Artists impression of a rogue planet — WikiCC

For a while now, astronomers have known that rogue planets wander through the galaxy unbridled by any star. At first, they seemed like a mere peculiarity of the universe; a nice-to-know-about fact. But a recent paper has suggested that we should consider taking these elusive objects far more seriously, as they could harbour advanced alien civilisations on the lookout for a new home planet. In other words, less “rogue planet” and more “mega alien spaceship that would dwarf the Deathstar.” So what makes rogue planets such an ideal form of transport for these extraterrestrials? And should we be worried about them?

Before we dive into the topic of alien lifeforms, we first need to understand what a rogue planet is, where they come from, and how they move through the galaxy.

A rogue planet is an exoplanet that has been ejected from its host star. This can either be through complex gravitational interactions, such as a slingshot interaction with a gas giant, or by a supernova pushing an outer planet away. In fact, astronomers now think that Jupiter’s immense gravity slingshotted a planet the same size as Neptune out of our solar system around 4 billion years ago.

To leave a gravitational body (such as a planet or a star) and no longer be on its surface or in orbit, you need to reach escape velocity. This is where the kinetic energy (Ke = 1/5*m*v²) of the exiting object is equal to (or higher) than its gravitational potential energy (Ge = m*g*h). If you do the maths, the escape velocity speeds tend to be incredibly high. For example, Earth’s escape velocity (from sea level) is 25,000 mph, and escaping the Sun from Earth’s orbit takes an escape velocity of 93,000 mph!

Now, these rogue planets may have come from a more giant star than out Sun or originate from a closer orbit, which would require even higher escape velocities. The few rogue planets that have been observed are all moving through space at tremendous speeds, some as high as 62,000 mph. But this speed is in relation to the surrounding stars, which is very important.

You see, stars orbit the centre of the galaxy in a giant glob, all moving at similar speeds and in the same direction. This is why our Sun is orbiting the galaxy at an astonishing 479,000 mph yet has next to zero chance of bumping into another system, given the other stars are also moving at similar speeds and in a similar direction. But because rogue planets have been forcibly ejected from these moving stars, they can catch up to stars or cross the flow of stars. This means rogue planets have a high chance of intercepting star systems and can travel between them relatively quickly as they shoot through the galaxy.

This is why a recent paper from Cambridge presented a hypothesis that advanced aliens may be using these rogue planets to move across the galaxy, as they are far more useful than any interstellar spaceship they could build. Let me explain.

Even for super-advanced civilisations, getting from one star system to another is an arduous journey. It will take thousands, if not millions of years, which means multiple generations will have to live on board. Furthermore, the ship needs all of the raw materials and fuel for those onboard to live safely through the depths of space, and to be able to disembark or slow down when they get to their destination. Such a vessel would be monumentally hard to build!

The other alternative is that they could hop onto a rogue planet that is going in the direction they want to go in. Some of these planets would be Earth-like and contain all the necessary materials and nutrients to allow life and civilisation to flourish. Sure, while they were in space, there would be no sun to power photosynthesis or heat the planet, but there would be hydrothermal vents which could support life, and nuclear or fusion power to heat themselves, provide power, and even grow food using artificial light. Then, once the rogue planet intercepted the star system they wanted to get to, they could simply build a load of rockets and leave.

Theoretically, this is a far easier method of traversing the cosmos than building your own ship from scratch. I say “in theory” because we don’t know enough about rogue planets yet to know for definite. As they don’t orbit a star, they are very dim, and we can’t infer their existence through the motions or brightness of stars like we would an exoplanet. Instead, we need to trawl through thousands of images taken over decades by different telescopes to find chance alignments with the rogue planet and background stars. Currently, we see them as a series of background stars in a line, slowly dimming and brightening one by one as the rogue planet passes by. But this means we don’t know how many of them are out there or their range of sizes, both of which are crucial pieces of information that could make or break this hypothesis.

For example, if there are too few rogue planets, they won’t intercept star systems frequently enough for an advanced civilisation to be able to use one. At the very least, the planets need to intercept star systems every few hundred thousand years or so. But if the number is closer to every few billion years, then the chances of them finding a rogue planet that will intersect their system, be aligned to where they want to go, and have the correct composition and size before their star dies or they, as a species, are wiped out are practically zero. Astronomers believe that there are billions, if not trillions, of rogue planets in the Milky Way, which should be more than enough for our aliens to take their pick, but because they are so hard to spot, we don’t yet know this for certain.

Even if the number of rogue planets is high enough for frequent intersections, size may still be a problem. You see, the vast majority of the rogue planets we currently know of are Jupiter-sized, which means they are gas giants and utterly inhospitable to life, even for really advanced aliens. This is likely due to the limitations of our observational methods, as we can’t see smaller rogue planets. But there could be some form of natural pressure, such as common exoplanet orbital set-ups that mean there are far more rogue gas-giant planets than Earth-like planets. After all, we have found evidence that our own system kicked out a gas giant. So if there are no Earth-like rogue planets, then this hypothesis can’t happen.

But it is still worth taking seriously. Organisations like NASA and SETI regularly scan stars to see if they have technosignatures. These are the breadcrumbs of artificial radio waves and artificial lights that tell us that a star system or exoplanet has an advanced civilisation. Sadly, the scientists have yet to find such tell-tale signs of aliens, but maybe if they pointed their hypersensitive telescopes at rogue planets, they would have better luck? We can’t restrict our search to pristine Earth-like copycat exoplanets, as it would mean that we would accidentally miss a massive number of potentially habitable places. So, who knows, one day we could find cosmic hitchhikers riding a rogue planet to a new home. Let’s just hope they are friendly or, if not, willing to avoid our pleasant corner of the cosmos.