HIP 65Ab

Lore & Background

HIP 65Ab is a massive super-Jupiter with a mass of about 3 Jupiter masses. Its unusually large radius of roughly 2 Jupiter radii is likely overestimated due to the grazing nature of its transit, where the planet only partially crosses the stellar disk. This grazing geometry also accounts for the large margin of error in the radius measurement. Based on the planet's mass and temperature, it is unlikely to be larger than 1.5 Jupiter radii, and a 2024 study agrees that the true radius is close to the presented lower limit.

The planet orbits extremely close to its star at a distance of 0.01782 AU, about twice the radius of the Sun. It receives 661 times the flux Earth does, placing its equilibrium temperature at 1411 K, just above the melting point of copper. Due to its proximity, tidal interactions are slowly causing its orbit to decay, and the planet is expected to spiral into HIP 65A and be destroyed by its Roche limit within somewhere between 80 million years and a few billion years.

In 2024, the planet's atmosphere was analysed spectroscopically for the first time, revealing absorption lines of water and carbon monoxide. Both molecules are less abundant than they would be if the planet had a Sun-like composition. The results are consistent with a sub-solar metallicity but an overabundance of carbon and oxygen, implying the planet may have formed beyond the snow line and migrated inward after the protoplanetary disk dissipated, or that some oxygen has precipitated out of the atmosphere.

Reader's Guide

HIP 65Ab's significance lies in its combination of extreme properties that make it a prime target for atmospheric characterization. Its large transit depth of about 8% and the star's high X-ray luminosity make HIP 65A one of the best candidates for transit observations using X-ray telescopes. The planet's 23.5-hour orbit and massive size facilitate accurate spectroscopic studies, as demonstrated by the 2024 detection of water and carbon monoxide in its atmosphere. The atmospheric composition, showing sub-solar metallicity but carbon and oxygen overabundance, provides clues about the planet's formation and migration history. The grazing transit, while complicating radius measurements, offers a unique geometric perspective. The planet's impending orbital decay and eventual destruction within a timescale of 80 million to a few billion years highlight the dynamic evolution of ultra-short-period planets. The host star's binary companion, HIP 65B, an M-type red dwarf separated by 269 AU, adds context to the system's architecture. HIP 65Ab serves as a benchmark for studying atmospheric processes, tidal interactions, and the fate of planets in extreme orbits.

Did You Know?

• HIP 65Ab completes one orbit every 23.5 hours, making it an ultra-short-period planet.
• The planet's radius is likely overestimated due to its grazing transit, where it only partially crosses the star's disk.
• Its equilibrium temperature of 1411 K is just above the melting point of copper.