Montreal scientists discover dozens of new “starless planets”

Astronomers from the Montreal Planetarium and the Trottier Institute for Research on Exoplanets (IREx) at the Université de Montréal (UdeM) have discovered a large population of objects similar to the planet Jupiter, freely drifting through space without a host star. These objects are extremely difficult to detect, but once identified they become valuable targets for in-depth study, for example with the James Webb Space Telescope (JWST), to better understand their atmospheres. The absence of a host star makes it possible to precisely characterize the physical conditions and cloud structures in their atmospheres, at a level of detail that remains out of reach for exoplanets orbiting stars. As a reminder, the prefix exo in exoplanet refers to a planet located beyond the Solar System.

The research team made this discovery by carrying out a comprehensive census of stellar families near the Sun, out to a distance of 1500 light-years. These stellar families were born at the same time and move together through our Galaxy. By identifying very cold objects — visible only with infrared cameras — that move along with these stellar families, the team was able to determine key properties such as their temperature, age, and mass. Knowing both their temperature and how long these “planets” have been cooling in space makes it possible to estimate their mass, and thus to identify the most promising targets whose masses are similar to those of gas giant planets like Jupiter. Their age therefore served as the key to identifying these “planets” wandering through space without orbiting a star, sometimes referred to as planemos.

“These objects are real treasures for us,” says lead researcher Jonathan Gagné, a member of IREx and Adjunct Professor at UdeM. “Once discovered, we can study their atmospheres in unprecedented detail, which allows us to better understand the vast diversity of planetary atmospheres.”

What are planemos?

The term planemo comes from a contraction of the English expression planetary-mass object. Planemos have properties very similar to those of exoplanets, but they do not orbit a star. The first planemos were discovered only recently, about 20 years ago, and there is still no official definition for them, which is why these objects are sometimes also called “rogue planets”, “isolated planets,” or even “sub-brown dwarfs.” How most planemos form remains unknown, but scientists suspect that they may form directly from cold clouds of gas, in the same way as stars. It is also possible that some planemos once orbited a star before being ejected; however, there is currently no clear example of this mechanism observed in action, simply because it would be extremely difficult to detect.

Tracing the evolution of exoplanets

The study not only detected new planemos, but also made it possible to precisely determine the age of about a hundred stars known to host exoplanets in orbit around them. This represents a significant increase in the number of exoplanets whose ages are known with such precision. This population will serve as a kind of collection of snapshots of planetary systems at different ages, allowing scientists to better understand how stellar activity, planetary orbits, and atmospheres evolve over time.

“In a way, we are building a family album of the exoplanets closest to us,” concludes Leslie Moranta, co-author of the study and Ph.D. student at IREx/UdeM. “And for the first time, we know exactly how old many of them are.”

The full scientific results are presented in a paper accepted for publication in The Astrophysical Journal Supplement Series.