Time for exoplanets
During the final two weeks of in-orbit commissioning, Cheops observed two exoplanet-hosting stars as the planets ‘transited’ in front of their host star and blocked a fraction of starlight. Observing transits of known exoplanets is what the mission was built for – to measure planet sizes with unprecedented precision and accuracy and to determine their densities by combining these with independent measurements of their masses.
One of the targets was HD 93396, a subgiant yellow star located 320 light-years away, slightly cooler and three times larger than our Sun. The focus of the observations was KELT-11b, a puffy gaseous planet about 30% larger in size than Jupiter, in an orbit that is much closer to the star than Mercury is to the Sun.
The light curve of this star shows a clear dip caused by the eight hour-long transit of KELT-11b. From these data, the scientists have determined very precisely the diameter of the planet: 181,600 km – with an uncertainty just under 4300 km.
The measurements made by Cheops are five times more accurate than those from Earth, explains Willy Benz, Principal Investigator of the Cheops mission consortium, and professor of astrophysics at the University of Bern. “That gives us a foretaste for what we can achieve with Cheops over the months and years to come,” he says.
A formal review of the satellite performance and ground segment operations was held on 25 March, and Cheops passed it with flying colours. With this, ESA handed over responsibility for operating the mission to the consortium led by Willy Benz.
Fortunately, the commissioning activities were not affected much by the ensuing emergency caused by the coronavirus pandemic, which resulted in social distancing measures and restrictions to movement across Europe to prevent the spread of the virus.
“The ground segment has been working very smoothly from early on, which enabled us to fully automate most of the operations for commanding the satellite and downlinking the data already in the first few weeks after launch,” explains Carlos. “By the time the crisis emerged in March, with the new rules and regulations that came with it, the automated systems meant that the impact on the mission was minimal.”
Cheops is currently transitioning towards routine science operations, which are expected to begin before the end of April. Scientists have started observing some of the ‘early science targets’ – a selection of stars and planetary systems chosen to showcase examples of what the mission can achieve: these include a ‘hot super-Earth’ planet known as 55 Cancri e, which is covered in a lava ocean, as well as the ‘warm Neptune’ GJ 436b, which is losing its atmosphere due to the glare from its host star. Another star on the list of upcoming Cheops observations is a white dwarf, the first target from ESA’s Guest Observers Programme, which provides scientists from beyond the mission consortium with the opportunity to use the mission and capitalize on its observational capabilities.