Exoplanets in reflected starlight with dual-field interferometry and a fifth Unit Telescope at VLTI

In this white paper, we propose an upgrade to the Very Large Telescope Interferometer (VLTI) consisting of the addition of a new 8m Unit Telescope (UT5). The primary goal of this upgrade is to optimise the VLTI for exoplanet detection by creating four additional baselines of approximately 200m oriented toward the north-west. The inclusion of this telescope would reduce the inner working angle and improve the achievable contrast of the VLTI, thereby enabling the detection of mature exoplanets in reflected light.

💡 Research Summary

The white paper proposes a strategic upgrade to the Very Large Telescope Interferometer (VLTI) by adding a fifth 8‑meter Unit Telescope (UT5). The primary motivation is to enhance VLTI’s capability to detect mature exoplanets in reflected starlight, a regime that will become increasingly important as space‑based coronagraphs (e.g., Roman) begin to image cold, temperate worlds. Current VLTI geometry provides four 100 m baselines, but the north‑west direction is limited to only ~50 m, resulting in a relatively large inner working angle (IWA) that hampers detection of planets at small angular separations.

UT5 would be sited south of UT4, aligned with the existing AT‑J band arm, and would create four new baselines of roughly 200 m oriented toward the north‑west. By extending the maximum baseline to 220 m and improving UV‑plane coverage, the IWA can approach the theoretical λ/D limit at 800 nm (I‑band), enabling direct detection of a 1 AU‑separation planet around a star at 10 pc across all position angles. The upgrade leverages the already‑planned GRAVITY+ instrument, which will provide extreme adaptive optics (ExAO) on all four UTs and dual‑field interferometry, delivering contrasts better than 10⁻⁶ in the near‑infrared.

To quantify the scientific return, the authors selected 450 known exoplanets within 30 pc from the NASA Exoplanet Archive. For each planet they generated 1 000 orbital realizations, computed the instantaneous angular separation and reflected‑light contrast assuming Lambertian scattering with a geometric albedo of 0.3, and estimated the integration time required for a 3‑σ detection using the instrumental model of Lacour et al. (2025). The simulations show that up to 48 planets would be detectable in the I‑band with ≤30 h of observing time, and 12 of them could be detected in ≤3 h. Notably, Proxima Centauri b, which lies below the current VLTI IWA, becomes observable thanks to the reduced IWA and improved contrast. The detectable sample spans from Neptune‑mass down to Earth‑mass regimes, opening a unique parameter space that complements upcoming facilities such as the Roman coronagraph, the ELT Planetary Camera and Spectrograph (PCS), and the mid‑IR interferometer LIFE.

From an engineering perspective, the additional baselines would be realized by serially using the two unused delay lines, avoiding major re‑configuration of the existing infrastructure. The new telescope would feed its beam into the same delay‑line tunnels as the other UTs, preserving the overall VLTI architecture. The enhanced UV coverage fills the north‑west gap, improving image reconstruction fidelity and contrast at many position angles.

In summary, adding UT5 offers a cost‑effective path to dramatically improve VLTI’s high‑contrast, high‑resolution capabilities. It reduces the IWA, boosts achievable contrast, and provides richer UV sampling, thereby enabling the direct detection and precise astrometric characterization of nearby, mature exoplanets in reflected light. This capability will be essential for multi‑facility, multi‑wavelength studies aimed at constraining planetary atmospheres, albedos, and orbital dynamics, and it aligns with the strategic priorities outlined in both the US Astro 2020 and ESA Voyage 2050 roadmaps.