A “rare exoplanet with no known equivalents” unexpectedly detected

IAC / Europa Press

Artist’s impression of the Nu2 Lupi planetary system

The Cheops exoplanet hunter satellite of the European Space Agency (ESA) has unexpectedly detected a “rare, no known equivalent” extrasolar planet. As reported this Monday by the Canary Islands Astrophysics Institute (IAC), which participates in Cheops among other European institutions, the detection occurred when two known exoplanets around the star Nu2 Lupi.

This transit, published this Monday in the magazine Nature Astronomy, will reveal “exciting” details about a rare planet “with no known equivalent”. The discovery is one of the first results of Cheops (Characterizing ExOPlanet Satellite) and it is the first time that an exoplanet with a period of more than one hundred days has been seen transiting a star that is bright enough as to be seen with the naked eye.

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 Accuracy Radial velocity Planet Searcher, Harps, from 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, the Transiting Exoplanet Reconnaissance Satellite (TESS) of the US space agency NASA 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.

Formation and evolution

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

“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 through planet d, which is further out in the system. “, Add.

Planetary transits create a valuable opportunity to study the atmosphere, orbit, size and composition of a planet, according to the note. 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.

A “real surprise”

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 reaching planet d is also slight compared to many other discovered exoplanets; in our Solar System, Nu2 Lupi d would orbit between Mercury and Venus ”, says Mahmoudreza Oshagh, IAC researcher and co-author of the work.

Oshagh adds along these lines that planet d is “enormously exciting” because it has no known equivalent, combining its bright parent star, long orbital period, and suitability for follow-up characterization. And, for all this, it will surely be a “primary objective” for future studies, he considers.

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.

“While none of these planets would be habitable, their diversity makes the system even more exciting and a great prospect for the future to test how these bodies form and change over time,” according to the IAC researcher and co-author of the work. , Enric Pallé.


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