Many astronomical objects play by clear rules and fit into neat categories, but brown dwarfs (sky objects too massive to be mere planets but too small to be true stars) continue to refuse to cooperate.
Astronomers recently studied a sample of 70 objects, ranging from Jupiter-mass planets to brown dwarfs who is right on the brink of stardom. By looking for a relationship between the mass of these objects and certain features of their star systems (such as whether the host star contained elements heavier than helium, or how round the objects’ orbits were), the researchers hoped to draw a clear line dividing massive objects that form as stars and smaller ones that form as planets. But they were destined for disappointment, because the actual universe is messy and complicated.
Planets and stars form differently – except for that group in the middle
Stars, by definition, boast at least 80 times the mass of Jupiter, and they form from the outside in. When a lump of gas in a molecular cloud collapses under its own gravity, the tightly packed atoms in its core begin to fuse together, releasing heat and light; a star is born.
Giant gas planets of sizes up to roughly Jupiter’s mass, on the other hand, form from the inside out. First, a few dust grains clump together in the disc of material around a newborn star, and their combined gravity is enough to start attracting even more dust. Material continues to accumulate, faster and faster, building up a rocky core surrounded by thick layers of gas.
In between, however, there are a whole host of objects that astronomers aren’t sure whether to classify as “failed stars” or “overgrown planets.”
With between 13 and 80 times the mass of Jupiter, brown dwarfs are not quite massive enough to fuse together hydrogen into helium like a real star, but they are just big enough to fuse deuterium, an isotope of hydrogen that includes a neutron along with the standard proton and electrons. (Oddly enough, deuterium requires less pressure to fuse into helium than straight hydrogen does.) And then there are “sub-brown dwarfs,” gas giants that are really gigantic by planetary standards, but they’re not big enough to be proper brown dwarfs.
Ideally, there should be a clear line: objects above a certain mass should be failed stars that formed from collapsing gas clouds, and objects below that mass should be overgrown planets that coalesced from planetary disks.
So far, however, astronomers haven’t had much luck finding such a line.
In 2024, astrophysicist Steven Giacalone, one of the co-authors of the current study, found a brown dwarf that appeared to have formed by nuclear accretionmaking it the largest planet ever. And some sub-brown dwarfs—giant planets not big enough to count as brown dwarfs—appear to have formed by gravitational collapse, meaning they failed so badly to be stars that they couldn’t even make it as brown dwarfs.
“Exactly how large of an object can be formed by core accretion or how small of an object can be formed by disc instability or cloud fragmentation remains to be determined,” Gilbert and his colleagues wrote in their recent paper.
“Perhaps … we have not yet investigated the right combination of parameters”
Gilbert and his colleagues used statistical models to test how the objects’ masses related to the chemical composition of their host stars and the shape of the objects’ orbits.
Looking at these objects’ orbital eccentricity (a measure of how close to a perfect circle an orbit is) tells much the same story. Less massive objects tend to have rounder orbits, while the most massive, brown-dwarf-like of these objects vary more in their eccentricity. However, Gilbert and his colleagues noted that the trend was very gradual.
“We can reasonably assume that as the mass of an object increases, the probability that it formed via nuclear accretion and the probability that it formed by gravitational instability (a gas cloud collapsing in on itself) decreases,” the researchers wrote in their recent paper, but it is more of a spectrum than a pure sorting of objects into two groups.
And then there is metallicity. A planet can only collect enough material, fast enough, to grow into a gas giant if it forms in a star system that is very metallic—meaning it’s packed with elements heavier than helium (mostly carbon, oxygen, and iron). So if there were a clear dividing line between more massive objects formed by collapsing molecular clouds and less massive objects formed by accretion, researchers like Gilbert and his colleagues would expect to see smaller sub-brown dwarfs forming only in metal-rich star systems. But that’s not what Gilbert and his colleagues actually saw in their data.
Instead, there appears to be no correlation between the mass of a gas supergiant and the metallicity of the stellar system. It suggests that some of these objects were formed by core accretion, while others formed more like stars – with the same end result and often the same mass. That means right now we can’t tell if something is a failed star or a very successful planet.
“Maybe a clear dividing line between formation channels exists, but we haven’t found it yet, either because we don’t have enough objects or because we haven’t yet investigated the right combination of parameters,” Gilbert and his colleagues wrote in their recent paper.
