On Wednesday, November 29, 2023, ESA released that, with the help of the Cheops mission, a rare star system with six exoplanets has been unlocked. The discovery is particularly valuable because the planets’ orbital configuration shows that the system has been largely unchanged since its formation, more than a billion years ago.
Cheops: the ESA’s CHaracterising ExOPlanet Satellite
Cheops is an acronym that stands for CHracterising ExOPlanet Satellite. It was launched on December 18, 2019, from ESA’s space center in Kourou (French Guiana) and is moving in an orbit 700 km above Earth. It is the first ESA space mission dedicated to studying bright, nearby stars already known to host exoplanets. The objective is to make high-precision observations of the planet’s size, to pursue it the mission uses the transit technique, one of the most renowned for detecting extrasolar planets.
The idea is to monitor the brightness of the host star, which will decrease as the planet passes in front of it, blocking some of the light, and creating a characteristic curve that can allow us to deduce some of the properties of the planet.
The mission focuses on exoplanets with orbital periods of around 50 days or less, typically ranging from Earth-sized to Neptune-sized planets around a variety of stars. By targeting planets that have short orbits, Cheops have access to multiple transits over the lifetime of the mission, making it easier to repeat observations in order to build up the precision of the measurement.
Its data enables the planets’ bulk density to be derived – a first-step characterization toward understanding these alien worlds. The mission is showing auspicious results, which is why it has just been extended: on March 7th, ESA’s Science Programme Committee confirmed its continued operations to 2026 and an indicative extension to 2029.
A mysterious planetary system that has been studied for years
Cheops recently supplied crucial data to unravel the mysteries of an enigmatic exoplanet system, centered around the star HD110067, situated approximately 100 light-years away in the Coma Berenices constellation. This story starts in 2020 when NASA’s TESS detected brightness dips in the star, suggesting planetary transits. Initial analysis proposed two planets, one with a 5.642-day orbital period and another with an undetermined period.
Two years later, TESS revisited the star, providing more certain detections but raising new questions. Intrigued by the uncertainties, Rafael Luque and colleagues from the University of Chicago turned to Cheops for a detailed study, deciding to explore potential signals among the planets’ orbital periods.
An intuition that paid off: a system unveiled
The idea of wanting to take a closer look at this star by guessing that the planets revolving around it might have peculiarities was correct. Thanks to the help of the satellite they confirmed a third planet in the system and realized that they had finally found the key to unlocking the whole system: the three planets were in an orbital resonance, an effect that occurs when two or more celestial bodies follow orbits whose ratio of orbital periods is a simple fraction.
The outer-most planet takes 20.519 days to orbit, which is extremely close to 1.5 times the orbital period of the next planet, 13.673 days. This in turn is almost exactly 1.5 times the orbital period of the inner planet, 9.114 days. Predicting other orbital resonances and matching them to the remaining unexplained data allowed the team to discover the other three planets in the system.
“Cheops gave us this resonant configuration that allowed us to predict all the other periods. Without that detection from Cheops, it would have been impossible.”– Rafael Luque, Department of Astronomy and Astrophysics, University of Chicago
Cheops has thus succeeded in revealing a cosmic secret.
But why is it important to find a system of planets in resonance?
Orbitally resonant systems provide crucial insights into the formation and evolution of planetary systems. While planets around stars often form in resonance, various factors like the presence of a massive planet, close encounters with passing stars, or giant impact events can disrupt this delicate balance.
Consequently, many known multi-planet systems appear close to resonance but have likely been perturbed. Maintaining resonance in multi-planet systems is rare, with only about one percent believed to do so. HD110067 stands out as a special case, offering a pristine glimpse into a planetary system that has remained untouched, making it a valuable source for scientific study.
Cheops, discoveries out of the ordinary
Cheops is therefore achieving incredible results:
“As our science team puts it: Cheops is making outstanding discoveries sound ordinary. Out of only three known six-planet resonant systems, this is now the second one found by Cheops, and in only three years of operations”– Maximilian Günther, ESA project scientist for Cheops
The newly revealed system, HD110067, is the brightest known system with four or more planets. Since those planets are all sub-Neptune-sized with atmospheres that are likely extended, it makes them ideal candidates for studying the composition of their atmospheres using the NASA/ESA/CSA James Webb Space Telescope and the ESA’s future Ariel and Plato telescopes. Cheops is therefore charting the course to indicate which systems could represent key points to be studied more specifically by future space missions focusing on exoplanets.