An exoplanet 'photobombs' CHEOPS satellite observations
The third planet in the Nu2 Lupi system has unexpectedly entered the field of view of ESA's CHEOPS satellite. With a long orbital period of more than 100 days, it was highly unlikely to be observed passing in front of its star (transit). The scientific team that made this important discovery, led by Laetitia Delrez, an astrophysicist at ULiège, has just published the results in the scientific journal Nature Astronomy. The planet, called Nu2 Lupi d, has rare characteristics with no known equivalent.
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bit like a passer-by inadvertently walks into the field of view of a photographer wishing to take a picture of a landscape, a planet unexpectedly appeared while ESA's CHaracterising ExOPlanet Satellite (CHEOPS) was taking images of the two inner planets orbiting the star Nu2 Lupi. Discovered in 2019 by HARPS (High Accuracy Radial velocity Planet Searcher) - an instrument on one of ESO's telescopes - this planetary system is located in the Wolf constellation (Lupus), about 50 light years from our solar system. It consists of a bright star similar to our Sun, around which three planets (b, c and d) are in orbit, with masses between those of Earth and Neptune and orbits lasting 11.6, 27.6 and 107.6 days. NASA's Transiting Exoplanet Survey Satellite (TESS) later discovered that the two innermost planets - b and c - transit Nu2 Lupi, making it one of only three stars visible to the naked eye known to host multiple transiting planets.
"Systems with several planets transiting a bright star, such as Nu2 Lupi, are of paramount importance for our understanding of the formation and evolution of planetary systems, as we can compare in detail the properties of the planets with each other," explains Laetitia Delrez, FNRS research fellow at the Astrobiology and STAR research units of the University of Liège, and lead author of this new discovery. "While studying the transits of planets b and c in front of their star, we serendipitously spotted the transit of the third planet in the system, Nu2 Lupi d, which is much further away from the star and was therefore not expected to transit! »
Planetary transits offer a valuable opportunity to study a planet's atmosphere, orbit, size and interior. A transiting planet blocks a small but detectable portion of its star's light as it passes in front of it - and it is this dimming that led Laetitia Delrez and her colleagues to their discovery. As long-period exoplanets orbit further from their star, the chances of them transiting are incredibly low, making the CHEOPS discovery a real surprise.
Using high-precision measurements from the CHEOPS satellite, the team of European researchers found that the third planet has a radius of about 2.5 times that of Earth, takes just over 107 days to orbit its star and, using archival observations from ground-based telescopes, has a mass of 8.8 times that of the Earth.
"The amount of stellar radiation reaching planet d is also mild compared to many other known exoplanets. In our solar system, Nu2 Lupi d would orbit between Mercury and Venus," adds Francisco J. Pozuelos, also a researcher at the University of Liège in the STAR and Astrobiology research units and co-author of the study. "The less radiation a planet receives, the less it changes over time. Therefore, Nu2 Lupi d could have preserved interesting information about its origin. »
Most of the long-period transiting exoplanets discovered so far have been found around stars that are too faint for detailed follow-up observations, which means that little is known about the properties of these planets. Nu2 Lupi is, however, close and bright enough to be an interesting target for atmospheric studies with other powerful telescopes - such as the Hubble Space Telescope or its successor, the James Webb Space Telescope (due to be launched later this year), or large ground-based observatories.
"Given its overall properties and its orbit, planet d is a particularly favourable target for studying an exoplanet with a mild-temperature atmosphere around a star similar to the Sun," says Laetitia Delrez. "It is an exceptional object with no known equivalent, and certainly a golden target for future studies. »
By combining the new CHEOPS data with archival data from other observatories, the researchers were able to accurately determine the mean densities of the three known planets in Nu2 Lupi, and constrain their possible compositions. The researchers found that planet b is predominantly rocky, while planets c and d appear to contain large amounts of water enshrouded in envelopes of hydrogen and helium gases. Planets c and d are thought to contain much more water than Earth. A quarter of the mass of each planet would be made up of water, compared to less than 0.1% for Earth. However, this water would not be liquid, but in the form of ice at high pressure or steam at high temperature.
"Although none of these planets is habitable, their diversity makes the system very interesting and offers an excellent future prospect for testing how these bodies form and evolve over time," explains Sébastien Salmon, researcher at the STAR research unit of the University of Liege and co-author of the study. "There is also the possibility to search for rings or moons in the Nu2 Lupi system, as the exquisite precision and stability of CHEOPS could allow the detection of bodies as small as Mars. »
This discovery is one of the first results of the CHEOPS satellite, an ESA space mission in which the University of Liège is involved, and it is the first time that an exoplanet with a period of more than 100 days has been found transiting a star bright enough to be visible to the naked eye. These exciting results demonstrate once again the enormous potential of CHEOPS* to not only better understand known exoplanets, but also to discover new ones and reveal their secrets.
*CHEOPS is an ESA mission in partnership with Switzerland, with major contributions from Austria, Belgium, France, Germany, Hungary, Italy, Portugal, Spain, Sweden, and the United Kingdom.
Scientific reference
DELREZ L. & al, Transit detection of the long-period volatile-rich super-Earth Nu2 Lupi d with CHEOPS, Nature Astronomy, June 2021. http://doi.org/10.1038/s41550-021-01381-5
The CHEOPS data were obtained during six rounds of observations between 4 April and 6 July 2020, targeting four transits of planet b and three of planet c.
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