ESA Declares CHEOPS Space Telescope Ready for Operation

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The CHEOPS space telescope was declared ready for space exploration by the European Space Agency (ESA) late last month and has begun collecting observations of known exoplanet-hosting stars. The telescope is a joint mission between the ESA and Switzerland under the leadership of the University of Bern in collaboration with the University of Geneva (UNIGE).

With this milestone, the ESA has handed over the responsibility of operation to the mission consortium, which consists of scientists and engineers from approximately 30 institutions in 11 European countries.

“We were thrilled when we realized that all the systems worked as expected or even better than expected,” said CHEOPS instrument scientist Andrea Fortier from the University of Bern, who led the commissioning team of the consortium.
The luminosity of the star HD 88111 as derived from each of the 5,640 photos taken by CHEOPS over 47 hours is shown as a 'light curve'. Courtesy of ESA/CHEOPS Mission Consortium.
The luminosity of the star HD 88111 as derived from each of the 5640 photos taken by CHEOPS over 47 hours is shown as a “light curve.” Courtesy of ESA/CHEOPS Mission Consortium.

The team began by focusing on the evaluation of the photometric performance of the space telescope. CHEOPS has been conceptualized as a device of exceptional precision capable of detecting exoplanets the size of Earth.

“The most critical test was in the precise measurement of the brightness of a star to a variance of 0.002%,” Willy Benz said.

This precision is required to clearly recognize the dimming caused by the passage of an Earth-size planet in front of a sun-like star, an event known as a “transit,” which can last several hours. CHEOPS was also required to demonstrate its ability to maintain this degree of precision for up to two days.

To verify this, the team focused on a star known as HD 88111, located in the Hydra constellation. The star is some 175 light-years away from Earth and it is not known to host planets. CHEOPS took an image of the star every 30 seconds for 47 consecutive hours. Every image was carefully analyzed, initially using a specialized automatic software package and subsequently by the team members, to determine in each image the brightness of the star as accurately as possible. The team had expected the brightness of the star to change during the period of observation due to a variety of effects, such as other stars in the field of view, the tiny jittering motion of the satellite, or the impact of cosmic ray hitting the detector.

The results of the 5640 photos taken by CHEOPS over 47 hours were compiled into a graph as a “light curve.” The curve depicts the change over time in the brightness measurements from all the images, showing a root-mean-square scatter of 0.0015%.

“The light curve measured by CHEOPS was pleasingly flat. The space telescope easily surpasses the requirement for being able to measure brightness to a precision of 0.002%,” said Christopher Broeg, mission manager for the CHEOPS mission at the University of Bern.

The team observed other stars, including some known to host planets. CHEOPS focused on the planetary system HD 93396, which is in the Sextans constellation, approximately 320 light-years away from Earth. This system consists of a giant exoplanet called KELT-11b, which was discovered in 2016 to orbit its star in 4.7 days. The star is almost three times the size of the sun. The team chose this particular system because the star is so big that the planet takes a long time to pass in front of it, in fact, almost eight hours.

“This gave CHEOPS the opportunity to demonstrate its ability to capture long transit events otherwise difficult to observe from the ground, as the ‘astronomical’ part of the night for ground-based astronomy usually takes less than eight hours,” said Didier Queloz, professor at the Astronomy Department of the Faculty of Science at the University of Geneva and spokesperson of the CHEOPS science team.

The first transit light curve of CHEOPS shows a dip approximately nine hours after the beginning of the observation.

The transit of KELT-11b measured by CHEOPS enabled the size of the exoplanet to be measured. It has a diameter of 181,600 km, which CHEOPS is able to measure with a 4290-km margin of error. The diameter of Earth, in comparison, is only approximately 12,700 km, while that of Jupiter — the biggest planet in our solar system — is 139,900 km.

Exoplanet KELT-11b may be bigger than Jupiter, but its mass is five times lower, which means it has an extremely low density.

“It would float on water in a big-enough swimming pool,” said David Ehrenreich, CHEOPS mission scientist from the University of Geneva. The limited density is attributed to the close proximity of the planet to its star.

Benz explained that the measurements by CHEOPS are five times more accurate than those from Earth. “That gives us a foretaste for what we can achieve with CHEOPS over the months and years to come,” Benz said.

Published: April 2020
An afocal optical device made up of lenses or mirrors, usually with a magnification greater than unity, that renders distant objects more distinct, by enlarging their images on the retina.
The scientific observation of celestial radiation that has reached the vicinity of Earth, and the interpretation of these observations to determine the characteristics of the extraterrestrial bodies and phenomena that have emitted the radiation.
Research & Technologytelescopespace observationexoplanetexoplanet discoveryexoplanet imagingEuropean Space AgencyEuropeCHEOPSastronomyTech Pulse

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