Nomads of the galaxy

Artist’s rendition of a nomad planet. Credit: NASA

Far from the warmth and light of any star, there are still planets…depending on what you call them. There is no doubt that objects the size of planets have been found floating in space by themselves. There are several in the Sigma Orionis Cluster. They are only a few times the mass of Jupiter, too small to be stars or even brown dwarfs, but what to call them?

Science fiction writers have long called these objects “rogue planets”, but many astronomers don’t like to call them planets because they aren’t orbiting a star. The International Astronomical Union (IAU), of “Dwarf Planet Pluto” fame, would prefer to call them “sub-brown dwarfs”. Other astronomers use “planemo”, short for “planetary mass object”. Around 2001, “planetar”, for “planet+star”, was in vogue, but that one has fallen by the wayside.

Meanwhile, there are astronomers who don’t mind using the word planet, but they like to throw in some adjectives, so you might have “rogue planets”, “interstellar planets”, “free-floating planets”, or “orphan planets”. But “nomad planets” seems to be the fad now, so let’s go with that.

Nomad planets may have formed orbiting a star, but they were kicked out of their solar system by the gravity of the other planets. Or they might have collapsed from clouds of gas, like stars do. Either way, there a lot of them. Several studies, like this one, say that there are twice as many nomad planets in our galaxy as stars, and at least one estimates that there are far, far more.

The surprising thing is that, even though nomad planets have no sun, they need not be frozen solid. With no sun to strip it away, even an Earth-sized nomad planet could hold on to a thick hydrogen atmosphere that would trap its internal heat and keep its interior warm. Some planetary scientists even speculate that a nomad planet could have a layer of liquid water deep within some part of its structure. It’s not likely, but it is possible, and that is something pretty amazing in itself.

About Alex R. Howe

I'm a full-time astrophysicist and a part-time science fiction writer.
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