On 17 December 2019 at 05:54 local time (09:54 CET), the European Space Agency (ESA) CHaracterising ExOPlanets Satellite (CHEOPS) space telescope is scheduled to lift off from Europe’s spaceport in Kourou, French Guiana, on board a Soyuz launcher. The mission will further extend the search for exoplanets, which was one of the topics of this year's Nobel Prize in physics. Didier Queloz, one of the Nobel Prize winners, is Chair of the CHEOPS Science Team. With the participation of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), CHEOPS will determine the radii and densities of a large number of exoplanets and investigate which of them might have an atmosphere. In addition to providing on-board hardware, DLR will contribute its extensive expertise in data analysis. The space telescope will examine exoplanets from a Sun-synchronous Earth orbit at an altitude of 700 kilometres.
"More than 4000 exoplanets have been discovered in the Milky Way, yet we still know far too little about these distant worlds in our cosmic neighbourhood," says Heike Rauer, Director of the DLR Institute of Planetary Research in Berlin. "We are all eager to see which 'faces' the planets characterised by CHEOPS will show us."
'Mini eclipse' reveals details
The new space telescope will study several hundred bright stars where orbiting planets have already been discovered by other surveys and missions. These include the Next-Generation Transit Survey (NGTS) telescope system in Chile and NASA’s Transiting Exoplanet Survey Satellite (TESS) all-sky survey mission. CHEOPS will measure the very small dips in apparent brightness that occur when a planet crosses its host star's disc, referred to as a 'transit'. "We could describe this fluctuation in brightness as a 'mini stellar eclipse', as the transiting exoplanet reduces the intensity of the light from the star for a short time," explains Juan Cabrera Perez, Head of the Extrasolar Planets and Atmospheres Department at the DLR Institute of Planetary Research. “This fluctuation can be measured and analysed – an area in which we can contribute suitable tools and many years of experience.”
The mission will focus on stars orbited by planets with sizes ranging between those of Earth and Neptune – in other words, planets with diameters of approximately 10,000 to 50,000 kilometres. The scientists can use their measurements of the transit light curve to determine the size of the planet passing in front of the star. These data, together with information about the masses of the planets already obtained using other observation techniques, will allow scientists to determine their density – one of the most important criteria for characterising an unknown planet. For the first time, it will be possible to understand these extrasolar worlds more precisely. A planet’s density provides important clues about its composition and structure – whether it primarily consists of rock, with a metal inner core, for instance, or whether the planet might even be home to vast oceans, or if it mainly gaseous.
In addition, CHEOPS will observe the planets both during transit and in orbit to the side of the star and illuminated by it, very similar to the situation when Earth's inner neighbouring planet Venus can be observed to the side of the Sun. The measured light curves will enable the scientists to reach conclusions about the existence of an atmosphere and, if possible, even find out whether the planet has clouds.
High sensitivity and stability with DLR technology
DLR is involved in the scientific analysis of the data, while its institutes of Optical Sensor Systems and Planetary Research in Berlin contributed the focal plane module for the data acquisition sensor and the sensor control module for the front-end electronics. One of the telescope’s outstanding features is its ability to maintain an extremely high pointing accuracy of ideally one arcsecond (1/3600 of a degree) over long periods of time. By way of comparison, when viewed from Earth, the Moon has a diameter of 1800 arc seconds or 30 arc minutes, and Venus, the brightest planet in the sky that can be seen with the naked eye, a diameter of a maximum of 60 arc seconds.
"The focal plane module ensures the high sensitivity and stability of the CHEOPS telescope," emphasises Gisbert Peter from the DLR Institute of Optical Sensor Systems. “The CHEOPS sensor is able to detect differences in brightness of 20 parts per million." The 1.2-metre-long telescope has a 30-centimetre aperture and a mass of almost 60 kilograms. Together with its platform, it weighs just 300 kilograms. For this reason, the space telescope does not require its own launcher for the journey into space, but rather will 'piggyback' with another payload being sent into orbit.
CHEOPS was developed as part of a partnership between ESA and Switzerland. A consortium of more than 100 scientists and engineers from eleven European countries, including DLR in Germany, is involved, led by the University of Bern and ESA. The mission is initially scheduled to last for three-and-a-half years. In addition to the measurements and observations to be carried out by the CHEOPS science team, 20 percent of the observation time is reserved for external scientists from all over the world.
Earth-like planets – extensive tour of discovery planned with PLATO
CHEOPS is the first of three European missions that will explore exoplanets. Together with CHEOPS, DLR is also extensively involved with the ESA 'Planetary Transits and Oscillations of stars' (PLATO) space telescope. PLATO is set to launch in 2026 and will represent a big step forward in the search for Earth-like planets. This much more elaborate space telescope, with its 26 individual optical systems and cameras, will be the first instrument to detect Earth-sized planets in the 'habitable' (that is, life-friendly) zone around Sun-like stars, where it is possible for liquid water to exist on the surface. PLATO is looking for a 'second Earth' and will also be able to pinpoint the age of planetary systems with greater precision than before. The scientific instrument is being developed by an international consortium led by Heike Rauer from the DLR Institute of Planetary Research. The entire mission includes not only discoveries from space using the transit method, but also follow-up measurements with other telescopes.
Finally, in 2028 ESA will launch the 'Atmospheric Remote-sensing Infrared Exoplanet Large-survey' (ARIEL) mission, another space telescope, which will focus on investigating the atmospheres of exoplanets. This will allow a comprehensive catalogue to be produced with precise details of the orbit, radius, mass, density and age of the planets, as well as information about the atmospheric composition of gas planets. In operation from 2007 to 2012 the French 'Convection, Rotation and planetary Transits' (CoRoT) space telescope discovered a total of 32 planets, with significant involvement from the DLR Institute of Planetary Research. As a result, DLR has many years of expertise in this dynamic field of research, which is set to permanently change our view of the cosmos.