For years, the study of Hot Jupiters—the giant planets that orbit closely to their parent stars—has yielded fundamental insights into how planetary systems evolve. These planets, often Jupiter-sized but significantly hotter due to their proximity to their stars, had long been thought to form at greater distances from their stars and migrate inward, potentially ejecting or disrupting any other planets that might have formed in their vicinity. However, a groundbreaking discovery has now challenged this long-standing theory and revealed that the process by which Hot Jupiters migrate might be more complex than originally thought.
This shift in understanding was spurred by a remarkable new study led by the University of Geneva (UNIGE), with significant contributions from the Universities of Bern (UNIBE) and Zurich (UZH), the National Center of Competence in Research (NCCR) PlanetS, and numerous international universities. This collaboration, published in Astronomy & Astrophysics, has uncovered a previously unknown multi-planetary system—WASP-132—that not only features a Hot Jupiter but also an inner Super-Earth and an outer icy giant planet. These findings challenge our previous models of planetary migration and offer new insights into the formation of planetary systems.
What are Hot Jupiters?
Before delving into the intricacies of this new discovery, it’s essential to understand what Hot Jupiters are and why they have long been a topic of fascination for astronomers. Hot Jupiters are giant gas planets that bear a close resemblance to Jupiter, the largest planet in our solar system. However, unlike Jupiter, which orbits at a distance of about 5.2 astronomical units (AU) from the Sun, Hot Jupiters reside very close to their parent stars. These planets often orbit at distances much smaller than Mercury’s orbit around the Sun, which leads to surface temperatures that can soar to thousands of degrees.
Because there is limited gas and dust in the inner regions of planetary systems, Hot Jupiters cannot form where they are observed. It is believed that these planets form farther out in the cooler regions of the system and subsequently migrate inward towards their parent stars. Migration models, driven by gravitational interactions and gas drag in the protoplanetary disk, have suggested that these planets might clear out or disrupt the planetary systems they pass through—ejecting smaller planets or causing planetary systems to be left empty in their wake.
This understanding, while supported by many observational data, has begun to shift with recent discoveries, including the discovery of WASP-132.
WASP-132: A New Kind of Planetary System
The WASP-132 system is exceptional because it defies the previously accepted paradigm of Hot Jupiter migration. The system features a Hot Jupiter, orbiting its host star in just 7 days and 3 hours. This planet, designated WASP-132b, is a gas giant with a size comparable to that of Jupiter. However, the system also contains two other surprising planets.
The first is an inner Super-Earth—a rocky planet six times the mass of Earth—that orbits its star in a swift 24 hours and 17 minutes. Unlike the Hot Jupiter, which is located at a relatively large distance from the other planets, this Super-Earth occupies a region closer to the star. The presence of this Super-Earth is particularly intriguing because it resides much closer to the star than WASP-132b, contrary to what one might expect from a system dominated by a migrating Hot Jupiter.
In addition to the Hot Jupiter and Super-Earth, the system includes a massive outer planet, five times the mass of Jupiter, which orbits the star on an extended 5-year cycle. This outer planet is likely a gas giant, perhaps similar to Neptune or Uranus. And perhaps most astonishingly, the system also features a distant companion—a brown dwarf, a celestial object with a mass between that of a planet and a star—that orbits the star at a much greater distance.
18 Years of Observation
The discovery of the WASP-132 system is the result of more than 18 years of careful monitoring and observation. The investigation began in 2006 with the Wide-Angle Search for Planets (WASP) program, which is aimed at identifying exoplanets through the detection of periodic dips in star brightness caused by transiting planets. In 2012, a series of over 23,000 photometric measurements pinpointed a planetary candidate, identified as WASP-132b.
The confirmation of WASP-132b as a planet occurred in 2016, following further analysis and mass measurement using the CORALIE spectrograph at the Swiss Euler telescope. These measurements showed that WASP-132b had a mass equal to about 0.41 times the mass of Jupiter and confirmed the existence of another giant planet in the system with a much longer orbital period.
In 2021, the Transiting Exoplanet Survey Satellite (TESS) revealed an exciting signal from a Super-Earth orbiting the same star—a planet with a diameter of 1.8 Earth radii and an orbital period of just over one day. The additional observations made with the HARPS spectrograph at the La Silla Observatory in early 2022 revealed that this Super-Earth has six times the mass of Earth, with a density comparable to that of Earth, suggesting a composition dominated by silicates and metals.
The fine-tuned measurements gathered during these observations were crucial in helping scientists piece together the architecture of the WASP-132 system, revealing the complex interplay between the planets.
A New Understanding of Planetary Migration
The surprising architecture of the WASP-132 system offers new perspectives on the migration processes of Hot Jupiters. Under the traditional models of planetary migration, it was believed that Hot Jupiters formed further from their stars and migrated inward, likely disturbing or ejecting any planets that formed inside their orbit. However, if this were the case, it would be challenging for systems like WASP-132 to exist, as the strong gravitational forces exerted by the migrating Hot Jupiter would destabilize the orbits of nearby planets, particularly those like the Super-Earth.
The discovery of a stable multi-planetary system containing a Hot Jupiter alongside an inner Super-Earth and an outer giant planet suggests that planetary migration may be more dynamic and diverse than we once thought. Specifically, the migration of WASP-132b may have followed a path that preserved the stability of the entire system. The system hints at a migration process that is gentler—what might be described as a “cooler” migration. This could involve a more gradual evolution within the protoplanetary disk rather than the catastrophic accretion or ejection processes that were previously proposed.
The observations also reveal that these planets maintain the integrity of their internal compositions. WASP-132b, for example, reveals an enrichment of heavy elements, which supports the current theories regarding the formation of gas giants. The Super-Earth, on the other hand, appears to be composed mainly of metals and silicates, suggesting a formation process more akin to that of rocky planets like Earth.
Implications for the Formation of Planetary Systems
The remarkable diversity in the architecture of the WASP-132 system has important implications for our understanding of planet formation. Traditional models suggested that planetary migration followed a predictable path of planet accretion or disruption. However, with evidence of stable multi-planetary systems like WASP-132, astronomers are now rethinking the role of migration in planetary formation.
The system demonstrates the necessity of very long-term, high-precision observations to fully capture the complexities of planetary dynamics. It serves as a stark reminder of the incredible diversity that exists among planetary systems. While migration models are an essential tool, they must now be adapted to account for the diverse ways in which planets can interact with one another, maintaining or altering their orbits as they form and evolve.
Conclusion: Looking Ahead
The study of the WASP-132 system continues to unfold and adds an exciting new chapter to the investigation of exoplanetary systems. As we continue to unravel the mysteries of Hot Jupiters, Super-Earths, and distant giant planets, the findings present us with a profound opportunity to refine our models of planetary migration and formation. They challenge us to consider new possibilities for how planets can form and coexist in stable multi-planetary systems.
Ultimately, the system embodies the complexity of planet formation processes. Each new discovery, like WASP-132, drives us further toward a more comprehensive understanding of how planets come into existence, evolve, and adapt within the unique environments of their stellar systems. The era of planetary discovery is far from over, and each system we observe deepens our understanding of the vast diversity that our universe holds.
Reference: Discovery of a cold giant planet and mass measurement of a hot super-Earth in the multi-planetary system WASP-132, Astronomy and Astrophysics (2025). DOI: 10.1051/0004-6361/202348177