The CHEOPS satellite unexpectedly spotted the third planet in the system crossing the star's face while studying two exoplanets within a nearby star system. The scientific team pointed out that this transit revealed exciting details about a rare, undiscovered planet.Every day, photobombs are a way for something or someone to enter a camera's field during the capture of a photograph. It could be a friend, a stranger, or a bird. It rarely, however, takes in a whole planet. This is what happened when CHEOPS (the Swiss-led space telescope) was photographing a planetary system located 50 light-years from Earth.A planet unlike any otherThe planetary system can be found in the constellation Lupus, Latin for Wolf. It is around the star Nu2 Lupi visible to the naked eye but not from Switzerland. Three exoplanets were discovered by Swiss astronomers around this Sun-like star in 2019. These three exoplanets are approximately 17 times larger than Earth and Neptune, and they take between 12, 28 and 107 days respectively to circle their parent star. Yann Alibert (Professor of Astrophysics at the University of Bern) is co-author of the Nature Astronomy study. "What makes these exoplanets so outstanding is that they can pass just in front of them star; it's said that they 'transit". We knew this already for the inner planets which is what led us to point CHEOPS at the system. Alibert says that the transit of the third planet was unlikely because it is so far from the star. It is less likely that a planet will transit if it is farther from its star.This was a major breakthrough as an exoplanet spotted transiting a star bright enough to be visible by the naked eye for the first time with a revolution duration of more than 100 days."Due its long duration, the amount of solar radiation reaching the planet has been relatively low in comparison to other exoplanets. A planet that receives less radiation will experience less changes over time. David Ehrenreich, professor of CHEOPS and mission scientist at the University of Geneva, co-signed the study. However, astronomers have only found a few exoplanets that orbit faint stars. This means that very little of their light reaches Earth, making them difficult to study. It is not so. "Since it bright host star is quite near to us, it's easier to study. Ehrenreich says this makes it an ideal target for future research with no equivalent."Additional insights are available from other telescopesThe third planet, nu2 Lupi d (high-precision measurements by CHEOPS), is approximately 2.5 times larger than Earth and nearly 9 times as large. Laetitia Derez, visiting researcher at University of Geneva, and the lead author of the study, was capable of accurately describing the composition and density of the planet and its neighbours by combining these measurements with archive data from other observatories as well as numerical models created by the University of Bern. Delrez says that the innermost planet is mostly rocky and the outer two are covered in hydrogen and helium gas envelopes, which contain large amounts of water. There is far more water than the Earth. A quarter of all planets' mass is actually made up of water. Earth, however, has less than 0.1%. However, this water is not liquid. Instead, it takes the form of high pressure ice or high temperature steam. This makes the planets inhospitable. These insights are only the beginning."Now that we have determined that all three planets transit, and have accurately measured their properties, it is time to study them using bigger and more powerful instruments that CHEOPS like the Hubble Space Telescope and its successor, James Webb Space Telescope. Ehrenreich says that they could provide additional information, including the composition of the atmosphere. Planet d, with its orbit and overall properties, is likely to be the most famous exoplanet with a mild-temperature atmosphere surrounding a star like the Sun.
