Can an object somewhat behave like both a planet and a star, especially when it is forming? Turns out, the answer is yes! Astronomers have reported a mysterious “rogue planet” gobbling 6 million tons of gas and dust per second — an unprecedented rate that blurs the line between planets and stars. This is the strongest growth rate ever recorded for a rogue planet, or a planet of any kind, providing valuable insights into how they form and grow. The discovery was published on October 2 in The Astrophysical Journal Letters.
You may wonder, what makes rogue planets “rogue”? Unlike Earth and other planets in our solar system, which orbit the sun, rogue planets float freely through the universe, not orbiting a host star. Scientists have discovered dozens of rogue planets before, but what makes this new one special is the fact that it exhibits phenomena that have previously only been observed in stars. “People may think of planets as quiet and stable worlds, but with this discovery we see that planetary-mass objects freely floating in space can be exciting places,” said Victor Almendros-Abad, an astronomer at the Astronomical Observatory of Palermo, National Institute for Astrophysics (INAF), Italy and lead author of the new study.
The rogue planet in the new discovery, officially called Cha 1107-7626, has a mass five to ten times bigger than Jupiter. It is around 620 light years from Earth in the constellation Chamaeleon. Being roughly one to two million years old, the planet is still forming and is fed by a surrounding disk of gas and dust constantly falling onto the free-floating planet for its growth, a process known as accretion. However, the team led by Almendros-Abad has now found that the rate at which the young planet is accreting is not steady. The discovery was made with the X-shooter spectrograph on European Southern Observatory’s (ESO) Very Large Telescope (VLT), located in Chile’s Atacama Desert. The team also used data from NASA’s James Webb Space Telescope and archival data from the SINFONI spectrograph on ESO’s VLT.
“The origin of rogue planets remains an open question: are they the lowest-mass objects formed like stars, or giant planets ejected from their birth systems?” asked co-author Aleks Scholz, an astronomer at the University of St Andrews, United Kingdom.
The findings indicate that at least some rogue planets may share a similar formation path to stars since similar bursts of accretion have been spotted in young stars before. By comparing the light emitted before and during the burst, astronomers gathered clues about the nature of the accretion process. Remarkably, magnetic activity appears to have played a role in driving the dramatic infall of mass. Moreover, the chemistry of the disc around the planet changed during the accretion episode, with water vapour being detected during it but not before. Both phenomena had been spotted in stars but never in a planet of any kind!
Still, Cha 1107-7626 is expected to have similar characteristics to huge planets, being similar in size. Unlike stars, it is “not massive enough to ever have fusion reactions in the core. It will cool inevitably as it gets older,” said Scholz, indicating that it will ultimately act like other planets.
Rogue planets are difficult to detect, as they are very faint, but ESO’s upcoming Extremely Large Telescope (ELT), operating under the world’s darkest skies for astronomy, could change that. Its powerful instruments and giant main mirror will enable astronomers to uncover and study more of these lonely planets, helping them to better understand how star-like they are. As co-author and ESO astronomer Amelia Bayo puts it, “The idea that a planetary object can behave like a star is awe-inspiring and invites us to wonder what worlds beyond our own could be like during their nascent stages.”
