image: This artist's impression shows the Nu2 Lupi planetary system, which was recently explored by ESA's exoplanet observer CHEOPS
The exoplanet satellite hunter CHEOPS of the European Space Agency (ESA), in which the Instituto de Astrofísica de Canarias (IAC) is participating along with other European institutions, has unexpectedly detected a third planet passing in front of its star while it was exploring two previously known planets around the same star. This transit, according to researchers, will reveal exciting details about a strange planet "without a known equivalent".
The discovery is one of the first results of CHEOPS (CHaracterising ExOPlanet Satellite) and the first time that an exoplanet has been seen with a period longer than 100 days transiting a star which is sufficiently bright to be seen with the naked eye. The discovery was published today in the journal Nature Astronomy.
This bright star similar to the sun, called Nu2 Lupi, is a little more than 50 light years from Earth, in the constellation of Lupus. In 2019, HARPS (High Accuracy Radial velocity Planet Searcher) of the European Southern Observatory (ESO) in Chile discovered three exoplanets in this system (called b, c, and d) with masses between those of the Earth and Neptune, and with orbital periods of 11.6, 27.6 and 107.6 days respectively. Afterwards NASA's TESS satellite, designed to detect transiting planets, found that the two interior planets, b and c, transit Nu2 Lupi, making it one of the only three naked eye stars which have more than one transiting planet.
"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.
Transits of planets give a valuable opportunity to study their atmospheres, their orbits, their sizes and their compositions. A transiting planet block out a tiny but detectable proportion of the light of its star when it passes in front of it, and it was this tiny drop in the light which led the researchers to their discovery. Because exoplanets with long periods orbit far away from their stars, the possibility of detecting a planet during transit is very small indeed, which makes the finding with CHEOPS a real surprise.
Using the high precision techniques of CHEOPS planet d was found to have some 2.5 times the radius of the Earth, and its orbital period around its star of a little over 107 days, was confirmed. In addition, using archive observations from terrestrial telescopes its mass could be estimated at 8.8 times that of the Earth.
"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. "Combined with its bright parent star, its long orbital period and its ideal situation for follow-up, this means that planet d is very exciting: it is an exceptional object, with no known equivalent, and it will certainly be a fundamental object for future studies".
The majority of long period transiting exoplanets discovered until now are orbiting stars which are too faint to allow detailed follow-up observations, which means that we know little about their properties. Nu2 Lupi is, however, sufficiently bright to be an attractive object for other powerful space telescopes such as the NASA/ESA Hubble Space Telescope, the future James Webb Space Telescope, as well as major observatories on the ground. "Given its general properties and its orbit, planet d will be an exceptionally favourable objective to study an exoplanet with a moderate atmospheric temperature around a star similar to the Sun", adds Laetitia Delrez.
Combining the new data from CHEOPS with archive data from other observatories, the researchers found that planet b is mainly rocky, while planets c and d appear to have large quantities of water surrounded by hydrogen and helium gas. In fact, planets c and d contain much more water than the Earth, a quarter of the mass of each of them is water, in comparison with less than 0.1% on Earth. But this water is not liquid, it is high pressure ice, or high temperature water vapour.
"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 designed to gather high precision data of individual stars known to harbour planets, rather than to make a more general survey of possible exoplanets around many stars. This approach and accuracy are proving exceptionally useful to understand the planetary systems around 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".