Webb discovers six new ‘rogue worlds’ that provide clues to star formation

Free-floating planetary-mass planets (FFPMOs) are planet-sized objects that originated in interstellar space or were part of a planetary system before being ejected by gravitational disturbances.

Since they were first observed in 2000, astronomers have discovered hundreds of candidates that are separated from a particular star and drifting through the interstellar medium (ISM) of our galaxy. Some scientists even estimate that there could be as many as 2 trillion (or more) stray planets wandering through the Milky Way alone.

In recent news, a team of astronomers working with the James Webb Space Telescope (JWST) announced the discovery of six candidate rogue planets in an unlikely location. The planets, including the lightest rogue planet ever identified (with a debris disk surrounding it), were discovered during Webb’s deepest survey of the young nebula NGC 1333, a star-forming cluster about 1,000 light-years away in the constellation Perseus. These planets could teach astronomers a lot about how stars and planets form.

The team was led by Adam Langeveld, an assistant research scientist in the Department of Physics and Astronomy at Johns Hopkins University (JHU). The paper describing the research findings has been accepted for publication in The Astronomical Magazine and is currently available on the arXiv preprint server.

Most of the rogue planets discovered so far were discovered using gravitational microlensing, while others were detected using Direct Imaging. The former method relies on “lensing events”, where the gravity of massive objects changes the curvature of spacetime around them and amplifies light from more distant objects. The latter consists of directly spotting brown dwarfs (objects that cross the boundary between planets and stars) and massive planets by detecting the infrared radiation produced in their atmospheres.

In their paper, the team describes how the discovery occurred during an extremely deep spectroscopic survey of NGC1333. Using data from Webb’s Near-Infrared Imager and Slitless Spectrograph (NIRISS), the team measured the spectrum of every object in the observed part of the cluster. This allowed them to reanalyze spectra of 19 previously observed brown dwarfs and led to the discovery of a new brown dwarf with a planetary-mass companion.

This latest observation was a rare find that challenges theories about how binary systems form. But the real kicker was the discovery of six planets with between five and ten times the mass of Jupiter (also called super-Jupiters).

This means that these six candidates are among the lowest-mass rogue planets ever found, which formed via the same process as brown dwarfs and stars. This was the goal of the Deep Spectroscopic Survey for Young Brown Dwarfs and Free-Floating Planets survey, which was to study massive objects that are not large enough to become stars.

The fact that Webb’s observations failed to detect objects less than five Jupiter masses (making their detection sensitive enough) is strong evidence that stellar objects less than five Jupiter masses are more likely to form in the manner of planets.

Lead author Langeveld said in a statement from JHU’s new source (the Hub):

“We are exploring the boundaries of the star formation process. If you have an object that looks like a young Jupiter, is it possible that under the right conditions it could have become a star? This is an important context for understanding both star and planet formation.”

The most intriguing of the rogue planets was also the lightest: an estimated five Jupiter masses (about 1,600 Earths). Since dust and gas generally fall into a disk during the early stages of star formation, the presence of this ring of debris around this one planet strongly suggests that it formed in the same way that stars do.

However, planetary systems also form from debris disks (also called circumsolar disks), suggesting that these objects may be able to form their own satellites. This suggests that these massive planets could be a nursery for a miniature planetary system, like our own, but on a much smaller scale.

Johns Hopkins Provost Ray Jayawardhana, an astrophysicist and lead author of the study (who also leads the research group): “It turns out that the smallest free-floating objects that form as stars overlap in mass with giant exoplanets orbiting nearby stars. It’s likely that such a pair formed in the same way as binary star systems, from a cloud that broke up as it collapsed. The diversity of systems that nature has produced is remarkable, and forces us to refine our models of star and planet formation…

“Our observations confirm that nature produces planetary masses in at least two different ways: by the collapse of a cloud of gas and dust, the way stars form, and in disks of gas and dust around young stars, as Jupiter did in our own solar system.”

In the coming months, the team plans to use Webb to conduct follow-up studies of the atmospheres of these rogue planets, comparing them to those of brown dwarfs and gas giants. They also plan to search the star-forming region for other objects with debris disks to investigate the possibility of mini-planetary systems.

The data they obtain will also help astronomers refine their estimates of the number of runaway planets in our galaxy. The new Webb observations indicate that such bodies make up about 10 percent of the bodies in the target cluster.

Current estimates place the number of stars in our galaxy at between 100 and 400 billion stars, and the number of planets at between 800 billion and 3.2 trillion. At 10%, that would mean there are somewhere between 90 and 360 billion rogue worlds floating around. As we’ve explored in previous articles, we may one day explore some of them, and our Sun may even capture a few.