Rogue Planets

Image by NASA/JPL-Caltech

Rogue planets are not planets that hide in plain sight and backstab you. But I bet you want to know what they really are.

Rogue planets?

Th Earth is a planet, and most of us know about the other planets in our solar system, from Mercury to Neptune. Those planets have formed from the accretion disc of the sun billions of years ago, and will continue to circle the sun for billions more.

Other planets exist around other stars. The list of exoplanets we’ve found is ever-growing. Scientists only discovered exoplanets in the eighties and confirmed them less than twenty years ago. Now scientists have confirmed nearly 5,000 exoplanets already and they have several thousand more candidates.

However, there is a special category of exoplanets that we’ll talk about today, the rogue planet. A rogue planet is a planet that is not gravitationally linked to a star. In other words, rogue planets float freely through interstellar space.

How does that happen?

There are two ways that rogue planets come about.

The first is similar to the way any planet comes about: it forms in the accretion disc of a star. However, the interplay of gravitational forces can fling a planet from a star system. Like a car skidding off the road in a too tight turn. And from then on it continues on into interstellar space.

The second way rogue planets come about is in the same way that stars are born: a gas cloud collapses in on itself due to gravity and clumps together. Normally that forms a star, but if there isn’t enough mass, the result is a planet.

You might be wondering how often this happens. The problem is, it’s hard to even see a rogue planet, let alone count them. They give of some radiation, affect things passing by with their gravity and can obscure light sources. However, those three things are much smaller than what we see for stars, and we’re talking about massive distances. Luckily, there have been sightings. Scientists found a number of rogue planets. Recently, even a clump of 70 of them, the reason for this post, actually. That allows statistic analysis, with broad margins.

The numbers range from one rogue planet for every four stars, to about two rogue planets for every star. Some studies say even more. Given there are some fifty to two-hundred billion stars in the milky way alone, that gives tens of billions of rogue planets in our milky way alone. The universe houses some two trillion galaxies such as the milky way, so that gives you an idea of how many rogue planets are out there. A whole freaking lot.

Can they support life?

You’d think that a planet floating through interstellar space would be incapable of sustaining life, since there is no power source to heat it. You’d expect them to be frozen balls of gas and rock. That’s not always true, though. In theory, at least.

Since the planets are ejected from their star, solar winds will not strip their atmospheres like they normally would. That could lead the internal pressure of the planet’s atmosphere to generate heat. In theory, a rogue planet could be hot enough to have liquid water, and thus, to sustain life. But not life as we know it; well, life as we do know it, but the kind of life we find around geothermal vents deep in the ocean.

A second way life could happen, is on the moon of a rogue planet. When a rogue planet is ejected into interstellar space, a large moon could be ejected with it. If that moon keeps a stable orbit and contains water, the tidal forces from the parent planet could warm it enough for life to exist.

Still, nobody has found life on any exoplanet to date, let alone life on a rogue planet.

Can a star catch rogue planets?

The short answer: yes. Of course, in reality, this is a complicated feat of orbital mechanics. For rogue planets to find a new home, they have to pass close enough to a star, at sufficiently slow speeds. Then, the star can catch them. Of course, there is a good chance the planet simply passes through the star system and only deviates from its course, or that it ends up plummeting to its fiery demise in the star. Still, models show that planets will sometimes be caught by a star. Like with many unlikely things in the universe: there are so many stars it’s bound to happen, and more than you might think.

That said, the chances of a rogue planet passing through our solar system are small, and the chances of one being caught are even smaller. It’s could happen next year, but it could also never happen during the remaining billions of years that the sun burns.

So, don’t hold your breath. And if you find the idea of migrating planets fun, go watch the Wandering Earth on Netflix.

Martin Stellinga Written by:

I'm a science fiction and fantasy writer from the Netherlands