While studying two exoplanets in a bright, nearby star system, the CHEOPS satellite identified the third known planet in the system, which unexpectedly crossed the star’s surface. This transit reveals exciting details about an “unprecedented planet”, as emphasized by the scientific team headed by the Universities of Geneva and Bern and members of the National Research Center PlanetS.
So-called photo bombs – when an object or person unexpectedly falls into the camera’s field of view while taking a photo – happen every day. Sometimes it’s an acquaintance, other times a stranger, or maybe a bird. However, it is seldom an entire planet. But that is exactly what happened when the CHEOPS space telescope, managed by Switzerland, recorded images of a planetary system 50 light years away.
A planet like no other
The planetary system is located in the constellation Lupus (Latin for wolf), around a star called Nu2 Lupi, which is visible to the naked eye (but not from Switzerland). In 2019, Swiss astronomers announced the discovery of three exoplanets around this bright, sun-like star. The three exoplanets have masses between those of Earth and Neptune (17 times that of Earth) and take 12, 28 and 107 days to orbit their parent star. “What makes these exoplanets really stand out is that we can see them pass by right in front of their star; one speaks of a ‘transit’ ”, says Yann Alibert, professor of astrophysics at the University of Bern and co-author of the study, which has just been published in the journal Nature Astronomy was published. “We already knew that about the two inner planets, which prompted us to focus CHEOPS on the system in the first place. The third planet, however, is quite a long way from the star, and nobody expected its transit, ”adds Alibert. The further away the planet is from its star, the less likely it will be a transit.
It is the first time that an exoplanet with an orbital period of over 100 days – which is a distance from the star somewhere between that of Mercury and Venus from the Sun – has been discovered that crosses a star that is bright enough to to be visible to the naked eye.
“Due to its relatively long orbital period, the amount of stellar radiation reaching the planet is mild compared to many other exoplanets discovered. The less radiation a planet receives, the less it changes over time. Therefore, a planet with a long period could have retained more information about its formation, ”says David Ehrenreich, professor at the University of Geneva and missionary scientist from CHEOPS, who was involved in the study. But the few such exoplanets that astronomers had found so far orbited weakly shining stars. In other words, little of its light reaches the earth, making it difficult to study. Not so this time: «Since its bright host star is very close to us, it is easier to examine.
Further findings from other telescopes
The high-precision measurements from CHEOPS show that the third planet, now called Lupi d, is around 2.5 times as large as the earth and almost 9 times as heavy. By combining these measurements with archive data from other observatories and numerical models developed by the University of Bern, Laetitia Delrez, visiting researcher at the University of Geneva and lead author of the study, was able to precisely characterize the density and composition of the planet and its neighbors. “The innermost planet is mainly rocky, while the two outer ones appear to be enveloped in envelopes of hydrogen and helium gases, among which they contain large amounts of water,” explains Delrez. This is far more water than the earth has: a quarter of the mass of any planet is water, compared to less than 0.1% in the case of the earth. However, this water is not liquid, but is in the form of high pressure ice or high temperature steam, which makes the planets uninhabitable. But these insights could only be the beginning.
“Now that we have discovered that all three planets are transiting and have precisely measured their properties, the next step is to study them with larger and more powerful instruments than CHEOPS. Like the Hubble Space Telescope or its successor, the James Webb Space Telescope. They could reveal further details, such as the composition of the atmosphere, ”says Ehrenreich. Given its overall properties and orbit, planet d will become the poster child for exoplanets with an atmosphere of mild temperature around a sun-like star.
Featured image: Artist’s impression of the Nu2 Lupi planetary system © ESA
L. Delrez et al .: Transit detection of the long-period volatile-rich super-Earth ν 2 Lupi d with CHEOPS, Nature Astronomy. https://doi.org/10.1038/s41550-021-01381-5
Provided by University of Bern