AF Leporis

Planetary system

In 2023, a super-Jupiter exoplanet, AF Leporis b, was discovered in orbit around AF Leporis by direct imaging using the NIRC2 instrument at the W. M. Keck Observatory and the SPHERE instrument at the Very Large Telescope. It was also detected in astrometric data from the Hipparcos and Gaia spacecraft, allowing an accurate measurement of its mass. AF Leporis b was later precovered in imaging data from 2011.

There have been multiple studies of AF Leporis b, which have found somewhat different parameters. Dynamical mass measurements range from 2.8 M_J to 5.5 M_J. Values for the planet's orbital inclination range from 50°+9°
−12° to ~98°, the former consistent with the stellar inclination of 54°+11°
−9° and suggesting an aligned system. Initial studies found a fairly eccentric orbit for the planet, but the precovery observations show that its orbit is nearly circular.

AF Leporis b has an effective temperature of about 750 K (477 °C; 890 °F), corresponding to an early-T spectral type. Spectroscopic evidence suggests that it has a metal-rich atmosphere with silicate clouds, though further studies are needed to confirm this.

AF Leporis b was observed with JWST NIRCam. The brightness of F444W is relative faint, indicating significant absorption due to carbon monoxide (CO). The strong CO absorption is explained with disequilibrium chemistry and high metallicity. The observations also rule out additional giant planets in the outer region. The study did not find any variability of AF Leporis b. A study with VLT/GRAVITY confirmed many of the previous observations. The GRAVITY instrument did add high precision astrometry, while at the same time providing a K-band spectrum. The new astrometry together with previous observations was able to constrain the orbit to a circular orbit with an inclination that is aligned with the inclination of the rotation axis of the host star. The K-band spectrum shows prominent methane (CH₄) absorption. The spectrum is also consistent with a metal-rich cloudy atmosphere, with [Fe/H] = 0.75±0.25, consistent with the formation via core accretion. The temperature was constrained to 800±50 Kelvin and the mass was re-estimated to 3.75 M_J. An observation with VLT/ERIS in the K-band detected CO and water vapor (H₂O), but not CH₄ or carbon dioxide (CO₂). Observations with VLT/HiRISE in the H-band on the other hand confirmed the detection of CH₄. Observation with JWST NIRSpec did detect CO₂, H₂S, CH₄, CO (including ¹²CO and ¹³CO) and H₂O. These observations did reveal that AF Leporis b has a sulfur-rich atmosphere due to the large amounts of hydrogen sulfide (H₂S) in the atmosphere, which makes up around 10% of all sulfur in the atmosphere. The high sulfur abundance is explained with significant accretion of solids from the circumstellar disk during the formation. The carbon abundance is low compared to other elements, meaning AF Leporis b formed within the CO snow line, but beyond the H₂O snow line. The researchers estimate that the planet contains 56±7 M_🜨 of solids. T-dwarfs do often have silicate clouds and the models favour enstatie (MgSiO₃) clouds, but mid-infrared spectroscopy is needed to confidently determine the cloud species.

A 2025 study assessed the feasibility of detecting exomoons by astrometry (i.e., from variations in the motion of a moon's host planet) around several directly imaged exoplanets with VLTI/GRAVITY. For AF Leporis b, it would theoretically be possible to detect a 0.14 M_J satellite orbiting at 0.39 AU, while a non-detection would rule out the existence of such a satellite.