What If Another Planet Joined The Solar System?

VOICE OVER: Noah Baum WRITTEN BY: Caitlin Johnson
There are 8 planets in the solar system... But what if you woke up tomorrow and there were 9? The solar system is very finely balanced, with every planet orbiting its central star, the sun. But, if another planet suddenly showed up, everything could be thrown into total chaos.

What If Another Planet Joined the Solar System?

Some facts seem concrete: water is wet, the sky is blue, and there are eight planets in the solar system. But what if that last one suddenly wasn’t true?

This is Unveiled, and today we’re answering the extraordinary question; What if another planet joined the solar system?

In recent years, the exact definition of “a planet” has been trickier to pin down. Pluto’s reclassification in 2006 saw it rechristened as a ‘dwarf planet’ - having been previously thought of as the ninth planet since it was discovered in 1930. There was an effort, also in 2006, to regain Pluto its original status by broadening our definition of what a planet was... But the move, which would’ve also seen Charon, Xena, and the large asteroid Ceres promoted to planethood, never came to fruition.

For today’s question though, we’re not so concerned with the various bodies that we already know to exist, which could suddenly become a planet if we changed our definition. Instead, we’re imagining that a true planet - one that conforms to today’s understanding of what a planet is - suddenly shows up. Today, we know that there are thousands more Trans-Neptunian Objects out there other than Pluto - so could there really be another world hiding just out of sight in the solar system?

There are theories that there at least was another planet, way back in ancient history. This mysterious hypothetical is nicknamed Planet V, under the assumption that it was once the fifth planet from the sun, operating as an additional inner planet in between Mars and Jupiter. The idea is that Planet V left our solar system around 3.8 billion years ago, with apparent evidence of its departure found on the inner planets and our own moon - which have similar impact craters dating back to that time.

According to Planet V advocates, it’s statistically unlikely that all of those craters could have been made in the same period without a specific event causing them. That event is now known as the Late Heavy Bombardment, where the inner planets were pelted with asteroids. We know LHB happened, but we’re not sure why. So, perhaps it was set off by Planet V, which for some reason found its orbit disrupted before either crashing out of the solar system via the asteroid belt; colliding with another planet; or heading straight into the sun. Any of those outcomes would’ve caused major upheaval in the solar system, and that upheaval was the Late Heavy Bombardment. Or, so the theory goes, anyway.

Back in the present day, the search for the so-called Planet Nine continues, and is in fact linked to the previously over-zealous discovery of Pluto. Because, Pluto was found when assumed discrepancies in the predicted orbits of Neptune and Uranus were attributed to another planet being nearby. Now we know that Pluto isn’t that planet, and we also believe that those orbital blips in Neptune and Uranus may have been miscalculated… But, astronomers currently have their eye set further afield, looking at similar problems in the troublesome orbits of Trans-Neptunian Objects in general.

In a bid to make sense of the chaotic cosmic landscape beyond Neptune, some onlookers believe the elusive Planet Nine could still exist, and there’s even a NASA project set up to encourage amateur stargazers all around the world to try and find it. If it exists where the experts think it might, planet Nine is most likely a Super Earth with about ten times more mass than our own earth and situated around 600 times further away from the sun - which would go some way to explaining why we haven’t actually seen it yet.

So, that’s Planet V and Planet Nine. But, arguably the most popular extra planet theory is the Nibiru theory, or Planet X. Actual evidence to back this one up is thin on the ground, but that hasn’t stopped it from gaining believers.

It’s essentially a doomsday prophecy that an alien world, Nibiru, will one day crash into Earth and destroy it. It was born out of the 1976 book, “The Twelfth Planet,” in which amateur astronomer Zecharia Sitchin pitches ancient Sumerian texts to apparently ‘prove’ Nibiru. From there, the idea got conflated with a story from a psychic, Nancy Lieder, who, in 1995, claimed aliens had told her that Planet X would wipe out humanity in 2003. When that didn’t happen, Planet X found itself pedalled as one of the 2012 apocalypse scenarios… and when that didn’t happen the date of collision was recalculated to 2017.

Clearly, the Niburu theory doesn’t have a great track record in terms of ‘coming true’. NASA has frequently spoken out against it, but the idea – which many still believe – does tap into our darkest concerns about what might happen if a new planet did join our corner of space.

Rogue planets are a very real and potentially dangerous thing. These are planets not attached to a star system, drifting aimlessly through space searching for a new home – if some interpretations of the Planet V theory are true, it could even be one of them. It’s not impossible that one such planet could get pulled in by the sun’s gravity, and if it was big enough – that is, bigger than Jupiter – it could pose monumental problems for the orbits of everything else in the solar system. The paths of most planets could stretch and become more elliptical, or just generally unpredictable, which would be bad news for Earth.

Such a change would most likely pull our planet out of its stabilising, life-supporting goldilocks zone. We’d suddenly face extreme winters and summers which, in just a couple of years, would turn our world into an uninhabitable husk of rock. But, what’s even worse is that we’d also be totally trapped. Even if the arrival of another dominant planet happened centuries or millennia in the future - at a time when humankind may have perfected commercial space travel - we wouldn’t be able to escape the consequences by planet-hopping elsewhere in the solar system, because the same thing could well be happening everywhere.

Say the planet was big enough to ruin established orbits, but still wasn’t able to find an orbit of its own, though. It’d plummet into the sun, potentially pulling us and others with it, and placing everything around our central star at the mercy of one very nasty solar flare. Of course, on its way it could also outright collide with Earth, obliterating us completely. Or, it could significantly disrupt the Oort Cloud, the Kuiper Belt or the Asteroid Belt, or crash into another terrestrial body, all of which would send other enormous projectiles flying through the skies.

Of course, all of this is a worst-case scenario. Like the rest of space, the solar system is mostly very empty, so there’s every possibility that even if a rogue planet did suddenly show, it’d simply drift in, possibly disturb certain orbits slightly, and then move on again peacefully - so long as it keeps far enough away from the sun. In fact, given that we know so little about what goes on out in the Oort Cloud, those very processes could be happening right now. And that’s what would happen if another planet joined the solar system.