# VLTI/GRAVITY Observations of AF Lep b: Preference for Circular Orbits, Cloudy Atmospheres, and a Moderately Enhanced Metallicity

**Authors:** William O. Balmer, Kyle Franson, Antoine Chomez, Laurent Pueyo, Tomas Stolker, Sylvestre Lacour, Mathias Nowak, Evert Nasedkin, Markus J. Bonse, Daniel Thorngren, Paulina Palma-Bifani, Paul Molliere, Jason J. Wang, Zhoujian Zhang, Amanda Chavez, Jens Kammerer, Sarah Blunt, Brendan P. Bowler, Mickael Bonnefoy, Wolfgang Brandner  
**arXiv:** [2411.05917](https://arxiv.org/abs/2411.05917)  
**Date:** 2024-11-08  
**Summary page:** https://www.summarizepaper.com/en/paper/2411.05917v1/

## Key points

- Direct imaging observations of exoplanets tend to favor wide-separation and massive companions with degenerate formation histories.
- Most exoplanets are believed to form through core accretion, but adherence to this pathway has not been definitively shown in most directly imaged exoplanets.
- New interferometric observations of the directly imaged giant planet AF Lep b were conducted using the VLTI/GRAVITY instrument, providing three epochs of relative astrometry and the K-band spectrum at a resolution of R=500 for the first time.
- Significant constraints on AF Lep b's orbit were placed by utilizing these measurements along with data from the Hipparcos-Gaia Catalogue of Accelerations, suggesting an effectively circular orbit and spin-orbit alignment with its host star.
- A dynamical mass estimate for AF Lep b was determined to be $M_\mathrm{p}=3.75\pm0.5\,M_\mathrm{Jup}$, and analysis of its spectrum revealed it is metal-rich ([M/H]$=0.75\pm0.25$) with a C/O ratio consistent with solar values.
- These findings strongly suggest that AF Lep b aligns with core accretion formation processes, shedding light on its nature and characteristics as a directly imaged giant planet.
- The study's results provide valuable insights into AF Lep b's formation history and composition, while also demonstrating the potential for advanced observational techniques like interferometry in studying exoplanetary systems in greater detail.

## Summary

Direct imaging observations of exoplanets are often biased towards wide-separation and massive companions with degenerate formation histories. However, the majority of exoplanets are believed to form through core accretion. Despite this, most directly imaged exoplanets have not definitively shown adherence to this formation pathway. In a recent study, new interferometric observations of the directly imaged giant planet AF Lep b were conducted using the VLTI/GRAVITY instrument. The research team presented three epochs of 50$\mu$as relative astrometry and the K-band spectrum of the planet at a resolution of R=500 for the first time. By utilizing these measurements, spanning less than two months, along with data from the Hipparcos-Gaia Catalogue of Accelerations, significant constraints were placed on AF Lep b's orbit. This development holds promise for future interferometric observations of planets discovered by Gaia DR4. Combining all available measurements, researchers inferred an effectively circular orbit for AF Lep b ($e<0.02$, $0.07$, $0.13$ at $1$, $2$, $3 \sigma$) in spin-orbit alignment with its host star. Furthermore, a dynamical mass estimate for AF Lep b was determined to be $M_\mathrm{p}=3.75\pm0.5\,M_\mathrm{Jup}$. Models analyzing the spectrum of the planet revealed that it is metal-rich ([M/H]$=0.75\pm0.25$) and possesses a C/O ratio consistent with solar values. Collectively, these findings strongly suggest that AF Lep b is in line with core accretion formation processes and shed light on its nature and characteristics as a directly imaged giant planet. The study's comprehensive results provide valuable insights into its formation history and composition. Moreover, the successful application of advanced observational techniques like interferometry opens up new possibilities for studying and understanding exoplanetary systems in greater detail moving forward.