Using NASA’s James Webb Space Telescope, or JWST, a team of researchers, including astronomers from the University of Michigan, are closing in on the answer to a looming cosmic question.

In probing the Flame Nebula, they’re finding out what’s the smallest celestial body that can form on its own from clouds of gas and dust in space.

The Flame Nebula, located about 1,400 light-years away from Earth, is a hotbed of star formation less than 1 million years old. Within the Flame Nebula, there are objects so small that their cores will never be able to fuse hydrogen like full-fledged stars—brown dwarfs.

Brown dwarfs, often called “failed stars,” become very dim over time and much cooler than stars. These factors make observing brown dwarfs with most telescopes difficult, if not impossible, even at cosmically short distances from the sun. When they are very young, however, they still are relatively warm and bright and, therefore, easier to observe despite the obscuring, dense dust and gas that comprises the Flame Nebula in this case.

JWST can pierce this dense, dusty region and see the faint infrared glow from young brown dwarfs. The team used this capability to explore the lowest mass limit of brown dwarfs within the Flame Nebula.

The researchers found free-floating objects roughly two to three times the mass of Jupiter, although they were sensitive down to 0.5 times the mass of Jupiter. The team’s study has been accepted for publication in The Astrophysical Journal Letters.

“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 lead author Matthew De Furio.

Now a postdoctoral fellow at the University of Texas, De Furio began working with JWST’s data as a doctoral student at U-M. His adviser, professor and department chair Michael Meyer, is also a senior author of the new study.

In fact, Meyer has been involved with JWST now for decades, well before its 2021 launch, helping plan and implement its scientific capabilities. That work is now helping a new generation of researchers, using cutting-edge instruments, deepen our understanding of the cosmos.

“All of us involved with JWST for a long time have imagined what the next generation of astronomers would be able to do with it,” Meyer said. “These results, generated by this team ably led by Matthew De Furio, are an example of its promise fulfilled.”

Smaller fragments

The low-mass limit the team sought is set by a process called fragmentation. In this process large molecular clouds, from which both stars and brown dwarfs are born, break apart into smaller and smaller units, or fragments.

Fragmentation depends on several factors with the balance between temperature, thermal pressure and gravity being among the most important. More specifically, as fragments contract under the force of gravity, their cores heat up. If a core is massive enough, it will begin to fuse hydrogen.

The outward pressure created by that fusion counteracts gravity, stopping collapse and stabilizing the object, then known as a star. However, fragments whose cores are not compact and hot enough to burn hydrogen continue to contract as long as they radiate away their internal heat.

“The cooling of these clouds is important because if you have enough internal energy, it will fight that gravity,” Meyer said. “If the clouds cool efficiently, they collapse and break apart.”

Fragmentation stops when a fragment becomes opaque enough to reabsorb its own radiation, thereby stopping the cooling and preventing further collapse. Theories placed the lower limit of these fragments anywhere between one and 10 Jupiter masses. This study significantly shrinks that range as JWST’s census counted up fragments of different masses within the nebula.

Previous research has shown that there appear to be more low-mass objects up to a certain point and, past that threshold, the trend reverses. That is, the lowest masses become increasingly rare.

“We find fewer five-Jupiter-mass objects than 10-Jupiter-mass objects, and we find way fewer three-Jupiter-mass objects than five-Jupiter-mass objects,” De Furio said. “We don’t really find any objects below two or three Jupiter masses, and we expect to see them if they are there.”

That has led the researchers to hypothesize they may have, in fact, found the lower limit they sought.

“Webb, for the first time, has been able to probe up to and beyond that limit,” Meyer said. “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.”

Building on Hubble’s legacy

Brown dwarfs, given the difficulty of finding them, have a wealth of information to provide, particularly in star formation and planetary research given their similarities to both stars and planets. NASA’s Hubble Space Telescope has been on the hunt for these brown dwarfs for decades.

Even though Hubble can’t observe the brown dwarfs in the Flame Nebula to as low a mass as JWST can, it was crucial in identifying candidates for further study. This study is an example of how JWST took the baton—decades of Hubble data from the Orion Molecular Cloud Complex—and enabled in-depth research.

“It’s really difficult to do this work, looking at brown dwarfs down to even 10 Jupiter masses, from the ground, especially in regions like this. And having existing Hubble data over the last 30 years or so allowed us to know that this is a really useful star-forming region to target,” De Furio said. “We needed to have Webb to be able to study this particular science topic.”

Astronomer Massimo Robberto of the Space Telescope Science Institute said, “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.”

This team is continuing to study the Flame Nebula, using JWST’s spectroscopic tools to further characterize the different objects within its dusty cocoon.

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

Written by Matthew Brown, Space Telescope Science Institute