# Exoplanets in reflected starlight with dual-field interferometry: A case for shorter wavelengths and a fifth Unit Telescope at VLTI/Paranal

**Authors:** S. Lacour, Ó. Carrión-González, M. Nowak  
**arXiv:** [2406.07030](https://arxiv.org/abs/2406.07030)  
**Date:** 2024-06-11  
**Summary page:** https://www.summarizepaper.com/en/paper/2406.07030v1/

## Key points

- Direct observation of cold and temperate exoplanets within 1 to 10 AU is challenging with current astronomical methods
- Ground-based optical interferometry, specifically dual-field interferometry using the VLTI, shows promise for studying distant exoplanets
- Analytical expressions and simulations were used to explore the limits of dual-field interferometry in characterizing exoplanets in reflected starlight
- Shorter wavelengths can improve detectability, enabling observation of Jupiter-mass exoplanets in the J band with current VLTI baselines
- Adding new baselines beyond 200 meters enhances VLTI's capabilities and increases the number of detectable exoplanets, potentially revealing habitable zone candidates like $\tau$ Ceti e and Proxima Centauri b
- Recommendations include developing instrumentation at wavelengths shorter than 1$\,\mu$m and adding a fifth Unit Telescope (UT5) to the VLTI
- Considerations were made regarding factors such as Strehl ratio's impact on contrast levels achievable through interferometry, signal injection into fibres, and stellar flux effectiveness

## Summary

The direct observation of cold and temperate exoplanets within 1 to 10 AU presents a significant challenge due to the limitations of current astronomical methods. However, ground-based optical interferometry, with its high angular-resolution imaging capabilities, offers a promising avenue for studying these distant worlds and complementing space-based observations. In this study, researchers aimed to explore the potential of dual-field interferometry in characterizing exoplanets in reflected starlight using the Very Large Telescope Interferometer (VLTI). By developing analytical expressions and integrating them with simulations of atmospheric wavefronts corrected by extreme Adaptive Optics, the team investigated the fundamental limits of dual-field interferometry. They also formulated an analytical solution for optimal phase apodization to enhance starlight rejection when injected into a single-mode fibre. The results of their analysis indicated that employing shorter wavelengths can significantly improve detectability, enabling the observation of at least seven Jupiter-mass exoplanets in the J band using current VLTI baselines. Furthermore, adding new baselines with lengths beyond 200 meters was found to enhance VLTI's capabilities, increasing the number of detectable exoplanets and potentially revealing habitable zone candidates such as $\tau$ Ceti e and Proxima Centauri b. To further advance exoplanet characterization capabilities, the researchers recommended developing instrumentation at wavelengths shorter than 1$\,\mu$m and proposed the addition of a fifth Unit Telescope (UT5) to the VLTI. Additionally, considerations were made regarding factors such as the Strehl ratio's impact on contrast levels achievable through interferometry and its influence on both signal injection into fibres and stellar flux effectiveness. These insights contribute to advancing our understanding of exoplanetary atmospheres and expanding our ability to study these distant worlds within our cosmic neighborhood.