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Astronomers discover dozens of "rogue planets" roaming the galaxy without a star

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It's not the first time astronomers have discovered so-called "rogue planets" — free-floating planets that wander aimlessly through space without a host star to orbit. But they thought it was a somewhat rare phenomenon, until now.

According to new research published in the journal Nature Astronomy, scientists have recently discovered an impressive number of these elusive exoplanets: 70 or more. It marks the largest such group ever spotted roaming the Milky Way — and it may be a crucial step in understanding the origins of the "mysterious galactic nomads," scientists say. 

"We did not know how many to expect and are excited to have found so many," Núria Miret-Roig, the first author of the study, said in a press release

This artist's impression shows an example of a rogue planet detected in the Rho Ophiuchi region, where ESO observations have recently helped uncover at least 70 of these objects. 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

Most exoplanets are spotted using observations of their host stars, so finding these orphaned planets is considerably more difficult. But using decades of research, the group of scientists saw infrared energy emitted by between 70 and 170 of the gas giants, young enough to still emit a detectable heat glow. 

"We measured the tiny motions, the colors and luminosities of tens of millions of sources in a large area of the sky," explains Miret-Roig. "These measurements allowed us to securely identify the faintest objects in this region, the rogue planets."

The planets were discovered using a series of telescopes, located both on Earth and in space, including the European Space Agency's Very Large Telescope and Gaia satellite. The planets, with masses comparable to that of Jupiter, are located within the Scorpius and Ophiuchus constellations. 

Artist’s animation of a rogue planet in Rho Ophiuchi by European Southern Observatory (ESO) on YouTube

"We used tens of thousands of wide-field images from ESO facilities, corresponding to hundreds of hours of observations, and literally tens of terabytes of data," said project leader Hervé Bouy. 

The findings indicate that there could be a treasure trove of cosmic wanderers just waiting to be found, Bouy added. "There could be several billions of these free-floating giant planets roaming freely in the Milky Way without a host star." 

This image shows the locations of 115 potential rogue planets, highlighted with red circles, recently discovered by a team of astronomers in a region of the sky occupied by Upper Scorpius and Ophiucus. The exact number of rogue planets found by the team is between 70 and 170, depending on the age assumed for the study region. This image was created assuming an intermediate age, resulting in a number of planet candidates in between the two extremes of the study. ESO/N. Risinger (

And finding more of these types of celestial travelers will help scientists understand their origins. Some hypothesize that they form from the collapse of a gas cloud that is too small to form a star companion, while others believe they could have been booted from their original parent system. 

Astronomers hope to continue their research using the forthcoming Extremely Large Telescope, or ELT, currently under constriction in Chile's Atacama Desert. 

"These objects are extremely faint and little can be done to study them with current facilities," said Bouy. "The ELT will be absolutely crucial to gathering more information about most of the rogue planets we have found."

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