Not all Hot Jupiters orbit solo

Not all Hot Jupiters orbit solo
by Robert Schreiber
Berlin, Germany (SPX) Jan 16, 2025

Hot Jupiters, once thought to orbit alone near their stars, have been revealed to exist within more complex planetary systems. A study led by the University of Geneva (UNIGE), in collaboration with the National Centre of Competence in Research (NCCR) PlanetS and other institutions, has uncovered the WASP-132 system. This remarkable discovery challenges existing models of planetary formation and migration. Published in *Astronomy and Astrophysics*, the findings reveal a Hot Jupiter accompanied by an inner Super-Earth and an outer icy giant planet.

Hot Jupiters are gas giants with masses akin to Jupiter, located extremely close to their stars-much closer than Mercury is to the Sun. Such proximity to their stars suggests these planets likely formed further away and migrated inward during their system's evolution. Traditionally, it was believed that this migration either absorbed or expelled other nearby planets, leaving Hot Jupiters isolated. However, emerging observations indicate that Hot Jupiters can coexist with other planets in multi-planetary systems.

The team, involving researchers from the Universities of Bern (UNIBE) and Zurich (UZH) and institutions such as the University of Warwick, confirmed this trend through their analysis of the WASP-132 system. The system includes a Hot Jupiter orbiting its star every 7 days and 3 hours, a Super-Earth in an even tighter 24-hour orbit, and a giant planet five times Jupiter's mass orbiting at a much greater distance with a 5-year period. Furthermore, a distant massive companion, likely a brown dwarf, is also present.

"The WASP-132 system is a remarkable laboratory for studying the formation and evolution of multi-planetary systems. The discovery of a Hot Jupiter alongside an inner Super-Earth and a distant giant planet calls into question our understanding of the formation and evolution of these systems," said Francois Bouchy, associate professor in the UNIGE Faculty of Science. "This is the first time we have observed such a configuration," added Solene Ulmer-Moll, a former postdoctoral researcher at UNIGE and UNIBE.

Observational Insights from Over 18 Years

The star WASP-132 has been under observation since 2006 through the Wide-Angle Search for Planets (WASP) program. By 2012, over 23,000 photometric measurements identified WASP-132b as a candidate planet with a radius 0.87 times that of Jupiter and an orbital period of 7.1 days. Follow-up monitoring using the CORALIE spectrograph on the Swiss Euler telescope confirmed the planet's existence and mass by 2016. Additionally, CORALIE data revealed the presence of a second massive planet on a longer orbit.

In late 2021, NASA's TESS space telescope detected a signal from a transiting Super-Earth with a radius of 1.8 Earth radii and a 1-day orbital period. Subsequent measurements in 2022 using the HARPS spectrograph at La Silla Observatory confirmed the planet's mass as six times that of Earth.

"The detection of the inner Super-Earth was particularly exciting," said Nolan Grieves, a postdoctoral researcher at UNIGE and lead author of the study. "We had to carry out an intensive campaign using HARPS and optimized signal processing to determine its mass, density, and composition, revealing a planet with Earth-like density."

ESA's Gaia satellite has been monitoring WASP-132 since 2014 to track variations in stellar positions caused by its planetary companions and the distant brown dwarf.

Rethinking Planet Formation and Migration

The discovery of the outer icy giant planet and the inner Super-Earth challenges the conventional theory of Hot Jupiter migration through dynamic perturbation, which would destabilize such a planetary arrangement. Instead, the evidence points to a more stable, gradual migration within a proto-planetary disk, allowing the Hot Jupiter to retain its neighboring planets.

Detailed mass and radius measurements also shed light on the planets' composition. WASP-132b, the Hot Jupiter, shows significant enrichment with heavy elements, consistent with gas giant formation models. Meanwhile, the Super-Earth's composition is dominated by metals and silicates, akin to Earth.

"The combination of a Hot Jupiter, an inner Super-Earth, and an outer giant planet in the same system provides important constraints on theories of planet formation and their migration processes," concluded Ravit Helled, professor at UZH and co-author of the study. "WASP-132 demonstrates the diversity and complexity of multi-planetary systems, underlining the need for very long-term, high-precision observations."

Research Report:Discovery of a cold giant planet and mass measurement of a hot super-Earth in the multi-planetary system WASP-132