independent– A team of astronomers discovered at least 70 ‘rogue’ planets in our galaxy, the largest collection ever found to date.
While conventional planets (like those in our Solar System) orbit a star, rogue planets roam freely without travelling around a nearby star.
“We did not know how many to expect and are excited to have found so many,” said Núria Miret-Roig, an astronomer at the Laboratoire d’Astrophysique de Bordeaux.
It would usually be impossible to detect rogue planets because they are hard to spot far from a star’s light. One key fact of their existence made them visible: these planets still give off enough heat to glow millions of years after their creation, making them visible to powerful telescopes.
This heat allowed the 70 planets – each with masses close to that of Jupiter – to be discovered in the Scorpius and Ophiuchus constellations.
“We measured the tiny motions, the colours and luminosities of tens of millions of sources in a large area of the sky,” explained Ms Miret-Roig. “These measurements allowed us to securely identify the faintest objects in this region, the rogue planets.”
The astronomers’ study suggests there could be many more elusive, starless planets yet to be discovered, numbering in the billions in the Milky Way alone.
By studying these planets, astronomers believe they could unlock clues as to how the mysterious objects come to be. It is hypothesised they are generated from the collapse of gas clouds too small to create stars, but they could also have been ejected from a parent system.
“These objects are extremely faint and little can be done to study them with current facilities,” says Hervé Bouy, another astronomer at the Laboratoire d’Astrophysique. “The ELT [Extremely Large Telescope, currently being built in Chile] will be absolutely crucial to gathering more information about most of the rogue planets we have found.”
The exact number of rogue planets discovered is vague, because the observations made by the researchers do not allow them to measure the mass of the objects. Bodies with a mass 13 times greater than that of Jupiter are unlikely to be planets, but relying on brightness makes this figure unclear.
The brightness of these objects is also related to age, as the older the planet is the dimmer it will be. The brightest objects in the sample could have a mass greater than the upper limit but be older and therefore dimmer. Researchers estimate there could be as many as 100 more planets yet to be discovered because of this uncertainty.