HAT-P-1b

Lore & Background

HAT-P-1b was detected by the HATNet Project using telescopes at the Fred Lawrence Whipple Observatory in Arizona and the Submillimeter Array facility in Hawaii, via the transit method—a 0.6% dip in starlight. Its orbital parameters were confirmed by radial velocity measurements at the 8.2 m Subaru and 10 m Keck telescopes, with the discovery announced on September 14, 2006. The planet orbits its star every 4.47 days at a distance of 8.27 million km, placing it in the hot Jupiter category. Tidal forces would circularize such a close orbit unless another perturbing body exists; existing measurements are insufficient to determine eccentricity, so a circular orbit is assumed, though eccentricity is no greater than 0.067. The mass of 0.53 ± 0.04 Jupiter masses was derived from radial velocity observations by the N2K Consortium combined with the transit inclination. As of August 2008, the spin-orbit angle was calculated as 3.7°.

Reader's Guide

HAT-P-1b is significant as one of the least dense exoplanets known, challenging theoretical models of planetary structure. Its inflated radius—larger than predictions—suggests an extra heat source, possibly tidal heating from an eccentric orbit, though this remains unconfirmed. Another inflated planet, HD 209458 b, is in a circular orbit, complicating the tidal heating hypothesis. An alternative explanation is a high axial tilt like Uranus, but such a configuration is thought difficult to achieve, making two such planets among known transiting planets problematic. The detection of emission from C2, CN, and CH radicals in its atmosphere in 2022, along with a hazy (not cloudy) atmosphere and 22% cloud area fraction, provides insights into its composition. Its discovery via the HATNet Project and confirmation by major telescopes exemplifies the synergy of transit and radial velocity methods. The planet's close orbit and low density make it a key object for studying atmospheric escape and internal heating mechanisms in hot Jupiters.

Did You Know?

• Its discovery was announced on September 14, 2006.
• The planet's atmosphere was found to contain C2, CN, and CH radicals in 2022.
• It orbits its star at only 8.27 million km, completing one orbit every 4.47 days.