“Like all animals, human beings have always taken what they want from nature. But we are the rogue species. We are unique in our ability to use resources on a scale and at a speed that our fellow species can’t.” -Edward Burtynsky
It’s really a romantic notion when you think about it: the heavens, the Milky Way, is lined with hundreds of billions of stars, each with their own unique and varied solar systems.
But beyond that — in addition to the stars — there are hundreds of billions of planets with no central stars at all: the rogue planets of our galaxy. We think this is true everywhere, from small star clusters to giant galaxies. As best as we can tell, there are at least as many starless planets wandering the cosmos as there are stars, meaning that for every point of light you see, there are probably more massive points that exist, but emit no visible light of their own.
We’ve recently discovered a number of possible rogue planet candidates, although since these are so difficult to detect (and are only visible from their heat signatures in the infrared) we know that there must be many, many more than what we’ve seen so far. You can’t help but wonder where these rogue planets come from!
One compelling source is near and dear to us all.
We know how solar systems like our own form: you get a central star with a protoplanetary disk around it. Gravitational perturbations in the disk attract more and more matter from their surroundings, while the heat from the newly formed central star gradually blows much of the lightest gas away into the interstellar medium. Over time, these gravitational perturbations grow into asteroids, rocky planets, and eventually — for the largest ones — gas giants.
The thing is, these worlds don’t just orbit their central star, they also gravitationally tug on one another! Over time, these planets migrate into the most stable configurations they can attain, and this usually mean the largest, most massive worlds migrating into their most stable configurations, often at the expense of other worlds.
A recent simulation shows that for every planet-rich solar system like our own (with gas giants) that forms, there’s likely to be at least one gas giant planet that gets kicked out, into the interstellar medium, where it’s doomed to wander the galaxy on its own as a rogue planet.
That’s almost definitely a major source of rogue planets.
But here’s the funny thing: when we work out the numbers of our best theoretical calculations, it’s far less than 50% of all rogue planets that are expected to be accounted for by this process. To figure out where the majority of starless planets come from, we have to look at a larger scale at around the same time: not just when the Solar System formed, but at the cluster of stars (and star systems) that all formed at around the same time!
Star clusters form from the slow collapse of cold, mostly hydrogen gas, typically within a galaxy. Within these collapsing clouds, gravitational instabilities form, and the earliest, most massive instabilities preferentially attract more and more matter. When enough matter gets together, and the densities and temperatures at the core of these clouds get high enough, nuclear fusion ignites!
This results in new stars and star systems, but something else happens, too. The biggest stars that form are also the hottest and bluest, meaning they emit the most ionizing, ultraviolet radiation.
So when you look inside a star-forming nebula in the Universe, you are actually watching two processes simultaneously competing:
- Gravity, as it attempts to pull matter in towards these young, growing gravitational overdensities, and
- Radiation, as it works to burn off the neutral gas and blow it back into the interstellar medium.
Who will win?
It depends what you mean by “win,” exactly. The biggest gravitational overdensities form the largest stars, but these are also the rarest of all stars. Smaller (but still large) ones form the other star types, but become more and more common as we get down to lower masses. This is why, when we look deep inside a young star cluster, it’s easiest to see the brightest (blue) stars, but they’re vastly outnumbered by lower mass, yellow (and especially red), dim stars.
The thing is, if it weren’t for the radiation, there dim, red-and-yellow stars would have grown more massive, brighter, and burned hotter! Stars (on the main sequence, which is most stars) come in a variety of types, O-stars being the hottest, largest and bluest and M-stars being the coolest, smallest, reddest and least massive.
Even though the vast majority of stars — 3 out of every 4 — are M-class stars, compared to less than 1% of all stars being O-or-B stars, there’s just as much total mass in O-and-B-stars as there are in M-stars.
And it turns out that some 90% of the original gas-and-dust that was in these star-forming nebula gets blown off back into the interstellar medium rather than forming stars. The most massive stars form the fastest, and then get to work blowing the star-forming material out of the nebula. By time a few million years go by, there’s less and less material to form new stars at all. Eventually, all the leftover gas-and-dust will burn off completely.
Well, guess what? Not only are M-class stars — stars between 8% and 40% of the Sun’s mass — the most common type of star in the Universe by far, but there are a whole lot more that would have been M-class stars if it weren’t for the high-mass stars burning off this material!
In other words, for every star that forms, there are many, many failed stars that didn’t quite make it; anywhere from tens to hundreds-of-thousands of them for each star that actually forms!
These nomad planets — or rogue planets — may or may not have atmospheres, and they may be incredibly difficult to detect, especially the (theoretically) more common ones: the smallest objects.
So we may have a few rogue planets that were ejected from young solar systems, and there may even be a couple that came from our Solar System, but the vast majority were never attached to stars at all! Rogue planets wander the galaxy, most of them to toil forever in loneliness, never knowing the warmth of a parent star, thwarted by stellar evolution from ever becoming stars themselves. What we have, instead, is a galaxy with trillions or possibly even quadrillions of these nomad worlds, objects which we’re only just beginning to discover. And that’s where rogue planets come from!