Not alone in solitude: a look into the surprising world of TOI-1130

After the detection of numerous planetary systems outside of our own Solar System which tend to be extremely diverse and which show a wide range of evolutionary states, the focus is now shifting to a characterization of their formation and evolution, as well as to the architecture of planetary systems and planet habitability.

One of the astonishing discoveries in exoplanet research has been the detection of Jupiter-like planets (in size and mass) that orbit their host star within 10 days. These so-called hot Jupiters are rarely accompanied by smaller close-in companions. TOI-1130 is one of very these rare systems. It hosts two transiting planets: a hot Jupiter and an inner low-mass planet that are near to the 2:1 period commensurability.

Planetary systems with transiting planets are markedly well-suited for a detailed characterization, since they allow the measurement of the planetary radius, which is essential to constrain the planet’s evolution and migration history, as well as to characterize the internal structure of the planet. The second fundamental parameter for the characterization is the planetary mass, that together with the radius allows the bulk density of a planet to be estimated, thereby constraining its composition.

In order to find out the history and future evolution of exoplanet systems, a complete knowledge of all orbital and planetary parameters with a high accuracy is crucial. However, precise measurements of the planetary mass are difficult to obtain, in particular for small planets. If there are multiple planets in a system close to a period commensurability, as is the case for TOI-1130, the planetary masses can be determined using the gravitational interactions leading to measurable transit timing variations.

TOI-1130 is a known planetary system consisting of a hot Jupiter, TOI-1130 c, on an 8.4-day orbit accompanied by an inner Neptune-sized planet, TOI-1130 b, with an orbital period of 4.1 days around a K-dwarf. As part of the ongoing radial velocity (RV) follow-up program carried out by our team with the HARPS spectrograph, we collected precise radial velocity measurements of TOI-1130. We perform a photodynamical modeling of the RVs, and the transit photometry from the Transiting Exoplanet Survey Satellite (TESS) and the TESS Follow-up Observing Program. We find that the two planets orbit with small eccentricities in a 2:1 resonant configuration. This is the first system where a hot Jupiter accompanied by an inner lower mass planet is locked in a mean-motion-resonance. We discuss possible formation scenarios.