Discovery: Hot Jupiters are not always alone

Hot Jupiters are giant planets originally known to orbit alone near their star. However, this paradigm has been undermined by recent observations and a new study, led by the University of Geneva (UNIGE), could definitively invalidate it. A team including the National Research Center PlanetS, the universities of Bern (UNIBE) and Zurich (UZH) as well as several foreign universities, has just announced the existence of a planetary system, WASP-132, with an unexpected architecture .

Hot Jupiters are planets with a mass similar to Jupiter, but which orbit very close to their star, at a much smaller distance than Mercury from the Sun. These giant planets can hardly form where they are observed, because there is not enough gas and dust so close to their star. They must therefore form far from the latter and migrate during the evolution of the planetary system.

Until recently, astronomers observed that these hot Jupiters were isolated around their star, with no other planets in their vicinity. This observation seemed all the more solid because a theory could explain it. The migration processes of giant planets towards their star generate accretion (capture and integration) or ejection of possible planets found in an internal orbit.

But recent observations suggest other scenarios. This trend is today confirmed by this new study, published in “Astronomy & Astrophysics”. The team discovered the existence of a multiplanetary system consisting of a hot Jupiter, an inner super-Earth (even closer to the star than the hot Jupiter), and an outer massive giant planet (well farther from the star than hot Jupiter). If hot Jupiters are not always alone in their planetary system, their migration process must then be different from that predicted by the models, in order to preserve the architecture of the system.

The WASP-132 system appears to be a unique multiplanetary system. Its hot Jupiter circles its star in 7 days and 3 hours; its super-Earth (a rocky planet 6 times the mass of Earth) goes around the star in just 24 hours and 17 minutes, and its giant planet (5 times the mass of Jupiter), revolves around the star in 5 years of the host star. In addition, a much more massive companion, probably a brown dwarf (a body whose mass is between that of a planet and that of a star), orbits at a very long distance.

“The WASP-132 system is a remarkable laboratory for studying the formation and evolution of multiplanetary systems. The discovery of a hot Jupiter alongside an inner super-Earth and a distant giant challenges our understanding of the formation and evolution of these systems,” says François Bouchy, associate professor in the Department of astronomy from the Faculty of Sciences of UNIGE and co-author of the study. “This is the first time that we have observed such a configuration!” adds Solène UlmerMoll, post-doctoral student at UNIGE and UNIBE at the time of the study.

Observations of WASP-132 are not over, however, because ESA’s Gaia satellite has been measuring the tiny variations in the positions of stars since 2014 in order to reveal their planetary companions and external brown dwarfs.

The discovery of an outer cold giant planet and an inner super-Earth adds a layer of complexity to the WASP-132 system. The standard hypothesis of a migration by dynamic disturbance of the hot Jupiter towards the interior does not hold, because this would have destabilized the orbit of the two other planets. Their presence rather suggests a more stable and dynamically “gentle” migration route, in a proto-planetary disk for the hot Jupiter, preserving its neighbors.

The combination of precise radius and mass measurements also made it possible to determine the density and internal composition of the planets. Hot Jupiter WASP-132b reveals heavy element enrichment of about 17 Earth masses, consistent with models of gas giant formation. The super-Earth has a composition dominated by metals and silicates quite similar to Earth.

“The combination of a hot Jupiter, an internal super-Earth and an external giant planet in the same system provides important constraints on theories of planet formation and in particular their migration processes,” says Ravit Helled, professor at UZH and co-author of the study. “WASP-132 demonstrates the diversity and complexity of multiplanetary systems, highlighting the need for very long-term and very high precision observations.”