Constraining exotrojans in hot-Jupiter systems using transit timing variation observations from TESS

Context . Co-orbital objects, also known as Trojans, are celestial bodies that share a 1:1 mean motion resonance orbit with a planet. Many planets in our Solar System hold Trojans, yet none has been confirmed in exoplanetary systems. While theoretical models suggest that hot Jupiters are unlikely to retain co-orbitals, their high transit cadence and deep transits make them accessible targets for constraining the existence of massive companions using transit timing variations (TTVs). Aims . The Exoplanet Ephemerides Change Observations (ExoEcho) project aims to study TTVs in exoplanetary systems using high-precision photometry from space- and ground-based telescopes. As the third paper in the ExoEcho project, we investigate the potential existence of exotrojans in hot-Jupiter systems through a TTV analysis of TESS observations. Methods . In this work, we analyzed TESS photometry for 260 confirmed hot Jupiters with published radial-velocity-based (RV-based) mass measurements to search for TTV signals compatible with Trojan companions. We derived transit times and assessed the compatibility of TTV residuals with co-orbital models through N-body simulation using the REBOUND N-body code. Accounting for the physical degeneracy between Trojan mass and libration amplitude, we placed upper mass limits on possible companions for typical libration amplitudes. Results . For a typical libration amplitude of 15°, we rule out exotrojans more massive than 1 Earth mass in 130 systems (∼50% of our sample). A more conservative χ 2 analysis incorporating observational uncertainties places this limit at 3 Earth masses. Additionally, we used the stability boundaries of the 1:1 resonance to exclude dynamically unstable configurations, ensuring our derived mass limits fall within the stable regime. Conclusions . Our results provide stringent constraints on co-orbital formation in short-period systems and establish a framework for future high-precision missions such as the PLAnetary Transits and Oscillations of stars (PLATO) or Earth 2.0 (ET) mission.