When, in Antiquity, the Greeks wanted to designate these stars which, unlike the stars – always motionless – changed place from week to week or from month to month, they named them “planets”. That is to say “the wanderers”, in Greek. We now know that these objects do not wander but follow an orderly circle around the Sun and that, conversely, there are real wanderers, solitary planets which do not revolve around any star. An international team led by astrophysicists from the University of Bordeaux has just discovered the richest sample of these vagabonds, described a study published on December 22, 2021 in Nature Astronomy.
To understand the approach of these researchers, we must begin by explaining how difficult it is to locate these celestial bodies, known by the English acronym FFP (for free-floating planets, or planets floating freely in space). In the absence of a host star, the classical methods of detecting exoplanets, which aim to measure the gravitational influence of these on their star or their passage in front of the stellar disk, are of no help.
Astronomers must therefore try to see them directly and are forced to target very large (several times the mass of Jupiter) and very young FFPs. Indeed, during their first millions of years of existence, these chubby babies still retain some of the heat that accompanied their birth and thus emit a little light.
Forty million objects, stars and galaxies in the background
The authors of the study looked at a cluster close enough to some 4,000 very young stars, hoping that in the cluster there would also be wandering planets of the same age, still bright enough to be seen. The problem is that this cluster is spread out over a region of the firmament which, in the background, also has around forty million objects, stars or distant galaxies!
To differentiate between the wheat and the heavenly chaff, astronomers have bet on the fact that the members of the cluster all have more or less the same color, related to their age, and that, “Being born together, they will all move at the same speed and in the same direction, which allows them to be identified with a lot of robustness, compared to the rest of the objects”, specifies Hervé Bouy, professor at the University of Bordeaux and co-author of the study.
The other problem, underlines the latter, is that the trips in question are tiny: “Either we are very patient and we take pictures several years apart to see these movements, or we take advantage of all the astronomical archives in the world. ” In addition to their own observations, Hervé Bouy and his colleagues have recovered images acquired by several telescopes over the past twenty years. In total, this represented 80,000 snapshots, or 120 terabytes of data to be calibrated to produce a catalog that the researchers then linked with those from the Gaia and Hipparcos satellites of the European Space Agency.
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Several dozen wandering planets discovered