Sub-Snowline Formation of Gas-Giant Planets in Binary Systems

Abstract

Giant planets are thought to build their cores beyond the snow line, where water ice solids catalyze efficient planetary core growth. In close binary star systems, however, the companion star gravity shrinks the region where orbits are long-term stable, effectively excluding the zone where giants should form. Nevertheless, here we show that such systems exist and are not rare. Among 393 binary systems with measured orbits and circumstellar gas giants, we identify 17 for which the snowline region is unstable. The distribution of their metallicities and eccentricities is consistent with the background population, making a capture origin or enhanced solids abundance unlikely causes for their formation. Instead, we show that the sub-snowline formation paradox can be resolved by the tidal torque from the companion, which truncates the protoplanetary disk and creates an inner dust trap. After removing evolved stellar systems (white dwarfs or red-giant stars) and higher-order systems, we find that gas giants position is a linear function of the truncation radius. Using this model, we can successfully predict the observed locations of gas giants from their host binary properties. Moreover, evolved systems are generally inconsistent with the predictions, which further supports the model and points to these being second-generation planets. Our results have therefore key implications for understanding planet formation processes and provide observational support for sub-snowline gas-giant formation and the role of trap-dusts in their formation.

Figures (7)

• Figure 1: All circumstellar gas giants for which $r_{\rm snow} > 0.8 a_{\rm c}$. The x-axis shows the current semi-major axis of the planets. The y-axis shows the system name. $a_{\rm c}$ and $r_{\rm snow}$ are marked as gray and blue vertical lines (see \ref{['sec:methods']} for details on how we calculate $r_{\rm snow}$ and $a_{\rm c}$). The pink shaded area shows the stable region for prograde orbits. The blue shaded area shows the ice formation regions. According to the basic theory, giants can form only at the intersection between the blue and pink shaded regions. Systems with an asterisk have no measured eccentricity. We used zero eccentricity for their stability calculation.
• Figure 2: $r_{\rm t}$ vs observed semi-major axis ($a_{\rm planet}$) of the $9$ gas giants with $r_{\rm snow} > 0.8a_c$. Systems with no measured eccentricity. Color represents the companion mass ratio $\mu$. evolved systems (host or companion), and systems that are known to be of higher order (triples or above) are not shown. Uncertainties on the x-axis were calculated based on uncertainties in the observed parameters. Uncertainties on the y-axis were taken to be $5\%$ when no uncertainty in $a_{\rm planet}$ was provided.
• Figure 3: Example of the evolution of snowline for HD 8673. The position of the snow line at different ages was calculated using parameters from MESA Paxton2011Jermyn2023. The shaded pink region marks the unstable region ($r_{\rm snow} > 0.8a_{\rm c}$).
• Figure 4: Same as Figure \ref{['fig:trap_vs_observed_goods']} but for the entire sample. Systems that are known to be of higher order (triplets and above) are marked with triangles. Evolved systems are marked with circles.
• Figure 5: $r_{\rm t}$ vs observed semi-major axis ($a_{\rm planet}$) for gas giants with stable snow line (upper panel) and terrestrial planets with unstable snow line. Only systems with measured eccentricity are shown.