HAT-P-32 b is a gas-giant exoplanet orbiting a late-F-to-early-G dwarf star in the constellation Andromeda, 943.26 light-years from Earth. It belongs to the class known as hot Jupiters: planets with masses in Jupiter’s range that sit so close to their host stars that a single year passes in a matter of days. What sets HAT-P-32 b apart from most of that already unusual class is a combination of extreme atmospheric loss, a wildly tilted orbit, and a bulk density of just 108 kg/m³ — so low that the planet is only 2% as dense as Earth.
Key facts
- Mass: 0.68 Jupiter masses (216.12× Earth)
- Body type: exoplanet
- Host star: HAT-P-32
- Radius: 141,554 km (22.22× Earth)
- Density: 108 kg/m³ (0.02× Earth)
- Temperature: 1,836 K (1,563 °C)
- Orbital period: 2.2 days
- Semi-major axis: 5,081,840 km
- Orbital eccentricity: 0.16
- Distance: 943.26 light-years
- Insolation: 2,505,601 W/m²
- Discovered: 2011
- Discovery method: Transit
Physical characteristics
HAT-P-32 b has a radius of 141,554 km — 22.22 times Earth’s radius — yet its mass is only 0.68 Jupiter masses (216.12 times Earth’s mass). The combination of that large volume and relatively modest mass produces a mean density of 108 kg/m³, which is just 0.02 times Earth’s mean density. That figure places the planet far below the density of any solid or liquid material familiar from everyday experience.
The planet’s temperature is measured at 1,836 K (1,563 °C). Spitzer Space Telescope secondary-eclipse photometry published in 2014 found planet-to-star flux ratios of 0.364% ± 0.016% at 3.6 μm and 0.438% ± 0.020% at 4.5 μm, and the data preferred models in which the dayside atmosphere carries a temperature inversion — a warm layer sitting above a cooler one — along with inefficient redistribution of heat from the dayside to the nightside.
Orbit and spin-orbit misalignment
HAT-P-32 b completes one orbit every 2.2 days at a semi-major axis of 5,081,840 km. The orbit carries a measured eccentricity of 0.16, giving it a notably non-circular path despite the strong tidal forces expected at such proximity.
In 2012, Albrecht et al. measured the sky-projected spin-orbit angle of HAT-P-32 b using the Rossiter-McLaughlin effect and found λ = 85° ± 1.5°. That figure means the planet’s orbital plane is almost perpendicular to the equatorial plane of its host star — a configuration described as a near-polar orbit. Most Solar System planets orbit within a few degrees of the Sun’s equatorial plane; HAT-P-32 b’s 85° tilt places it in a fundamentally different dynamical regime, one that likely points to gravitational scattering or secular perturbations in the system’s history.
The helium tidal tails
In June 2023, Zhang et al. published in Science Advances the detection of giant helium tidal tails escaping from HAT-P-32 b. The observation was made with the Hobby-Eberly Telescope at McDonald Observatory and carried a statistical significance of 14σ — well beyond the threshold conventionally required to claim a detection. The tails, both leading and trailing the planet in its orbit, span a projected length more than 53 times the planet’s own radius.
These structures rank among the largest known features associated with any exoplanet.
The host star and binary companion
HAT-P-32 b orbits the star designated GSC 3281-00800, a late-F-to-early-G dwarf with a visual magnitude of V = 11.289. At that brightness, the star is too faint to see without optical aid from Earth.
The HAT-P-32 system is a binary. A second star, classified as an M1.5 dwarf and designated HAT-P-32 B, was resolved using adaptive optics imaging at a projected separation of approximately 2.9 arcseconds, corresponding to a projected physical separation of about 830 astronomical units. Its measured effective temperature is 3,565 ± 82 K. The companion is cool and faint, but not negligible: it contributes roughly 4% of the light in the K-band, a detail that observers had to account for carefully when deriving the secondary eclipse depth of HAT-P-32 b.
Discovery and confirmation
The HATNet survey first flagged HAT-P-32 b as a planetary candidate in 2004, but the host star showed radial velocity jitter of approximately 80 m/s — a level of stellar noise large enough to swamp the planet’s own gravitational signal on the star. Confirmation required roughly three additional years of observations before the stellar and planetary signals could be separated with confidence.
The discovery paper, authored by Hartman et al. and submitted on 6 June 2011, announced HAT-P-32 b together with a second planet, HAT-P-33 b, and reported both as having radii among the largest measured at that time for any transiting exoplanet. The discovery method was the transit technique: the planet’s periodic passage across the face of its star produces a measurable dimming of the starlight, from which the planet’s size and orbital period can be derived.
Transit timing variation (TTV) analysis drawing on 45 transit observations collected through the Young Exoplanet Transit Initiative (YETI) network found no TTV amplitude greater than about 1.5 minutes. That result rules out a nearby planetary companion massive enough to gravitationally perturb HAT-P-32 b’s orbit by a detectable amount, leaving the planet as the dominant known body in the inner system.
Did you know
- The M-dwarf companion star HAT-P-32 B orbits the planet-hosting star at a projected separation of about 830 astronomical units and contributes roughly 4% of the combined light in the K-band. That relatively small but non-trivial fraction had to be carefully subtracted from the observations before astronomers could measure HAT-P-32 b’s secondary eclipse depth accurately. (iopscience.iop.org)
Data sources
Data updated: 2026-10-28