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A “herd” of rogue planets spotted in our galaxy!

​An international collaboration involving the CEA-Irfu has discovered the largest group of "rogue" planets to date, thanks in particular to a very sensitive "mega-camera" built by the CEA and deployed since 2003 on the Canada-France-Hawaii Telescope.

Published on 5 January 2022

A rogue planet is not gravitationally bound to a star, allowing it to move freely. It is also not illuminated like a classical planet by its star, making it much more difficult to image. However, for a few million years after its formation, it is just bright enough to be detectable by the most sensitive telescopes. Until now, only a very small number of such planets could be identified.

An international scientific team has just discovered no less than 70 new rogue planets in our galaxy, only a few hundred light-years away. With masses comparable to that of Jupiter, they are Located in a star-forming region (80 light-years in size) in the constellations of Upper Scorpius and Ophiuchus.

How was this challenge overcome? The researchers used data acquired over two decades by several terrestrial and space-based telescopes (including the European Space Agency's Gaia satellite). They measured the minute movements, color variations and luminosity of tens of millions of sources in a vast region of the sky. Thanks to this methodical and meticulous approach – which involved hundreds of hours of observation and dozens of terabytes of data – they were able to reliably identify very discrete cosmic objects, i.e. rogue planets.

In particular, they used data from the Canada-France-Hawaii Telescope (CFHT) equipped with MegaCam, a highly sensitive panoramic camera designed and built by the Irfu. "Its wide field of view, its sensitivity in the near infrared, and its high resolution were one of the keys to our success," explains Jean-Charles Cuillandre, a researcher at the Irfu. "This is an achievement that we could not have imagined back then, when it was designed to study the distant Universe!"

The study suggests that there could be several billion of these giant free-floating planets in the Milky Way, all without a host star.

The astronomers hope to find clues on the origin of these mysterious objects. Some believe that rogue planets can arise from the collapse of a gas cloud that is too small to form a star, while others imagine that they begin as an ejection of matter from the parent system. Whichever origin is more likely remains unknown.

The James Webb Space Telescope will offer new possibilities for observing dim planets in the infrared, which have the advantage of not being drowned in the brightness of a star. "These are excellent targets for the Webb's Miri instrument, which we contributed greatly to," explains Pierre-Olivier Lagage, a researcher at the Irfu. "We will be able to characterize their atmosphere in an unexplored wavelength domain, and possibly discover ever colder planets."

This work received support from the European Research Council (ERC) and the French "Investissements d'avenir" program.

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