Unique exoplanet photobombs CHEOPS study of nearby star system

Artist’s impression of the Nu2 Lupi planetary system. Credit: © ESA

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Photobombs, when something or someone unexpectedly enters a camera’s field of sight during the taking of a photograph happen every day. Sometimes it is a friend, other times a stranger or perhaps a bird. Rarely, however, is it a whole planet. Yet, this is exactly what happened while CHEOPS, the Swiss-led space telescope, was taking pictures of a planetary system 50 light-years away.

The planetary system is located in the constellation Lupus (Latin for Wolf), around a star called Nu2 Lupi, visible to the naked eye (but not from Switzerland). In 2019, Swiss astronomers announced the detection of three exoplanets around this bright, Sun-like star. The three exoplanets have masses between those of Earth and Neptune (17 times the Earth) and take 12, 28, and 107 days to circle their parent star.

Alibert, professor of astrophysics at the University of Bern and co-author of the study which has been published in Nature Astronomy said, what makes these exoplanets really outstanding is that we can see them passing just in front of their star; they’re said to ‘transit. We knew that already for the two inner planets, which led us to point CHEOPS to the system in the first place. However, the third planet is quite far away from the star, no one was expected to see its transit!. In fact, the farther away the planet is from its star, the less likely it is to transit.

This proved a game-changer, as it is the first time an exoplanet with a revolution period of over 100 days which corresponds to a distance from the star somewhere between that of Mercury and Venus from the Sun has been spotted transiting a star that is bright enough to be visible to the naked eye.

David Ehrenreich, professor at the University of Geneva and mission scientist of CHEOPS, who co-signed the study said, due to its relatively long period, the amount of stellar radiation reaching the planet is mild in comparison to many other discovered exoplanets. The less radiation a planet receives, the less it changes over time. Therefore, a planet with a long period could have retained more information about its origin.

But so far, the few such exoplanets astronomers had found orbited faint stars. In other words: little of their light reaches Earth and therefore makes them difficult to study. Not this time: Since its bright host star is quite close to us, it is easier to study. This makes it a golden target for future study with no known equivalent.

The high-precision measurements of CHEOPS reveal the third planet, called nu2 Lupi d, to be about 2.5 times the size of Earth and almost 9 times its mass. By combining these measurements with archival data from other observatories and numerical models developed by the University of Bern, Laetitia Delrez, a visiting researcher at the University of Geneva and lead author of the study, was able to accurately characterize the density and composition of the planet and its neighbors.

The innermost planet is mainly rocky, while the outer two appear to be enshrouded in envelopes of hydrogen and helium gases beneath which they hold large amounts of water. Far more water, in fact, than the Earth, has a quarter of each planet’s mass is made up of water, compared to less than 0.1% for Earth. This water, however, is not liquid, instead of taking the form of high-pressure ice or high-temperature steam, making the planets uninhabitable. But these insights could only be the beginning.

Now that we discovered that all three planets transit and have precisely measured their properties, the next step is to study them with bigger and more powerful instruments than CHEOPS, like the Hubble Space Telescope or its successor, the James Webb Space Telescope. They could reveal further details, such as the composition of the atmosphere. Given its overall properties and orbit, planet d is going to become the poster-child of exoplanets with a mild-temperature atmosphere around a star similar to the Sun.

Journal Information: “Transit detection of the long-period volatile-rich super-Earth ν2 Lupi d with CHEOPS” by Laetitia Delrez, David Ehrenreich, Yann Alibert, Andrea Bonfanti, Luca Borsato, Luca Fossati, Matthew J. Hooton, Sergio Hoyer, Francisco J. Pozuelos, Sébastien Salmon, Sophia Sulis, Thomas G. Wilson, Vardan Adibekyan, Vincent Bourrier, Alexis Brandeker, Sébastien Charnoz, Adrien Deline, Pascal Guterman, Jonas Haldemann, Nathan Hara, Mahmoudreza Oshagh, Sergio G. Sousa, Valérie Van Grootel, Roi Alonso, Guillem Anglada-Escudé, Tamás Bárczy, David Barrado, Susana C. C. Barros, Wolfgang Baumjohann, Mathias Beck, Anja Bekkelien, Willy Benz, Nicolas Billot, Xavier Bonfils, Christopher Broeg, Juan Cabrera, Andrew Collier Cameron, Melvyn B. Davies, Magali Deleuil, Jean-Baptiste Delisle, Olivier D. S. Demangeon, Brice-Olivier Demory, Anders Erikson, Andrea Fortier, Malcolm Fridlund, David Futyan, Davide Gandolfi, Antonio Garcia Muñoz, Michaël Gillon, Manuel Guedel, Kevin Heng, László Kiss, Jacques Laskar, Alain Lecavelier des Etangs, Monika Lendl, Christophe Lovis, Pierre F. L. Maxted, Valerio Nascimbeni, Göran Olofsson, Hugh P. Osborn, Isabella Pagano, Enric Pallé, Giampaolo Piotto, Don Pollacco, Didier Queloz, Heike Rauer, Roberto Ragazzoni, Ignasi Ribas, Nuno C. Santos, Gaetano Scandariato, Damien Ségransan, Attila E. Simon, Alexis M. S. Smith, Manfred Steller, Gyula M. Szabó, Nicolas Thomas, Stéphane Udry and Nicholas A. Walton, 28 June 2021, Nature Astronomy.
DOI: 10.1038/s41550-021-01381-5