What Are Rogue Planets?

(Last Updated On: June 9, 2020)

What Are Rogue Planets?

What Are Rogue Planets?

If you heard the term “rogue planet” and leapt at the chance to see an Earth-sized rock committing galactic-scale crimes, you might be disappointed. That’s because rogue planets are not criminals (nor do they have any relation to a former Alaskan politician). Simply put, a rogue planet is a planet that travels through interstellar space without orbiting a host star. Think the Earth if it didn’t revolve around the Sun. And it turns out these rogue planets can be kind of wild–some might even harbor life. What do we mean by that? And how do rogue planets form in the first place? Let’s explore.

What Makes a Rogue Planet?

Well, for starters, you need to have a regular planet. Planets typically form around a star by way of accretion. Leftover material from the formation of a star clumps together over time to make really big clumps. These clumps become so large that they just end up being planets at some point. That, or these clumps formed independent of a star’s leftover stuff. The universe is super big, and anything can happen. It’s kind of like how with 8 billion people on Earth, you know there’s definitely at least 1 person who has done that weird thing you thought nobody has ever done. 

Planets that form without a parent star or celestial body to orbit start off as rogue planets. And fun fact, rogue planets have many alternate names. Some of these include: interstellar planet, nomad planet, free-floating planet, unbound planet, orphan planet, wandering planet, starless planet, or sunless planet. The latter two names really tell you all you need to know.

Before we talk about actually going rogue, you should probably know a key quirk to orbits. Orbit is just a strange way of falling or being flung outwards into space. You probably know that that’s how the International Space Station orbits Earth. It’s moving almost perfectly perpendicular to the direction directly towards Earth at any given time, and so doesn’t get to fall straight down–yet. 

The Moon, on the other hand, is actually slowly leaving Earth. Probably because humans don’t make for great celestial neighbors.

Going Rogue

So as you’ve probably figured out, part of “going rogue” as a planet is just getting flung off into the void of space by happenstance. Other times, another body in a solar system can slam into a planet, pushing it far enough out to leave the orbit of its home star. 

Stars also move around a lot in space, and as such can get pretty close to one another. When this happens, gravitational imprints the stars have start to mess with each other. Sometimes they orbit each other and you get binary stars. They’re as cool as they sound. Though this is a super unlikely way to form a binary star system, but that’s neither here nor there.

But, when stars start messing with each other, even if they pass without ever making contact, any planets get caught in the crossfire. At that point, all bets are off. They can swap parent stars, collide with other planets and be destroyed, or be flung off into space. Thus, rogue planet.

There’s also a supernova, which could either incinerate a planet or slingshot it into the cold blackness of space. We’re not sure we’d like those odds.

Rogue Planets and Life

Alright, we’re not going to make the claim that rogue planets harbor life. Though there could be more than 100,000 times more rogue planets than there are stars in the Milky Way–which already harbors hundreds of billions of stars. So statistically speaking, it’s probably not a 0% chance.

You may or may not know that some planets are really hot on the inside. Earth is one of them, with a molten iron core estimated to be like 6,000 degrees Celsius.

To have life, our hypothetical life-bearing rogue planet would need some system of energy for their critters to access. On Earth, over 99% of that comes from the Sun. But we actually do have the perfect case for life that exists independent of the Sun. Stuff deep in the ocean rely on hydrothermal vents for their heat. Those ecosystems, then, get all of their energy from the Earth’s core rather than the Sun’s rays. Thus, we actually can have an ecosystem that can exist independent of a home star.

If you do some chemistry shenanigans, you can even retain enough heat on a planet to keep liquid water! Tidal forces offered by moons are another way to get energy into a planet.

Our most likely candidates for rogue planets harboring life are balls of ice. A thick glacial sheet with an ocean underneath would basically give you what we have deep near the Earth’s seabeds. It’s not like celestial bodies like this are unlikely–we see them in our own system! Jupiter’s moon Europa is a prime example. 

Imagine being an intelligent species on one of these planets though. You’d probably never know there was an entire universe beyond your kilometers-thick ice wall.


You might know the planets, but what about the planets by size? Test yourself, here.

About the Author:

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Kyler is a content writer at Sporcle living in Seattle, and is currently studying at the University of Washington School of Law. He's been writing for Sporcle since 2019; sometimes the blog is an excellent platform to answer random personal questions he has about the world. Most of his free time is spent drinking black coffee like water.