 526 light-years from Earth and was discovered in 2010 using the transit method.
How Big Is HAT-P-18 b?
HAT-P-18 b has a radius of 10.70 times that of Earth, or 0.96 times the radius of Jupiter. Its mass is 55.5 times that of Earth, giving it a density of 0.25 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is HAT-P-18 b in the Habitable Zone?
The position of HAT-P-18 b relative to the habitable zone of HAT-P-18 cannot be precisely determined from the available orbital data.
Orbit and Year Length
A year on HAT-P-18 b — one full orbit around HAT-P-18 — lasts 5.51 Earth days, shorter than Mercury's 88-day year. Its orbit is nearly circular (eccentricity 0.030).
How Was HAT-P-18 b Discovered?
HAT-P-18 b was discovered in 2010 using the transit method, with observations from HATNet.
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 HAT-P-18 b?
HAT-P-18 b is 526.4 light-years (161.4 parsecs) from Earth. Light arriving here tonight left the planet around the year 1500. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 9,264,640 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. HAT-P-18 b scores 0.09, ranking #4,508 of 5,565 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: HAT-P-18
HAT-P-18
Planetary System
HAT-P-18 b is the only planet known to orbit HAT-P-18 so far.
HAT-P-18 b — Complete Data
| Radius | 10.705 Earth radii (0.955 Jupiter radii) |
|---|---|
| Mass | 55.52 Earth masses (0.175 Jupiter masses) |
| Density | 0.25 g/cm³ (Earth: 5.51) |
| Orbital period | 5.51 days |
| Eccentricity | 0.030 |
| Earth Similarity Index | 0.09 |
| Distance from Earth | 526.4 light-years (161.4 parsecs) |
| Constellation | Hercules |
| Discovery method | Transit |
| Discovery facility | HATNet |
| Discovery year | 2010 |
Data: NASA Exoplanet Archive, last updated 2026-09-03. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HAT-P-18 b
Is HAT-P-18 b habitable?
HAT-P-18 b is not known to orbit within the habitable zone of HAT-P-18, and as a hot Jupiter it is an unlikely candidate for life as we know it.
How far away is HAT-P-18 b?
HAT-P-18 b is about 526 light-years from Earth in the constellation Hercules. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 9,264,640 years to get there.
How big is HAT-P-18 b compared to Earth?
HAT-P-18 b has 10.70 times the radius of Earth and about 55.5 times its mass.
How long is a year on HAT-P-18 b?
One orbit around HAT-P-18 takes 5.5 Earth days — short enough that 66 of its years would fit into one Earth year.
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