What catches your eye in this description of an exoplanetary system? Start with a ‘hot Jupiter,’ with a radius 0.87 times that of our Jupiter and an orbit of 7.1 days. This is WASP-132b, confirmed in 2016, and first discovered through the labors of the Wide-Angle Search for Planets program. Subsequent confirmation came through the CORALIE spectrograph installed on the Euler telescope at the European Southern Observatory’s La Silla site. This world orbits a K-class star 403 light years out in Lupus.
The CORALIE measurements gave hints of another giant planet in a long period orbit. The system came still further into focus in 2021, when observations from TESS (Transiting Exoplanet Survey Satellite) showed a transiting super-Earth with a diameter of 1.8 Earth radii in a tight orbit of 1.01 days. The mass of the planet, as measured by the HARPS spectrograph at La Silla, is six times that of Earth. So we have both a hot-Jupiter and a super-Earth hugging the star, along with an outer gas giant.
Image: The WASP-132 system was known to harbour WASP-132b, here in the foreground, a hot Jupiter planet orbiting around a K-type star in 7.1 days. New data confirms the system has more planets, including an inner super-Earth, here seen transiting in front of the orange host-star. Visible as a pale blue dot near the top right corner is also the giant planet WASP-132d discovered in the outskirts of the system. © Thibaut Roger – Université de Genève.
While the European Space Agency’s Gaia satellite continues to take astrometrical data on WASP-132, follow-up work has shown the super-Earth to have a density similar to Earth’s and a composition of metals and silicates fairly similar to our planet (remember, we have both radius and mass measurements to work with because of the multiple datasets from different detection methods). Meanwhile, the problem here should be apparent.
Ravit Helled (University of Zurich) and a co-author of the study offers this:
‘‘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 in particular their migration processes. WASP-132 demonstrates the diversity and complexity of multi-planetary systems, underlining the need for very long-term, high-precision observations.’’
All true, of course, but it doesn’t get across how unusual this finding is. For hot Jupiters as thus far observed have been relatively isolated from planets further out in their systems. That makes sense because the model for their formation involves migration, with the giant worlds forming far enough out from the star to feed off plentiful gas and dust in the protoplanetary disk, and then moving inward as the system takes form. Woe to inner planets, whose fate might include ejection from the system entirely.
WASP-132 shouldn’t have the system architecture it does given this theory of migration, meaning we have to re-examine the nature of migration, or ponder ways to achieve a planet of this size in a tight stellar orbit that leave migration behind altogether. The hot Jupiter here leads François Bouchy (University of Geneva), a co-author of the study, to say this:
“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.”
To my knowledge, WASP-132 is the second example of a planetary system in this configuration. WASP-47 takes precedence in terms of discovery, having been first analyzed by the WASP team in 2012 (discovery of the hot Jupiter) and then expanded through work with K2 data in 2015. WASP-47, a G-class star in Aquarius some 880 light years away, hosts a super-Earth inside the hot Jupiter’s orbit, a hot Neptune outside its orbit, and an outer gas giant (‘warm Saturn’) within the habitable zone. The discovery paper of the smaller worlds at WASP-47 is worth quoting:
The continued existence of the companions in this system indicates that HEM [high eccentricity migration] ] cannot serve as the sole formation mechanism for hot Jupiters. HEM would likely have disrupted the orbits of the smaller planets. It is quite possible that there is more than one potential formation mechanism for hot Jupiters. Additionally, recent observations have identified an additional Jupiter-mass planet in a 571-day orbit (called WASP-47c; Neveu-VanMalle et al. 2015) in this system, making this the first hot Jupiter with both close-in companions and an external perturber. Future dynamical work will place limits on the architecture of this system.
The paper is Thibaut et al., ”Discovery of a cold giant planet and mass measurement of a hot super-Earth in the multi-planetary system WASP-132′,” Astronomy and Astrophysics Vol. 693 (15 January 2025), A144 (full text). On WASP-132, see Becker at al., “WASP-47: A Hot Jupiter System With Two Additional Planets Discovered by K2,” Astrophysical Journal Letters Vol. 812, No. 2 (12 October 2015), L18 (full text).
