Department of Astronomy

The orbital planes of the planets in our Solar System are aligned with the Sun’s equator within six degrees. However, for exoplanets, this alignment is not always the case. In particular, hot Jupiters—exoplanets with a normalized semi-major axis (a/R*) less than 11 and masses greater than 0.3 Jupiter masses—can have a wide range of angles between their orbital axes and the stellar spin axes (hereafter, spin-orbit angle). These angles can vary from prograde to polar, and even retrograde orbits. Moreover, in hot-Jupiter systems, the spin-orbit angle is strongly correlated with stellar effective temperature: cool stars tend to be aligned, while a significant fraction of hot stars exhibit spin-orbit misalignment. The origin of this misalignment has puzzled researchers since its discovery. 

One prevailing theory is that the mechanism for misalignment is universal, applicable to all types of exoplanets—not only hot Jupiter systems but also warm Jupiter systems (a/R* ≥ 11) and even multi-planet systems. However, for cool stars, due to the strong tidal interaction between the star and the hot Jupiter, hot-Jupiter systems become tidally realigned over time. If this is true, we should be able to observe a considerable number of misaligned warm Jupiter systems, especially around hot stars, since they are likely not affected by tides due to the relatively large separation between them and their hosts. 

To verify this, Prof. Songhu Wang at the IU Astronomy Department and Prof. Malena Rice at Yale University initiated the Stellar Obliquities in Long-period Exoplanet Systems (SOLES) survey (https://ui.adsabs.harvard.edu/public-libraries/lZh6ZrxxQhiE0tBZANrHfg). The survey utilizes instruments like NEID at WIYN, PFS at Magellan, and HIRES at Keck, among others, to expand the sample of warm-Jupiter systems with measured spin-orbit angles. 

The most recent work, led by postdoc Dr. Xian-Yu Wang, added six additional aligned warm-Jupiter systems discovered by the TESS satellite to the sample, primarily using NEID data, confirming that warm-Jupiter systems tend to be aligned, regardless of stellar effective temperature. Additionally, Brandon Radzom’s recent work confirmed that multi-planet systems tend to be aligned (see https://astro.indiana.edu/news-events/news/2024-brandon-radzom-primordial.html). This observational evidence indicates that the mechanism of misalignment is not universal and is confined to hot-Jupiter systems. 

This paper has been published (https://iopscience.iop.org/article/10.3847/2041-8213/ad7469) and highlighted by AAS Nova (https://aasnova.org/2024/10/02/cool-it-down-warm-jupiters-are-better-aligned-than-hot-jupiters/).