Exoplanet photobombs observations of its star system

29 June 2021

CHEOPS studies Nu2 Lupi Exoplanets

  • While ESA’s Cheops mission was studying two exoplanets in a nearby system, a third one “photobombed” the observations

  • The mission that includes ICE researchers detected the surprising and improbable transit of a third exoplanet orbiting far away from the star

  • As the star is visible to the naked eye and the system is close to Earth, the planet will be an invaluable target for future studies 

Researchers involved in the European Space Agency’s exoplanet-hunting Cheops mission spotted a surprise transit in front of a nearby star. While exploring two exoplanets of the system, the satellite has spotted a new planet, the system’s third one known until date, crossing the face of the star. 
This is the first time an exoplanet with a period of over 100 days has been found transiting a star that is bright enough to be visible to the naked eye. As long-period exoplanets orbit so far from their stars, the chances of seeing one during a transit are incredibly low, making this finding, one of the firsts from ESA’s Cheops (CHaracterising ExOPlanet Satellite), a real surprise.
Named Nu2 Lupi, this bright, Sun-like star is located just under 50 light-years away from Earth in the constellation of Lupus (the Wolf). The system hosts three known exoplanets and the two innermost planets (designated planet b and planet c) had been previously observed transiting the star. As such, the star was one of only three naked-eye stars known to host multiple transiting planets.
"Transiting systems such as Nu2 Lupi are of paramount importance in our understanding of how planets form and evolve, as we can compare several planets around the same bright star in detail," says Dr. Laetitia Delrez, researcher at the University of Liège (Belgium), and lead author of the new finding. "We set out to build on previous studies of Nu2 Lupi and observe planets b and c crossing the face of the star with Cheops, but during a transit of planet c we spotted something amazing: an unexpected transit by planet d, which lies further out in the system."
Planetary transits create a valuable opportunity to study a planet’s atmosphere, orbit, size and interior. A transiting planet blocks a tiny but detectable proportion of its star’s light as it crosses in front of it – and it was this drop in light that led Dr. Delrez and colleagues to their discovery. 
Using the high-precision capabilities of Cheops, planet d was found to be about 2.5 times the radius of Earth, confirmed to take just over 107 days to loop once around its star and, using archival observations from ground-based telescopes, found to have a mass 8.8 times that of Earth. 
Most long-period transiting exoplanets discovered to date have been found around stars that are too faint to allow detailed follow-up observations, meaning that little is known about their planets’ properties. Nu2 Lupi, however, is bright enough to be an attractive target for other powerful telescopes based in space — such as the NASA/ESA Hubble Space Telescope or the forthcoming NASA/ESA/CSA James Webb Space Telescope — or large observatories on the ground. 
Cheops, the ESA mission in which the Institute of Space Sciences participates as part of the Science Team and the Mission Board, is designed to collect ultra-high precision data of individual stars known to host planets, rather than sweeping more generally for possible exoplanets around many stars – and this focus and precision are proving exceptionally useful in understanding the star systems around us. 
“While still at the beginning of its mission of measuring exoplanets and identifying interesting targets for further studies, Cheops already lives up to the expectations and is delivering impressive results”, said Prof. Ignasi Ribas, member of the Cheops mission team, researcher at the Institute of Space Sciences (ICE-CSIC) and director of the Institut d’Estudis Espacials de Catalunya (IEEC).

Observatories and Instruments

Cheops is an ESA mission developed in partnership with Switzerland, with a dedicated consortium led by the University of Bern, and with important contributions from Austria, Belgium, France, Germany, Hungary, Italy, Portugal, Spain, Sweden and the UK. ESA is the Cheops mission architect, responsible for procurement and testing of the satellite, the launch and early operations phase, and in-orbit commissioning, as well as the Guest Observers’ Programme through which scientists world-wide can apply to observe with Cheops. The consortium of 11 ESA Member States led by Switzerland provided essential elements of the mission. The prime contractor for the design and construction of the spacecraft is Airbus Defence and Space in Madrid, Spain.
The Cheops mission consortium runs the Mission Operations Centre located at the Instituto Nacional de Técnica Aeroespacial (INTA), in Torrejón de Ardoz (Madrid, Spain), and the Science Operations Centre, located at the University of Geneva (Switzerland). 

More information

This research is presented in the paper "Transit detection of the long-period volatile-rich super-Earth Nu2 Lupi d with CHEOPS" by Delrez, L. et al. (2021), published in Nature Astronomy.
IEEC Communication Office / ICE Communication & Outreach Office


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