Could these weird stars just be overgrown planets?

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Many astronomical objects obey clear rules and fit into specific categories, but brown dwarfs (celestial objects too massive to be simple 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 are on the verge of stardom. By looking for a relationship between the mass of these objects and certain characteristics of their star systems (such as whether the host star contained elements heavier than helium or the roundness of the objects’ orbits), the researchers hoped to draw a clear line separating massive objects that form as stars and smaller ones which form like planets. But they were doomed to disappointment, because the current universe is messy and complicated.

It turns out that the boundary between stars and planets might be more of a gray, fuzzy continuum, according to University of California, Los Angeles astrophysicist Gregory Gilbert and colleagues in a recently published paper. in The Astronomical Journal.

Planets and stars form differently – except for this group in the middle

Stars, by definition, have at least 80 times the mass of Jupiter and form from the outside in. in a molecular cloud collapses under its own gravity, the densely packed atoms at its core begin to fuse, releasing heat and light; a star is born.

In contrast, giant gas planets up to about the mass of Jupiter form from the inside out. First, a few dust grains clump together in the disk of material around a newborn star, and their combined gravity is enough to start attracting even more dust. The materials accumulate faster and faster, forming a rocky core surrounded by thick layers of gas.

In between, however, there are a whole host of objects that astronomers are unsure whether to classify as “failed stars” or “invaded planets”.

Between 13 and 80 times the mass of Jupiter, brown dwarfs are not massive enough to hydrogen fuse in helium like a real star, but they are just big enough to fuse deuterium, an isotope of hydrogen that includes a neutron as well as the standard proton and electrons. (Oddly, deuterium requires less pressure to fuse into helium than pure hydrogen.) And then there are “sub-brown dwarfs,” gas giants that are truly gargantuan by planetary standards, but they’re not big enough to be true brown dwarfs.

Ideally, there should be a clear line: objects above a certain mass should be failed stars formed from collapsing gas clouds, and objects below that mass should be overgrown planets merged 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 present study, found a brown dwarf that appeared to have formed by central accretionwhich basically makes it the largest planet of all time. And some sub-brown dwarfs – gargantuan planets not big enough to be considered brown dwarfs – appear to have formed by gravitational collapse, meaning they failed so badly at becoming stars that they couldn’t even become brown dwarfs.

“The exact size of an object can be formed by core accretion or the size of an object can be formed by disk instability or cloud fragmentation,” Gilbert and colleagues wrote in their recent paper.

“Perhaps…we haven’t looked at the right combination of settings yet”

Gilbert and his colleagues used statistical models to test the link between the mass of their objects and the chemical composition of the host stars and the shape of the objects’ orbits.

The orbital eccentricity of these objects (a measure of how close an orbit is to a perfect circle) tells much the same story. Less massive objects tend to have rounder orbits, while more massive objects, resembling brown dwarfs, 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 by core accretion decreases and the probability that it formed by gravitational instability [a gas cloud collapsing in on itself] increases,” the researchers wrote in their recent paper, but it’s more of a spectrum than a neat sorting of objects into two groups.

And then there is the metallicity. A planet can only accumulate enough material, quickly enough, to become a gas giant if it forms in a very metallic star system, meaning it is full of elements heavier than helium (mainly carbon, oxygen, and iron). So if there were a clear dividing line between more massive objects formed by the collapse of 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 found in their data.

Instead, there appears to be no relationship between the mass of a gas supergiant and the metallicity of its star system. This suggests that some of these objects formed by core accretion, while others formed more like stars – with the same end result and, often, the same mass. Which means that right now we can’t tell by looking whether something is a failed star or a wildly successful planet.

“There may be a clear dividing line between the training channels, but we have not yet found it, either because we do not have enough objects or because we have not yet examined the right combination of parameters,” Gilbert and colleagues wrote in their recent paper.