Yesterday's Part II of this article finished explaining how rogue planets are formed, their relationship to brown dwarfs, and how brown dwarfs are formed. It also detailed how since 2000, when astronomers found the first free-floating objects of planetary mass, about 50 such objects have been observed and confirmed as rogue planets. But, in that time, there has been strong debate both about what these objects should be called, which has sparked another debate about the exact definition of the word “planet”.
The thing is, before we found any planet-like objects outside of our solar system, a lot of questions about the term “planet” just never came up. Planets were those nine round things that orbit the sun and don’t orbit anything else. That was, of course, before the whole Pluto controversy. But once we started finding things that were just like the planets here in our solar system, but they weren’t in our solar system the question arose: can we call these objects planets?
In 2003, the International Astronomical Union (by the way, this is the same organization that demoted Pluto to a dwarf planet) stated that for a planet to be a planet it must orbit the sun. Not some other star, our sun. According to the IAU planet-like objects that orbit other stars should be called extrasolar planets, or exoplanets. But this next part seems a little … off the mark? The IAU thinks that objects with planetary mass that do not orbit a star should not be called “planets” of any sort. The IAU thinks they should be called “sub–brown dwarfs”. This is a logical title: they’re objects that aren’t large enough to be brown dwarfs, but it is a mouthful. Also, if you think about it a little, it’s kind of … heliocentric? as in the idea that the stuff that we’re most familiar with is normal, and everything that we’re not familiar with is something else. Once we establish communication with another inhabited planet (yes, I used the word “planet”) doesn’t it seem likely that terms like “exoplanet” will soon start to be seen as politically incorrect? (if that’s still a thing). And, not for nothin’, but if we’re being grammatically correct, shouldn’t it be: brown dwarves?
Anyway, though the IAU seemed to be trying to draw a clear dividing line between stars, brown dwarfs, sub–brown dwarfs and planets, as Michael Liu, the leader of the team that spotted PSO J318.5-22, the first rogue planet to be observed with a telescope says, “… in reality, the galaxy’s contents are much more complicated.” He should know.
When he and his team first found PSO J318.5-22, they thought it might be a brown dwarf. But, says Liu, PSO J318.5-22 was “like nothing we’d seen before.” It was dimmer than a brown dwarf, and it wasn’t brown. It was red, extremely red, certainly redder than any brown dwarf. As Liu and his team examined the object more closely with larger telescopes, they found that it was less like a brown dwarf, and much more like a planet. PSO J318.5-22 is about 6.5 Jupiter masses, much smaller than a brown dwarf. In fact, they found, the object’s brightness, color, atmosphere and mass to be quite similar to those of a number of young, exoplanets that orbit the nearby star HR 8799, according to a report the team published in the Astrophysical Journal Letters in 2013.