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Nu2 Lupi d, an unusual planet

While observing two known planets orbiting a star, the CHEOPS astronomical satellite of the European Space Agency (ESA) has unexpectedly detected a third planet crossing in front of the star. This transit, according to scientists who have analyzed the sighting data, will reveal exciting details about the planet, which is entirely unusual for lacking a known equivalent.

The discovery is one of the first results of CHEOPS (Characterizing ExOPlanet Satellite), an exoplanet hunter satellite (planets from outside our solar system) and the first time that an exoplanet with a period of more than 100 days transiting a star that is bright enough to be visible with the naked eye. The finding has been published in the academic journal Nature Astronomy, under the title “Transit detection of the long-period volatile-rich super-Earth Nu2 Lupi d with CHEOPS”.

This bright sun-like star, called Nu2 Lupi, is located just under 50 light-years from Earth in the constellation Lupus (the Wolf). In 2019, the high-precision radial velocity planet searcher HARPS (High Accuracy Radial velocity Planet Searcher) of the European Southern Observatory (ESO) in Chile discovered three exoplanets in the system (called ‘b’, ‘c’ and ‘d’ ) with masses similar to those of Earth and Neptune and orbits of 11.6, 27.6 and 107.6 days. Subsequently, NASA’s Transiting Exoplanet Reconnaissance Satellite (TESS) discovered that the two innermost planets, b and c, were transiting Nu2 Lupi, making it one of only three naked-eye stars that host multiple transiting planets.

“Transiting systems like Nu2 Lupi are of utmost importance in our understanding of how planets form and evolve, as we can compare several planets around the same bright star in detail,” explains Laetitia Delrez, researcher at the University of Liège. (Belgium) and main author of the new finding.

“We set out to build on previous studies of Nu2 Lupi and observe planets b and c crossing in front of Nu2 Lupi with CHEOPS, but during a transit of planet c we saw something amazing: an unexpected transit by planet ‘d’, which is further out in the system, “he adds.

An artist’s recreation of the Nu2 Lupi planetary system, which was recently observed by ESA’s CHEOPS astronomical satellite. (Image: ESA)

Planetary transits create a valuable opportunity to study the atmosphere, orbit, size, and composition of a planet. A transiting planet blocks a small but detectable proportion of its star’s light as it crosses in front of it, and it was that drop of light that led the researchers to their discovery. Since long-period exoplanets orbit so far from their stars, the chances of seeing one during a transit are incredibly low, making the CHEOPS find a real surprise.

Using CHEOPS ‘high-precision capabilities, planet d was found to be about 2.5 times the radius of Earth and it was confirmed that it takes just over 107 days to circle its star. Furthermore, using archival observations from ground-based telescopes, it was found to have a mass 8.8 times that of Earth.

“The amount of stellar radiation that reaches planet d is also slight compared to many other discovered exoplanets; in our Solar System, Nu2 Lupi d would orbit between Mercury and Venus”, indicates Mahmoudreza Oshagh, senior postdoctoral researcher at the Instituto de Astrofísica de Canarias (IAC) in Spain and co-author of the work. “Combined with its bright parent star, long orbital period, and suitability for follow-up characterization, this allows planet d to be enormously exciting: it is a rare object with no known equivalent, and it will surely be a prime target for future studies.”

Most of the long-transiting exoplanets discovered to date have been found around stars that are too faint to allow detailed follow-up observations, meaning little is known about the properties of their planets. Nu2 Lupi, however, is bright enough to be an attractive target for other powerful telescopes in space, such as ESA’s Hubble Space Telescope, the upcoming James Webb Space Telescope, or large ground-based observatories. “Its general properties and orbit make planet d an exceptionally favorable target for studying an exoplanet with a relatively mild atmosphere around a sun-like star,” adds Laetitia Delrez.

By combining new CHEOPS data with archival data from other observatories, the researchers discovered that planet b is primarily rocky, while planets c and d appear to contain large amounts of water encased in hydrogen and helium gas envelopes. In fact, planets c and d contain much more water than Earth: 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, but takes the form of high pressure ice or high temperature steam.

“While none of these planets would be habitable, their diversity makes the system even more exciting and a great prospect for the future to determine how these bodies form and change over time,” says Enric Pallé, IAC researcher and co-author of the job. “There is also the possibility of looking for rings or moons in the Nu2 Lupi system, since the exquisite precision and stability of CHEOPS could allow the detection of bodies roughly the size of Mars,” he says.

CHEOPS is designed to collect high-precision data from individual stars known to host planets, rather than searching more generally for possible exoplanets around many stars. This approach and its precision are proving to be exceptionally helpful in better understanding the star systems around us.

“These exciting results demonstrate once again the enormous potential of this satellite – adds Enric Pallé -; CHEOPS will allow us not only to better understand known exoplanets, as shown in this and other initial results of the mission, but also to discover exoplanets new ones and reveal their secrets. ” (Source: IAC)

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