CHEOPS characterises three planets with some extraordinary properties
June 28, 2021 | A super-Earth in the constellation Lupus
CHEOPS characterises three planets with some extraordinary properties
Artist's impression of the 'ν² Lupi' planetary system
This artist's impression shows the 'ν² Lupi' planetary system recently studied by CHEOPS, ESA's exoplanet space telescope. This bright, Sun-like star is located just under 50 light-years from Earth in the constellation Lupus (Wolf) and is orbited by three planets named 'b', 'c' and 'd' (the star is considered to be object 'a'). The three planets have masses between those of Earth and Neptune and orbital periods of 11.6, 27.6 and just over 107 days respectively.
Comparison of the sizes, orbits, orbital periods and compositions of the three exoplanets orbiting the star Nu2 Lupi in the constellation Lupi (Wolf)
Illustration of the possible arrangement of the three planets orbiting the Sun-like star Nu2 Lupi. The three planets have an iron core and a rocky mantle, but planet c and d have a high water content and a gas shell of hydrogen-helium mixture. Planet d moves in an orbit that would lie between Mercury and Venus in the Solar System and therefore only receives moderate radiation from its parent star.
The CHEOPS space telescope has observed a known planetary system in the constellation 'Lupus' (Wolf) and has discovered a 'super-Earth'.
The radius of the 'ν² Lupi d' super-Earth is two and a half times larger than that of Earth. Its mass is approximately 8.8 times that of Earth.
The results were published in the journal Nature Astronomy on 28 June 2021. Planetary researchers from DLR were involved in the work.
Focus: Space, planetary research, exoplanets
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 'ν² Lupi d' 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 d' is an example of how different observation methods and instruments complement each other, thus providing important new insights.
The ESA CHEOPS mission
CHEOPS is a European Space Agency (ESA) mission that was developed in close cooperation with Switzerland. The scientific consortium is led by the University of Bern, with significant contributions from Austria, Belgium, France, Germany, Hungary, Italy, Portugal, Spain, Sweden, and the United Kingdom. The German Aerospace Center (DLR) is involved in the scientific evaluation of the CHEOPS data. DLR's Berlin-based Institutes of Optical Sensor Systems and Planetary Research contributed two electronic modules, including the heart of the satellite telescope – the focal plane module containing the CCD detector – the thermo-mechanical stability of which enables high-precision measurements. DLR also developed algorithms for the scientific evaluation of the measurement data.
