Why Hasn’t a Rogue Planet Destroyed the Solar System Yet?

Everything in the solar system is finely balanced. Planets orbit the sun; moons orbit the planets; asteroids speed through space. And, thankfully for us, Earth’s particular position is just about right to allow for life to thrive. But will it always be this way? What would happen if, one day, this supreme, cosmic balance just… disappeared?

This is Unveiled, and today we’re answering the extraordinary question; why hasn’t a rogue planet destroyed the solar system yet?

Rogue planets are the rebels of the universe. While billions of other worlds in trillions of other galaxies find themselves gravitationally bound to a particular orbit around a particular star, rogue planets unashamedly avoid all of that. They move through space of their own accord and have therefore proven fairly difficult to monitor in the past… but we’re today learning more and more about them.

It’s thought that a rogue planet is most likely to have originally formed as part of a regular star and planetary system, before being ejected out of that system for a number of reasons - including, for example, a powerful enough collision with another astronomical body. However, rogue planets can form independently of a star, too, in some cases, and can exist never having been gravitationally tied to anything. The first strong, rogue planet candidates were registered around the turn of the millennium, with the majority being officially charted in the years since 2010. But, still, only a small percentage of the almost five thousand confirmed exoplanets out there are believed to be rogue. There are various studies tabled for the future that should reveal many more of these interstellar enigmas to us, though. And the early predictions are that we’ll soon discover how rogue planets are actually very common.

The question of atmosphere on a rogue planet is another that scientists are now busy getting their heads around. At first, it might seem that they should be cold, stark and empty worlds… because without a star to provide them with energy, or to inspire a regular orbital period, what prevents these planets from becoming just bigger-than-average asteroids? It all depends on their history. On how far, and in which ways, they developed before being pushed out. As far back as in 1999, a study was published in “Nature”, by the planetary scientist David J. Stevenson, to suggest that rogue planets could maintain hydrogen-rich atmospheres. And that, in doing so, they could theoretically retain heat, enough even for bodies of liquid water on the surface. Again, it all depends on how they evolved before being ejected from their star system.

Meanwhile, a headline-making 2011 study, by astrophysicists Dorian Abbot and Eric Switzer, titled “The Steppenwolf: A Proposal for a habitable planet in interstellar space”, arrived at a similar conclusion. It found that a rogue planet with various Earth-like qualities, that’s also roughly 3.5 times more massive than Earth is, could maintain at least a subglacial liquid ocean. Ultimately, then, rogue planets could have their own part to play in our search for habitable worlds and perhaps even alien life. Although they do still remain fiendishly difficult to spot… 1) because of the lack of light that these sunless bodies emit; 2) because of the fact that they don’t pass in front of a parent star, so astronomers can’t chart them that way, either; and 3) because compared to conventional planets, orbiting around a star… they’re journey is unpredictable. That unpredictability, that unknowable-ness, is a key part to their nature, though, meaning that they’re often assumed to be hidden in plain sight… and, therefore, a potential threat.

So, how many of these things could there actually be? And how big is the threat, really? Well, according to the implications of one 2019 study, titled “Survivability of planetary systems in young and dense star clusters”, there could be millions (maybe billions!) of rogue planets in just our Milky Way galaxy. Led by astronomers at Leiden University in the Netherlands, the study ran simulations on the future of the Orion Trapezium Cluster of stars, which is about 1,300 lightyears away from Earth. It calculated that almost three hundred planets from within its sample would eventually break away from their star - making them rogue planets. Various analysts have since applied these results to the rest of the galaxy, with fifty billion being the most often-cited total number of potential rogue planets there are. And remember, that’s fifty billion in just the Milky Way galaxy, an average-sized galaxy in a universe with billions more just like it. And suddenly, our general understanding of the universe existing in perfect balance begins to fall apart… with so many of these errant worlds zooming through it and breaking the rules!

Astronomers remain divided on exactly how concerned we should be about rogue planets, though… with the majority consensus tending toward not that concerned at all. The fact remains that the galaxy is huge, and easily huge enough to contain fifty billion planets on the loose. In some ways, the phenomenon can be approached as we do with the question of black holes. We know black holes exist… and in the twenty-first century, we know there are examples of them far closer than we had previously thought. But most scientists and astronomers assure us that the chances of one ever devouring the solar system remain seriously low. Space is just too massive, no matter how many rogue planets (or black holes) there are predicted to be.

And, ultimately, even if a rogue planet did (against the odds) enter the solar system today, it needn’t be the end of the world. The solar system, too, is mostly empty space, and the chances of any one object colliding with another are just not very high at all. Even in the asteroid belt, which is typically painted as being crammed full of space debris, there can be literally thousands of miles of nothingness between any two asteroids. What’s most likely to happen with a rogue planet, then, is that it would enter the solar system, be spotted moving across our skies, and then leave on its merry way through the interstellar realm. No harm done.

In the unlikely event that it did crash into another object, however, then we could see massive, possibly devastating changes to our star system. Even if it doesn’t necessarily exist in the universe as a whole, the balance that certainly does exist in the solar system could be shot to pieces. Planetary masses could potentially be altered, orbits could be changed, and a cosmic, catastrophic domino effect could take hold. In fact, even without a direct collision, a massive enough rogue world might still serve to disrupt how the planets of our star system move around the sun, resulting in much the same level of carnage. Earth could be pushed closer to our star, or tugged further away from it… and neither of those scenarios is good for the continuation of life. But this really is a worst-case scenario, and not at all likely to actually happen.

The chances of a rogue planet entering the space between the sun and Neptune, the solar system’s outermost planet, are very low. And the chances of one doing so and then crashing into something else are even lower. So, you can sleep easy. And, what’s more, we can find further relief in how science is hoping to track rogue planets more closely in the future. The long-term hope is that if there ever was something approaching us from outside the sun’s influence, then we’ll know about it long in advance.

Much of what we discover in the years to come could be down to NASA’s Nancy Grace Roman Space Telescope. Also known as the Wide-Field Infrared Survey Telescope, it’s been in development since 2016 and is currently scheduled to launch in 2025. Planned to start operations just four years after the James Webb Space Telescope, the Roman will peer through space in near-infrared, revealing the universe to us in greater detail than ever before. Roman will have particular capabilities, though, including that it’ll be built to monitor gravitational microlensing - which involves the most intricate of changes and distortions in space, caused by the movements of comparatively small objects like rogue planets. According to an August 2020 article by NASA, Roman’s microlensing survey will “detect hundreds of rogue planets, even though it will search only a relatively narrow strip of the galaxy”.

So, it seems as though we are destined to find out more about these mysterious, untethered worlds. For now, the message is to watch with interest, but not with fear. Yes, there could be billions of rogue planets inside our galaxy, but the chances of any one of them ever crossing our path are slim. And that’s why a rogue planet hasn’t destroyed the solar system yet.

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