Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:

BROWN DWARF

In 1962, Indian astronomer Shiv S. Kumar theorized that there could be objects out there in space too big to be considered planets but too small to become stars. Since main sequence stars are referred to as dwarfs of various colors (our own Sun is a yellow dwarf), Kumar called his theoretical objects “black dwarfs.”

It turned out that the term “black dwarf” was already taken, so in 1975 American astronomer Jill Tarter (best known for her work with the SETI Institute) suggested the name “brown dwarf” instead. In an article from Universe Today, Tarter is quoted as saying: “it was obvious that we needed a color to describe these dwarfs that was between red and black.”

The term stuck, despite the fact that “brown” is a very misleading description. It’s not clear to us what color these objects are or would appear to be to the human eye. They do radiate light, but it’s mostly infrared light. In the visible spectrum, they might appear to be purple or magenta, or perhaps a rather dull red or orange. In fact they may come in all sorts of colors, depending on their metallicity. But astronomers do seem to agree about one thing: brown dwarfs are definitely not brown.

Today, brown dwarfs are typically described as failed stars.

Stars are defined scientifically as objects massive enough to cause nuclear fusion in their cores—specifically, to be classified as a star an object must be able to fuse hydrogen into helium. Brown dwarfs can’t do that.

But while this distinction between stars and brown dwarfs is fairly straightforward, the distinction between brown dwarfs and planets can get pretty murky. We actually don’t know enough yet about either brown dwarfs or exoplanets to be sure where to draw the line separating one from the other.

One of the leading proposals would define brown dwarfs based on their formation. If an object coalesces from a molecular cloud, as a star would, but fails to initiate hydrogen fusion, that object would be a brown dwarf. If an object forms in the accretion disk surrounding a star, the way planets form, then that object would not be a brown dwarf.

Another leading proposal would define brown dwarfs based on their internal physics. If an object can’t fuse hydrogen but can fuse other elements like lithium or oxygen, then that object would be a brown dwarf. (For more about these two competing proposals, click here.)

Eventually the International Astronomy Union will have to step in and set an official definition. But they’re not ready to do that. Not yet. Not until we’ve learned a lot more. In the meantime, ongoing observational research of objects like Gliese 504b (which I’ve nicknamed “Pinkie Pie”) may help the I.A.U. figure out which definition makes the most sense.