Hot Jupiters are a class of giant exoplanets that have fascinated astronomers since their discovery. They are similar in mass to Jupiter but orbit much closer to their host stars, sometimes completing their orbits in just a few days. Initially, it was believed that these planets, in their migration toward their stars, would either accrete any other planets present in the system or eject them from orbit altogether, leading to a system with a solitary Hot Jupiter. However, this long-standing paradigm has been challenged by new observations that suggest Hot Jupiters may not always be isolated. One of the most significant studies shedding light on this anomaly comes from a new discovery led by the University of Geneva (UNIGE), supported by a collaborative team that includes the National Center of Competence in Research (NCCR) PlanetS, the Universities of Bern (UNIBE), Zurich (UZH), and several international institutions. Their research introduces a multi-planetary system, WASP-132, which contains not just a Hot Jupiter, but also an unexpected inner Super-Earth and a distant icy giant planet.
Hot Jupiters and their Challenging Formation
To understand the significance of this new discovery, it’s essential to first examine the unique characteristics of Hot Jupiters. These planets have masses comparable to Jupiter’s but orbit at remarkably close distances to their stars—far closer than Mercury’s orbit around the Sun. The proximity of these planets to their stars creates harsh conditions for their formation. Traditionally, it is thought that Hot Jupiters must have originally formed much farther from their stars, where gas and dust were abundant, and then migrated inward due to gravitational interactions within the planetary system.
What makes this migration process so intriguing is that it has been widely assumed that Hot Jupiters tend to migrate alone. According to prevailing models, any planets that might be located within their orbits would either be gravitationally perturbed or ejected as the Hot Jupiter made its journey inward. This led to the expectation of planetary systems with a Hot Jupiter as the singular planetary occupant within a close orbit. This was consistent with early observations, reinforcing the theory that Hot Jupiters functioned as lone giants in their respective systems.
Overturning the Old Paradigm
In light of this, the recent discovery of the WASP-132 system challenges traditional assumptions. This system contains not only a Hot Jupiter but also an unexpected ensemble of planets: an inner Super-Earth and an outer giant planet. Specifically, the WASP-132 system consists of three distinct planets:
- A Hot Jupiter (WASP-132b) that orbits its host star in just 7.1 days, a typical characteristic for Hot Jupiters.
- An inner Super-Earth, located even closer to the star than the Hot Jupiter, which completes an orbit in only 24 hours and 17 minutes.
- A massive giant planet, much further out in the system, that completes an orbit in around five years.
To add further complexity, the system is home to a very distant object, likely a brown dwarf—a celestial body that falls between a planet and a star in mass.
These new findings, published in Astronomy & Astrophysics, indicate that Hot Jupiters are not always isolated but can exist alongside other planetary bodies in their star systems, despite the close proximity of their orbits. This suggests that the migration of Hot Jupiters must occur through processes that do not disrupt the entire planetary system. Instead of a chaotic migration that ejects other planets or consumes them, a more stable form of migration appears to be at play in such systems, allowing other planets to coexist in an orbitally stable configuration.
A Multi-Planetary System that Challenges Current Theories
The WASP-132 system provides a unique opportunity to study how planetary systems form and evolve. Researchers have noted the configuration of this system as a highly informative case. As François Bouchy, associate professor in the Department of Astronomy at UNIGE and co-author of the study, pointed out, the presence of both an inner Super-Earth and an outer giant planet, alongside a Hot Jupiter, defies previous expectations regarding planetary migration. “The WASP-132 system is a remarkable laboratory for studying the formation and evolution of multi-planetary systems,” Bouchy stated.
The detection of this planetary system provides further evidence that there are multiple possible scenarios for how planetary systems can develop and persist. Contrary to previous beliefs that Hot Jupiters’ migration would clear their orbits, it seems that migration might occur along a more carefully regulated path that can maintain a system with multiple planets of varying sizes and orbital distances.
WASP-132: A Comprehensive View of Planetary Evolution
The study of the WASP-132 system was not an overnight success. Astronomers began their observations of this particular star in 2006 as part of the Wide-Angle Search for Planets (WASP) program. In 2012, with over 23,000 photometric measurements of the star, a planetary candidate was identified, WASP-132b, which had a radius approximately 0.87 times that of Jupiter and an orbital period of just 7.1 days.
Further observations with the CORALIE spectrograph installed on the Swiss Euler Telescope, starting in 2014, continued to refine the characteristics of the candidate, confirming the presence of the Hot Jupiter in 2016. During this time, the spectrograph revealed another massive planet on a long orbit around the same star.
The next breakthrough came with the TESS (Transiting Exoplanet Survey Satellite) space telescope, which, in 2021, revealed the signal of a transiting Super-Earth orbiting the same star, with a mass of six times that of Earth. This exciting find was followed by precise measurements using the HARPS spectrograph, which confirmed the mass and density of the Super-Earth and provided insights into its composition. The data showed that, like Earth, this Super-Earth is dominated by silicate and metal-rich material.
As research continued, observations from ESA’s Gaia satellite revealed further details about the long-term motion of the star and its possible additional companions, such as the distant brown dwarf.
Understanding the Composition of the Planets
The WASP-132 system’s combination of diverse planetary types offers critical insights into the formation processes of giant planets, Super-Earths, and cold giant planets. For instance, the Hot Jupiter WASP-132b was found to have an unusual heavy element enrichment, with a composition corresponding to around 17 Earth masses of heavy elements. This result aligns well with current models for the formation of gas giants, which posit that such planets accumulate solid materials and ice before growing into large gaseous worlds.
Meanwhile, the Super-Earth in the system shows a composition dominated by metals and silicates, similar to that of Earth. This composition hints that the Super-Earth formed through processes different from those that created gas giants and provides valuable insights into the diversity of planet types and their formation environments.
The Future of Planetary System Studies
The discovery of WASP-132 highlights the ongoing need for detailed, long-term, and high-precision observations in the field of exoplanet research. According to Ravit Helled, professor at the University of Zurich and co-author of the study, this system’s configuration underscores the complexity of multi-planetary systems. Helled emphasized, “WASP-132 demonstrates the diversity and complexity of multi-planetary systems, underlining the need for very long-term, high-precision observations.”
The discovery also suggests that our understanding of planetary migration is still evolving. As astronomers develop more sophisticated instruments and observational techniques, the study of systems like WASP-132 will continue to refine our models of planetary formation and migration, challenging existing assumptions and offering new insights into the nature of distant worlds.
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
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