While exploring two previously known planets around the same star, the European Space Agency’s (ESA) exoplanet-hunting satellite CHEOPS (Characterising ExOPlanet Satellite), in which the Instituto de Astrofísica de Canarias (IAC) and other European institutions are participating, unexpectedly detected a third planet passing in front of its star. According to scientists, this transit will provide fascinating facts about a weird planet “without a known equivalent.”
The finding is one of the first CHEOPS (Characterising ExOPlanet Satellite) finds, and it is the first time an exoplanet with a period longer than 100 days has been spotted transiting a star bright enough to be seen with the naked eye. The findings were published in the journal Nature Astronomy today.
Nu2 Lupi, a brilliant star comparable to the sun, is located in the constellation Lupus, around 50 light-years from Earth. In 2019, the European Southern Observatory (ESO) in Chile’s HARPS (High Accuracy Radial Velocity Planet Searcher) identified three exoplanets in this system (dubbed b, c, and d) with masses between Earth and Neptune with orbital periods of 11.6, 27.6, and 107.6 days, respectively.
Transiting systems such as Nu2 Lupi are of great importance in our understanding of how planets form and evolve, because we can compare several planets around the same bright star in detail.
Laetitia Delrez
Following that, NASA’s TESS satellite, which is designed to identify transiting planets, discovered that the two inner planets, b and c, transit Nu2 Lupi, making it one of only three naked-eye stars with multiple transiting planets.
“Transiting systems such as Nu2 Lupi are of great importance in our understanding of how planets form and evolve, because we can compare several planets around the same bright star in detail,” explains Laetitia Delrez, a researcher at the University of Liege (Belgium) and first author of the article.
“Our idea was to follow up previous studies of Nu2 Lupi and to observe planets b and c passing in front of Nu2 Lupi with CHEOPS, but during a transit of planet c we were amazed to see an unexpected transit of planet d, which is further out within the system,” she adds.
Planetary transits provide a unique chance to investigate their atmospheres, orbits, sizes, and compositions. When a transiting planet passes in front of a star, it blocks off a little but detectable fraction of its light, and it was this tiny dip in brightness that led the researchers to their findings.
Because exoplanets with lengthy periods orbit distant from their stars, the chances of detecting one during transit are extremely slim, making the discovery with CHEOPS all the more surprising.
Planet d was discovered to have a radius of 2.5 times that of the Earth, and its orbital period around its star of a little over 107 days was validated using the high accuracy techniques of CHEOPS. Its mass might also be approximated at 8.8 times that of the Earth using archival images from terrestrial observatories.
“The amount of radiation from the star which falls onto planet d is quite small compared to many other known exoplanets. If it were in our own solar system Nu2 Lupi d would orbit between Mercury and Venus,” says Mahmoudreza Oshagh, a senior postdoctoral researcher at the IAC, and a co-author of the paper. “Planet d is particularly interesting because of its brilliant parent star, lengthy orbital period, and great location for follow-up: it is an extraordinary object with no known parallel, and it will undoubtedly be a fundamental item for future investigations.”
The bulk of long-period transiting exoplanets identified so far orbit stars that are too dim to allow extensive follow-up investigations, thus we know very little about their features. Nu2 Lupi, on the other hand, is bright enough to attract other strong space telescopes, like the NASA/ESA Hubble Space Telescope, the future James Webb Space Telescope, and significant ground-based observatories.
“Given its general properties and its orbit, planet d will be an exceptionally favorable objective to study an exoplanet with a moderate atmospheric temperature around a star similar to the Sun,” adds Laetitia Delrez.
The researchers discovered that planet b is mostly stony, whereas planets c and d appear to contain substantial amounts of water surrounded by hydrogen and helium gas, based on fresh data from CHEOPS and archival data from other telescopes.
In reality, planets c and d have significantly more water than the Earth, with water accounting for a quarter of each planet’s mass, compared to less than 0.1 percent on Earth. This water, however, is not liquid; it is high-pressure ice or water vapor at a high temperature.
“Although none of these planets would be habitable, their diversity makes the system very exciting and a great future perspective to show how these bodies formed and how they have changed with time,” explains Enric Pallé, an IAC researcher and a co-author of the article. “We can also look for rings or moons within the Nu2 Lupi system because the extreme accuracy and stability of CHEOPS could allow us to detect bodies close to the size of Mars.”
CHEOPS is intended to collect high-precision data on specific stars known to have planets, rather than a broader survey of potential exoplanets orbiting multiple stars. This method and precision are proving to be quite valuable in gaining a better understanding of the planetary systems that orbit the stars around us.
“These exciting results show, yet again, the major potential of this satellite,” says Enric Pallé. “CHEOPS will not only give us a better understanding of known exoplanets, but as shown by this result and others in the initial phase of the mission, it will enable us to discover new ones, and to reveal their secrets.”
