Ten Aligned Orbits: Planet Migration in the Era of JWST and Ariel

Abstract Understanding the diverse formation and migration pathways that shape exoplanetary systems requires characterizing both their atmospheric properties and their orbital dynamics. A key dynamical diagnostic is the projected spin–orbit angle—the alignment between the stellar spin and the planetary orbit—which provides crucial tests for theoretical models. This angle can be determined using the Rossiter–McLaughlin effect. Although measurements exist for over 200 planets, the overall distribution of these angles is not yet fully understood, motivating further observations across the full parameter space. We analyze archival HARPS and HARPS-N spectroscopic transit time series of nine gas giant exoplanets on short orbits and one brown dwarf. We derive their projected spin–orbit angle λ . We find aligned projected orbits for all nine gas giants as well as the brown dwarf. Furthermore, we are able to derive the true spin–orbit angle for the brown dwarf EPIC 219388192b, ψ = 2 2 − 13 + 10 deg. These projected prograde orbits are consistent with quiet disc migration disfavoring violent events exciting the orbits in the history of these systems. Finally, we investigate the current overlap between spin-orbit angle measurements and atmospheric characterization targets. While we find no strong observational biases due to the spin–orbit angle, we note that the majority of planets with atmospheric data still lack spin–orbit measurements. This incompleteness of the dynamical information may limit the interpretation of upcoming atmospheric surveys.