HAT-P-2b

Lore & Background

HAT-P-2b was detected via the transit method by the HATNet Project in May 2007. Its official name, Magor, was chosen by Hungary during the NameExoWorlds campaign on the 100th anniversary of the International Astronomical Union, referencing a legendary ancestor of the Magyar people. The host star HAT-P-2 is named Hunor, after Magor's brother in the same legend.

The planet's mass is 8.7 times that of Jupiter, yet its diameter is only 1.157 times Jupiter's, resulting in a mean density twice that of Earth and a surface gravity about 24 times Earth's—nearly equal to the Sun's. Despite atmospheric bloating, its scale height is the smallest measured among exoplanets with detectable atmospheres as of 2021, at just 26 km. Tidal heating, in addition to stellar radiation, is thought to have influenced its evolution.

The orbit is highly eccentric, ranging from 4.90 million to 15.36 million miles from the star, with a period of 5 days 15 hours. The planet transits its star as seen from Earth. Measurements of the spin-orbit angle have been disputed: Winn in 2007 reported +1 ± 13 degrees, while Loeillet in 2008 contested that value. A 2012 study found the orbit is probably aligned with the star's equatorial plane, with a misalignment of 9°.

Reader's Guide

HAT-P-2b stands out among transiting exoplanets for its extreme physical properties. Its high mass and relatively small radius yield a density twice that of Earth and a surface gravity rivaling the Sun's, making it one of the densest known gas giants. The planet's atmospheric scale height of only 26 km—the smallest among exoplanets with measurable atmospheres as of 2021—reflects the strong gravitational compression that counteracts thermal expansion.

The highly eccentric orbit, varying from 4.90 million to 15.36 million miles from its star, subjects the planet to dramatic changes in insolation and likely contributes to tidal heating, which has played a role in its evolution. The disputed spin-orbit angle measurements (Winn 2007 vs. Loeillet 2008, later refined to a probable 9° misalignment in 2012) illustrate the challenges in characterizing planetary system architecture.

The system also hosts a second confirmed planet, HAT-P-2c, announced in 2023, which may perturb HAT-P-2b's orbit. The naming of the planet Magor and its star Hunor, drawn from Hungarian legend, ties the discovery to cultural heritage. HAT-P-2b remains a benchmark for studying high-density, high-gravity exoplanets and the effects of eccentric orbits on planetary atmospheres and evolution.

Did You Know?

• HAT-P-2b has a surface gravity approximately 24 times that of Earth, nearly equal to the Sun's.
• Its atmospheric scale height of 26 km is the smallest among exoplanets with measurable atmospheres as of 2021.
• A second planet, HAT-P-2c, was confirmed in the system in 2023.