Largest ever group of lonely rogue planets discovered in Milky Way

Deep in the cold, dark emptiness of interstellar space, you can find some lonely planets roaming freely and not orbiting a star. Known as rogue planets, these objects are elusive and are rarely discovered due to being difficult to spot — but a new study has found the largest collection of rogue planets to date, located in a region of the Milky Way called the Upper Scorpius OB stellar association.

Finding rogue planets is hard because, unlike stars, planets are dim and give off very little light, and these tiny points have to be picked out from a background of bright stars. But an international team was able to spot this group of rogue planets by using a combination of both new observations and archival data from a large number of sources including telescopes of the European Southern Observatory, the Canada-France-Hawaii Telescope, and the Subaru Telescope. In total, the data they used added up to 80,000 wide-field images taken over 20 years of observations.

Artist’s impression shows an example of a rogue planet with the Rho Ophiuchi cloud complex visible in the background.
This artist’s impression shows an example of a rogue planet with the Rho Ophiuchi cloud complex visible in the background. Rogue planets have masses comparable to those of the planets in our Solar System but do not orbit a star, instead, roaming freely on their own. ESO/M. Kornmesser

“We measured the tiny motions, the colors and luminosities of tens of millions of sources in a large area of the sky,” explained lead author Núria Miret-Roig. “These measurements allowed us to securely identify the faintest objects in this region.” Using this technique, the researchers found at least 70 rogue planets from the data. “We did not know how many to expect and are excited to have found so many,” said Miret-Roig.

Studying these rogue planets, or Free-Floating Planets (FFPs), in more detail could help us learn about planet composition and formation, according to project leader Hervé Bouy: “The FFPs we identified are also excellent targets for follow-up studies. In particular, they will be essential to study planetary atmospheres in the absence of a blinding host star, making the observation far easier and more detailed. The comparison with atmospheres of planets orbiting stars will provide key details about their formation and properties. Additionally, studying the presence of gas and dust around these objects, what we call ‘circumplanetary discs’, will shed more light on their formation process.”

This could be just the tip of the iceberg where rogue planets are concerned. There could potentially be billions of them in our galaxy. “Assuming the fraction of FFPs that we measured in Upper Scorpius is similar to that of other star-forming regions, there could be several billions of Jupiters roaming the Milky Way without a host star. This number would be even greater for Earth-mass planets since they are known to be more common than massive planets.”

The research is published in the journal Nature.

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