Hot Jupiters are massive planets that were originally thought to orbit solo in close proximity to their host stars. It was believed that during their migration toward their stars, they would either absorb or eliminate any other nearby planets. However, recent findings have challenged this view, particularly highlighted by a new study revealing the presence of the WASP-132 planetary system, which has an intriguing structure. This system not only features a Hot Jupiter but also includes an inner Super-Earth and an outer icy giant planet.
Initially, Hot Jupiters were classified as large planets that orbit very close to their host stars. Researchers believed that as these planets migrated inward, they either captured or expelled any neighboring planets. Recent observations have overturned this established perspective. A study conducted by the University of Geneva (UNIGE), in collaboration with the National Centre of Competence in Research (NCCR) PlanetS, the Universities of Bern (UNIBE) and Zurich (UZH), and several international institutions, has confirmed the existence of a remarkable planetary system named WASP-132. This system not only features a Hot Jupiter but also hosts an inner Super-Earth and an icy giant planet. These findings are detailed in the journal Astronomy & Astrophysics.
Hot Jupiters are gigantic planets with masses comparable to Jupiter’s, yet they orbit at much closer distances to their stars than Mercury does to the Sun. The formation of these giant planets in their observed locations is perplexing because there isn’t sufficient gas and dust available near the star. Consequently, they must form at greater distances and migrate inward as the planetary system develops.
Until recently, astronomers found that Hot Jupiters tended to occupy isolated orbits around their stars, with no other planets nearby. This seemed to confirm a theory explaining that the processes during their inward migration would lead to the addition or removal of any inner orbiting planets. However, new observations are indicating alternative possibilities.
Led by the Astronomy Department of the UNIGE Faculty of Science, a partnership encompassing UNIBE, UZH, and other international institutions, including Warwick University, has validated this shift in understanding. The researchers have discovered a multi-planetary system that includes a Hot Jupiter, a Super-Earth (even closer to the star than the Hot Jupiter), and another giant planet located much further from the star. Hence, if Hot Jupiters are not always alone, then their migration must follow a different mechanism to maintain the system’s structure.
A unique multi-planetary system
The WASP-132 system stands out as an extraordinary multi-planetary arrangement. It features a Hot Jupiter that completes an orbit around its star every 7 days and 3 hours; a Super-Earth (a rocky planet 6 times larger than Earth) that orbits the star in 24 hours and 17 minutes; and a giant planet (5 times the mass of Jupiter) that takes 5 years to orbit its host star. Additionally, there is a much more massive companion, likely a brown dwarf (a celestial entity with a mass that falls between that of a planet and a star), which orbits at a significantly larger distance.
”The WASP-132 system serves as an exceptional laboratory for examining the formation and evolution of multi-planetary systems. The coexistence of a Hot Jupiter with an inner Super-Earth and a distant giant planet challenges our conventional notions of these systems’ formation and evolution,” states François Bouchy, an associate professor in the Department of Astronomy at UNIGE and a co-author of the study. ”This configuration is unprecedented in our observations!” adds Solène Ulmer-Moll, a postdoctoral researcher at UNIGE and UNIBE at the time of the study, and also a co-author of the paper.
Eighteen years of observation
The investigation of the star WASP-132 by exoplanet researchers commenced in 2006 as part of the Wide-Angle Search for Planets (WASP) initiative. By 2012, over 23,000 photometric observations led to the identification of a planetary candidate, WASP-132b, which exhibited a radius 0.87 times that of Jupiter and an orbital period of 7.1 days. In 2014, the CORALIE spectrograph on the Swiss Euler telescope, operated by UNIGE, began tracking this candidate. By 2016, WASP-132b was confirmed, and its mass was determined to be 0.41 times that of Jupiter. Furthermore, CORALIE data suggested the existence of another distant giant planet.
Later, in late 2021, the TESS space telescope detected signals from a transiting Super-Earth measuring 1.8 times the radius of Earth and completing an orbit in just 1.01 days. In early 2022, the HARPS spectrograph at the La Silla observatory measured the mass of this Super-Earth, establishing it to be six times that of Earth, within a study led by David Armstrong from the University of Warwick.
”The discovery of the inner Super-Earth was particularly thrilling,” remarks Nolan Grieves, a postdoctoral researcher in the Astronomy Department at UNIGE during the study and the paper’s lead author. ”We had to undertake an extensive campaign using HARPS along with optimized signal processing to accurately determine its mass, density, and composition, ultimately revealing a planet with an Earth-like density.”
Further observations of WASP-132 are ongoing, as ESA’s Gaia satellite has been monitoring minute positional changes in stars since 2014, with the aim of uncovering their planetary companions and distant brown dwarfs.
A fresh perspective on planet formation
The identification of an outer cold giant planet and an inner Super-Earth introduces critical complexity to the WASP-132 system. The traditional theory positing the migration of a Hot Jupiter through dynamic disturbances does not hold in this case, as it would have destabilized the orbits of the other two planets. Instead, the coexistence of these planets indicates a more stable and “cool” migratory pathway within a protoplanetary disk for the Hot Jupiter, preserving the integrity of the other planets’ orbits.
Combining precise measurements of radius and mass has allowed researchers to ascertain the density and internal structure of the planets. WASP-132b, the Hot Jupiter, displays an enrichment of heavy elements equivalent to about 17 Earth masses, aligning with gas giant formation models. In contrast, the Super-Earth’s composition is predominantly made up of metals and silicates, resembling that of Earth.
”The combination of a Hot Jupiter, an inner Super-Earth, and an outer giant planet within the same system imposes significant constraints on our theories regarding planet formation, particularly concerning migration processes,” concludes Ravit Helled, a professor at UZH and a co-author of the study. ”WASP-132 exemplifies the diversity and complexity of multi-planetary systems, highlighting the necessity for extensive, high-precision long-term observations.”
