Tiny brown dwarfs discovered inside the Flame Nebula

Deep within the Flame Nebula, about 1,400 light-years away in the Orion constellation, hot cosmic gas and dust are fueling the birth of new stars. This area is less than a million years old and contains not only newly forming stars but also so-called “failed stars” known as brown dwarfs. 

Because brown dwarfs lack the mass to fuse hydrogen in their cores, they gradually cool down and fade, making them difficult to detect with most telescopes.

Yet, at this early stage, they remain warm enough that NASA’s James Webb Space Telescope (JWST) can pierce through the dense dust to observe them in infrared light.

Probing the low-mass limit

A team of astronomers recently tapped into JWST’s capabilities to explore the smallest objects in the Flame Nebula, with a particular focus on free-floating brown dwarfs. 

The experts wanted to investigate just how little mass a newly formed brown dwarf can have before it falls below the threshold needed to be called a star. 

After analyzing Webb’s data, the team spotted objects around two to three times the mass of Jupiter, though their observations could, in principle, detect masses down to half of Jupiter’s mass.

“The goal of this project was to explore the fundamental low-mass limit of the star and brown dwarf formation process. With Webb, we’re able to probe the faintest and lowest mass objects,” said study lead author Matthew De Furio of the University of Texas at Austin.

How brown dwarfs form

Understanding the lower mass boundary for brown dwarfs is tied to fragmentation – the process in which large molecular clouds split into smaller clumps under gravity. 

If a fragment accumulates enough material, its center becomes hot and dense enough to initiate hydrogen fusion, stabilizing it as a star. If not, it contracts further, lacking the spark to shine like a star.

The tricky question is: How small can these fragments be? Previous theories placed the cutoff between one and ten times Jupiter’s mass, which is a wide window. With the new Webb findings, that window seems to be narrowing significantly.

Fewer objects below three Jupiter masses

The researchers discovered that although there are more low-mass objects than heavier ones up to about ten Jupiter masses, the number of objects declines sharply at even smaller masses.

“We find fewer five-Jupiter-mass objects than ten-Jupiter-mass objects, and we find way fewer three-Jupiter-mass objects than five-Jupiter-mass objects,” explained De Furio.

“We don’t really find any objects below two or three Jupiter masses, and we expect to see them if they are there, so we are hypothesizing that this could be the limit itself.”

Because JWST is exceptionally sensitive and should, in theory, have registered any objects below two Jupiter masses if they existed, the team posits that this boundary might be where the fragmentation process breaks down.

“Webb, for the first time, has been able to probe up to and beyond that limit. If that limit is real, there really shouldn’t be any one-Jupiter-mass objects free-floating out in our Milky Way galaxy, unless they were formed as planets and then ejected out of a planetary system,” remarked Michael Meyer of the University of Michigan, who was also involved in the work.

Brown dwarfs in the Flame Nebula

For years, NASA’s Hubble Space Telescope has been searching for brown dwarfs, but it struggled to detect such small, dim objects, especially in areas like the Flame Nebula.

However, it provided long-term data about star-forming regions, helping guide Webb’s deeper look. 

Combining Hubble’s archival observations with Webb’s infrared clarity allowed the team to identify and characterize precisely these faint brown dwarfs.

“It’s a quantum leap in our capabilities between understanding what was going on from Hubble. Webb is really opening an entirely new realm of possibilities, understanding these objects,” explained astronomer Massimo Robberto of the Space Telescope Science Institute.

Rogue planets and brown dwarfs

Another intriguing implication of this research is the overlap between very small brown dwarfs and rogue planets. 

Rogue planets may have formed around a star and later been ejected, making them free-floating and easily mistaken for brown dwarfs. Discriminating between these two categories is a core aim of future observations.

“There’s a big overlap between the things that could be planets and the things that are very, very low mass brown dwarfs,” Meyer noted. “And that’s our job in the next five years: to figure out which is which and why.”

Faint objects in the Flame Nebula

To refine their findings, the team plans to use JWST’s spectroscopic capabilities on the Flame Nebula’s faintest objects, hoping to learn more about each object’s composition, temperature, and precise mass. 

Spectroscopy can help discern whether these bodies formed from collapsing clouds (like stars and brown dwarfs) or might have originated in a planetary disk.

As the boundary between stars and planets becomes clearer, astronomers will gain deeper insights into the nature of celestial formation – and how often the cosmos crafts these elusive “in-betweens.” 

Published in The Astrophysical Journal Letters, these results mark a vital step toward comprehending the smallest objects that bridge the gap between fully-fledged stars and giant planets.

Image Credit: NASA, ESA, CSA, STScI

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