Department of Astronomy

A paper led by IU Astronomy Post-Baccalaureate Researcher Jace Rusznak describing the orbital alignment trends of the most massive known exoplanets has been accepted to the Astrophysical Journal Letters for publication. 

The paper investigates a massive planet 13 times the mass of Jupiter, XO-3b. This target is the first known planet to have an orbit misaligned with the rotational axis of its host star, and has been the subject of controversy over the past 17 years due to disagreements in the measured level of misalignment. 

To determine the misalignment of XO-3b, the authors took radial velocity measurements obtained using the NEID spectrograph mounted on the WIYN telescope as well as light curve data from TESS. By analyzing photometric light curves, radial velocity data, the Rossiter-McLaughlin effect, and Doppler tomography, the team finds that XO-3b is misaligned, with a sky-projected spin-orbit angle of 40.2 ± 2 degrees and no evidence of time-dependent variation. 

To place XO-3b's misalignment in the broader context of exoplanet alignment trends, the authors examine alignment angles as a function of planet-to-star mass ratio—a parameter more relevant to a system’s dynamical history than the planet’s mass alone. 

  In Jupiter mass systems, planets around cool stars exhibit alignment, while their hot star counterparts are misaligned. This discrepancy indicates that tidal damping is occurring in cool star systems. However, the alignment of high mass-ratio planets around both types of stars suggests that they were aligned primordially, since hot stars do not share the tidal realignment mechanism. 

While XO-3b was the only outlier in this trend, analysis of images of XO-3 by Prof. Adam Kraus at the University of Texas Austin revealed that there is a stellar companion to XO-3b, whose gravitational influence could have produced the misalignment seen in the system. By removing XO-3b from their analysis, the significance of their findings goes from 3.7 to 4.1σ, further suggesting that there is a significant population boundary between misaligned low mass-ratio systems and aligned high mass-ratio systems. 

The full article can be found at https://iopscience.iop.org/article/10.3847/2041-8213/adc129