Five planets revealed after 20 years of observation

A team of astronomers led by the UNIGE has discovered five new planets with periods of revolution between 15 and 40 years. It took 20 years of regular observations to achieve this result.


The first evidence of an exoplanet was noted in 1995. A giant planet was found in a four-day orbit around the nearby star 51 Pegasi. After that, more than 4000 exoplanets have been discovered, but the vast majority of them orbit their stars with relatively short periods of revolution.

Indeed, to confirm the existence of the planet, it is essential to wait for a few days, until it makes one or more revolutions around its star. This can take from a few days for the closest to the star to decades for the furthest away: Jupiter, for example, takes 11 years to go around the Sun. Such planets with long periods of revolution are of particular interest to astronomers because they are part of a poorly known but unavoidable population to explain the formation and evolution of planets.

Being farther away from their stars, they can be observed using direct imaging techniques. Indeed, to date, almost all planets have been discovered using the two main indirect methods: radial velocities, which measure the gravitational influence of a planet on its star, and transits, which detect the mini-eclipse caused by a planet passing in front of its star.

Only a telescope dedicated to the search for exoplanets can carry out such measurements over such long periods of time, which is the case of the EULER telescope of the Geneva University (UNIGE), Switzerland, located at the Silla Observatory in Chile.

The EULER telescope is mainly dedicated to the study of exoplanets. The telescope is a national, fully automatic 1.2-meter (3.9 ft) reflecting telescope, built and operated by the Geneva Observatory. Since its commissioning in 1998, it has been equipped with the CORALIE spectrograph which allows measuring radial velocities with an accuracy of a few meters per second, allowing for the detection of planets which mass is as small as Neptune’s.

Emily Rickman, first author of the study and researcher in the Astronomy Department of the UNIGE Faculty of Science said, “It took 20 years and many more observers. This result would have been impossible without the availability and reliability of the CORALIE spectrograph installed on the EULER telescope, a unique instrument in the world.”

“As early as 1998, a planetary monitoring programme was set up and carried out scrupulously by the many UNIGE observers who took a turn every two weeks in La Silla for 20 years. The result is remarkable: five new planets have been discovered and the orbits of four others known have been precisely defined. All these planets have periods of revolution between 15.6 and 40.4 years, with masses ranging approximately from 3 to 27 times that of Jupiter. This study contributes to increasing the list of 26 planets with a rotation period greater than 15 years, “but above all, it provides us with new targets for direct imaging!”

An article published by the journal Astronomy & Astrophysics.

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