This autumn will see the launch of a ‘S-class’ mission into space. In this case, however, the ‘S’ does not stand for ‘supersize’ or ‘super performance’, but for ‘small’. CHEOPS – CHaracterising ExOPlanet Satellite – is the first small-scale mission by the European Space Agency (ESA). Its primary objective is to investigate ‘exoplanets’ discovered by other satellites, or from Earth-based telescopes using the radial velocity method.
This technique measures tiny changes in a star’s light spectrum caused by the motion of the star and one or more planets around their common centre of mass. This oscillation manifests itself in an el ongation (redshift) or compression (blueshift) of the starlight wavelengths in a phenomenon known as the Doppler effect, named after the Austrian physicist Christian Doppler (1803–1853).
Investigating promising candidates
The discovery of the fi rst exoplanet orbiting a sun-like star in 1995 spawned a revolution in astronomy. Located approximately 50 light years from Earth, 51 Pegasi b was considered an astronomical sensation. This discovery ushered in a whole new discipline within the oldest of all the sciences. Didier Queloz of the University of Geneva – one of the two scientists who discovered the planetary companion of the star Helvetios in the Pegasus constellation – calls it ‘exoplanetology’. The word is a combination of the terms ‘planetology’ – which until then had been limited to the Solar System – and ‘extrasolar planets’, or exoplanets for short. These are the new kids on the block.
Today, we know of more than 4000 exoplanets, which have been discovered by ground-based telescopes or by space telescopes such as Kepler, TESS and CoRoT. The Kepler mission has been particularly prolific in revealing planetary candidates, but many of the Kepler ‘candidates’ still remain to be checked and confirmed. As such, CHEOPS’ main task is to determine the size, orbital period and physical characteristics of these planets by measuring the light curves of bright stars during so-called transits – the path of the exoplanet in its orbit when it crosses the star’s disc – and their associated dimming.
The key lies in density
The main objective of the mission – which was selected in 2012 – is to investigate the structure of exoplanets larger than Earth but smaller than Neptune, or with a diameter of between 10,000 and 50,000 kilometres. The technique used – transit photometry – is highly precise. The prerequisite is a favourable observation geometry. The planet identified by the Doppler effect must pass in front of its star in the observation plane of the CHEOPS telescope. Only then can a light curve be recorded using the transit method.
The planet’s size can be determined by observing the dimming of the starlight during a transit. When combined with the mass – determined from the radial velocity measurements – it provides a measure of the planet’s density. This is one of the most important parameters for characterising the star and determining the nature of the planets in orbit around it. For example, it will be possible to distinguish Earth-like planets with solid rock surfaces from gas planets or ocean worlds. The telescope has a spectral range of 400 to 1100 nanometres.
CHEOPS will also observe planets while they orbit their star – and are thus illuminated by its light. The researchers hope that this will allow them to draw conclusions about the existence of an atmosphere, and perhaps even find out whether the exoplanet has clouds. Unlike earlier missions, CHEOPS is not a ‘discovery instrument’, but rather a follow-up mission that will focus on individual stars that are already known to host one or more planets. ESA’s much larger PLATO mission, which will be equipped with 26 individual telescopes and cameras, is tasked with finding new exoplanets, especially Earth-like planets, starting in 2026.