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The European Space Agency (ESA) space telescope Characterising ExOPlanet Satellite (CHEOPS) has been observing a previously known planetary system. With the high-precision measurements performed by CHEOPS, its three planets have been described in detail for the first time. The outermost of the three planets has particularly extraordinary properties; it is what is referred to as a 'super-Earth', with an orbital period of 108 days and a high proportion of volatile substances. It will be the focus of further measurements in the future. The CHEOPS measurements were evaluated in 2020, with input from four scientists based at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) Institute of Planetary Research. The findings were published on 28 June 2021 in the journal Nature Astronomy.

Since 2019, scientists have known that the star 'ν² Lupi' – a bright, G-type main-sequence star with a mass approximately the same as that of the Sun – has three planets. This star in the Lupus (Wolf) constellation is only 48 light years away from Earth and is just visible to the naked eye. It was observed with the High Accuracy Radial velocity Planet Searcher (HARPS) instrument on the 3.6-metre telescope operated by the European Southern Observatory (ESO) in La Silla, Chile. In the acquired data, which was obtained using the radial velocity method, three planets were discovered and their orbital periods were determined: 12 days, 28 days and 108 days. After that, the NASA Transiting Exoplanet Survey Satellite (TESS) started looking specifically at this planetary system and was able to identify the two inner planets, 'ν² Lupi b' and 'ν² Lupi c', as transit planets. This means that when they are observed from Earth, they pass in front of the star's disc at regular intervals, dimming the star's brightness in the process. However, TESS only measures in observation cycles of 28 days, which did not allow the third planet to be found in the data. Therefore, it was impossible to determine whether this planet, which is located furthest from the host star, would pass in front of the star for an Earth-based observer – that is, whether the orbit of this outer, third planet was inclined in such a way that a transit event could even take place.

Six observation sequences by CHEOPS, referred to as 'visits', were planned between 4 April and 6 July 2020, during which four transits of planet 'b' and three of planet 'c' were expected. In a very fortunate set of circumstances, during a transit event of planet 'c' there was an additional drop in the star's brightness, which was thought likely to have been caused by a transit of the third planet. The investigation was led by Laetitia Delrez (University of Liège and Geneva Observatory).

Like a 'giant' between Mercury and Venus

This third exoplanet has unique properties. With its long orbital period of 108 days, if it were in the Solar System, its orbit would lie between those of Mercury and Venus. In contrast to many other exoplanets that, like the other two Lupi planets, orbit their star much more closely, only moderate amounts of radiation reach it. Its radius is 2.5 times that of Earth, and its mass about 8.8 times that of Earth. These properties put it into the super-Earth category. As it is orbiting the bright star 'ν² Lupi', it is an ideal object for follow-up observations.

Transit planets are an incredibly valuable category of planets for exoplanet research because they offer the opportunity to study the planets' atmosphere, determine their orbit and measure their size with precision. This also gives an insight into the interior, as these measurements narrow down the parameters for the composition of the planet.

Taking a close look has paid off for exoplanetary researchers studying observations from the Earth-orbiting CHEOPS space telescope. These scientists do not just want to find the planetary companions of stars and add to the list of exoplanets – although this itself is far from an easy task. Exoplanetary researchers are aiming at more than just a new discovery and an entry in the exoplanet catalogue, which now comprises almost 4800 exoplanets. They want to find out the composition of the planets and the underlying rules according to which planetary systems are formed.

The precise characterisation of a planet cannot be accomplished by a single team or a single observation. This analysis of a super-Earth orbiting 'ν² Lupi' is an example of how different observation methods and instruments complement each other, thus providing important new insights.