In six new rogue worlds, Webb telescope finds new clues about how stars are born

The James Webb Space Telescope has spotted six potentially rogue worlds – objects with planet-like mass but not bound to the gravity of a star – including the lightest one yet identified, surrounded by a dusty disk.

These elusive objects offer new evidence that the same cosmic processes that give birth to stars may also play a common role in the creation of objects only slightly larger than Jupiter.

“We’re probing the very limits of the star formation process,” said Adam Langeveld, an astrophysicist at Johns Hopkins University and lead author of the study. “If you have an object that looks like a young Jupiter, is it possible that it could have become a star under the right conditions? That’s important context for understanding both star and planet formation.”

The findings come from Webb’s most extensive study of the young nebula NGC1333, a star-forming cluster located about a thousand light-years from Earth in the constellation Perseus. A new image from the study released today by the European Space Agency shows NGC1333 glowing spectacularly with interstellar dust and clouds. A paper detailing the study’s findings has been accepted for publication in The Astronomical Journal.

Webb’s data suggest that the discovered worlds are gas giants 5 to 10 times more massive than Jupiter. That means they are among the lowest-mass objects ever discovered, formed by a process that would typically produce stars and brown dwarfs, objects somewhere between stars and planets that never initiate hydrogen fusion and fade away over time.

“We used Webb’s unprecedented sensitivity to infrared wavelengths to search for the faintest members of a young star cluster, seeking to answer a fundamental question in astronomy: How faint can a faint object form like a star?” said Ray Jayawardhana, Johns Hopkins dean, astrophysicist and lead author of the study. “It turns out that the smallest free-floating objects that form like stars overlap in mass with giant exoplanets orbiting nearby stars.”

The telescope’s observations did not reveal any objects with a mass less than five times that of Jupiter, although its sensitivity is sufficient to detect such bodies. This clearly indicates that any stellar object lighter than this threshold is more likely to form in the same way as planets, the authors conclude.

“Our observations confirm that nature produces planetary-mass objects in at least two different ways: from the contraction of a cloud of gas and dust, as stars form, and in disks of gas and dust around young stars, as Jupiter did in our own solar system,” Jayawardhana said.

The most intriguing starless object is also the lightest, with an estimated mass of five Jupiters (about 1,600 Earths). The presence of a dusty disk almost certainly means the object formed as a star, because space dust typically orbits a central object in the early stages of star formation, said Langeveld, a postdoctoral researcher in Jayawardhana’s group.

Disks are also a prerequisite for planet formation, suggesting that the observations may also have important implications for potential “mini” planets.

“These tiny objects, with masses comparable to a giant planet, could themselves form their own planets,” said Aleks Scholz, co-author of the study and an astrophysicist at the University of St Andrews. “They could be the nursery for a miniature planetary system, on a scale much smaller than our solar system.”

Using Webb’s NIRISS instrument, astronomers measured the infrared light profile (or spectrum) of each object in the observed portion of the star cluster and reanalyzed 19 known brown dwarfs. They also discovered a new brown dwarf with a planetary-mass companion, a rare discovery that challenges theories about how binary systems form.

“It is likely that such a pair formed in the same way that binary star systems do, from a cloud that fragmented as it contracted,” Jayawardhana said. “The diversity of systems that nature has produced is remarkable and challenges us to refine our models of star and planet formation.”

Wandering planets can arise from the collapse of molecular clouds that lack the mass needed for the nuclear fusion that powers stars. They can also form when gas and dust in disks around stars coalesce to form planetary orbs that are eventually ejected from their star system, likely due to gravitational interactions with other bodies.

These free-floating objects confuse the classification of celestial bodies because their masses overlap those of gas giants and brown dwarfs. Although these objects are considered rare in the Milky Way, Webb’s new data show that they make up about 10% of the celestial bodies in the targeted star cluster.

In the coming months, the team will further study the atmospheres of these faint objects and compare them to those of brown dwarfs and heavier gas giant planets. They will also have time to study similar objects with dusty disks on the Webb telescope to explore the possibility of forming mini-planetary systems similar to the many moons of Jupiter and Saturn.

Other authors are Koraljka Mužić and Daniel Capela of the Universidade de Lisboa; Loïc Albert, René Doyon and David Lafrèniere of the University of Montreal; Laura Flagg of Johns Hopkins; Matthew de Furio of the University of Texas at Austin; Doug Johnstone of the Herzberg Center for Astronomy and Astrophysics; and Michael Meyer of the University of Michigan in Ann Arbor.