What If the Solar System Started Losing Planets? | Unveiled

What if the Solar System Started Losing Planets?

Over the years, we’ve discovered and added new planets to the solar system, and demoted one from planet status. But we’ve never really lost a planet. However, all those rogue planets in the galaxy have to have come from somewhere. So, would it ever be possible for one of the solar system worlds to just up and leave?

This is Unveiled, and today we’re answering the extraordinary question; what if the solar system started losing planets?

Interestingly, there is some evidence that the solar system may have lost planets in the past. Where the inner solar system is concerned, NASA scientists have suggested the existence of “Planet Five”, a fifth terrestrial planet that may have once orbited between Mars and the asteroid belt. The theory surrounds the “Late Heavy Bombardment”, a celestial event that happened roughly four billion years ago. It's technically hypothetical, and remains disputed, but many scientists believe that there were a significantly higher number of asteroids striking the inner planets at this time. Much of the evidence used to support the idea of the “LHB” comes from material collected by the Apollo program in the 60s and 70s. Proponents believe that this fifth planet had an orbit so eccentric and unstable that it eventually wandered into the asteroid belt, and disrupted it so dramatically that it actually caused the Late Heavy Bombardment. In this case, then, the disappearance of a planet may have triggered a higher-than-average number of asteroid impacts, but ultimately left the solar system a more stable place than it was before. That’s IF this particular lost planet existed at all.

There’s another theory about a lost inner planet, known as “Phaeton”, although it’s less credible. It’s rooted in the Titius–Bode law, formulated in the 18th century, which held that each planet should be twice as far from the Sun as the one before. When the dwarf planet Ceres was discovered in the asteroid belt, it was thought to be the “missing planet” predicted by this law. However, the discovery of the asteroid Pallas in around the same position led to the theory that Ceres and Pallas represented fragments from a planet that had been destroyed long ago, creating the asteroid belt. There are various theories about how this destruction occurred. Some say it was torn apart by the gravitational influence of Jupiter. Others that it collided with another celestial body. Whatever happened, it supposedly left enough debris to create the asteroid field.

However, there are some major flaws with this theory. Because the asteroid belt is actually tiny. It’s spread over a vast area of space, but the amount of mass there is minuscule. In fact, the total mass of the asteroid belt is equivalent to just 3% of the moon’s. That means that even if it was all put together, you’d have a celestial body 97% less massive than the moon is. That’s significantly smaller than even Pluto, although it WOULD still be big enough for dwarf planet status. Altogether then, while it’s not totally beyond the realm of possibility, most scientists agree that the asteroid belt was probably born in a different way - without another lost planet.

There are similar ideas pertaining to the outer solar system, too, however - including that there was once a fifth gas giant. Some cosmological simulations of the early solar system predict that one of the four gas giants should have been ejected, rather than all of them settling into stable orbits. And so, as we still have Jupiter, Saturn, Uranus, and Neptune, some have suggested that there must have been another gas giant there at some point - one that was ejected. If the models ring true, it will have been sacrificed to ensure the stable orbits in the solar system today… again meaning that the loss of a planet may have actually benefited our system as a whole.

The problem is that in each of the above cases, the potential disappearance of a planet hasn’t left behind enough traces for us to prove that it did happen… and, so we may never know. But today’s question can also be viewed from the other side; i.e., what if one of the eight planets we have now were to disappear?

First, we have to think about HOW this could happen. A collision is the most obvious answer, but for that to work, you’d need a big enough impact object. An object large enough to destroy an entire planet would likely have to be another planet. Potentially, for example, two gas giants colliding could merge into one huge one. If this happened in our solar system, say between Jupiter and Saturn, we’d have one less planet but no real change in the amount of mass. So, gravitationally speaking, it wouldn’t affect us all that much. The solar system might even keep the same number of moons - more or less. Failing that, there’s also the possibility that a particularly fast collision between Jupiter and Saturn would destroy both of them, and likely also their moons, leaving us down two planets rather than one. But we - on Earth - would still be orbiting the sun quite peacefully.

Another way that a planet could disappear is if it were to fall into the sun, but physics makes this event unlikely. We know that, really, Earth (and all planets) are actually falling into the sun all the time… it’s just that they do it quickly enough that their orbits remain stable and they (and we) never actually get pulled in. Another way of thinking about it is that, throughout the year, we know the Earth slowly gets nearer to the sun and then further away again… but that’s a normal part of having an elliptical orbit. If something were to destabilize us – for instance, another collision – then we potentially could fall in and burn up. But the problem is that any object big enough to knock Earth out of orbit would also shatter it into pieces in the process, so Earth wouldn’t really exist anymore TO fall into the sun.

Finally, a wandering black hole could (theoretically) start destroying our planets, as well, but much would depend on its size. Wandering “orphan” black holes do exist, and many are massive enough, but they’re also not all that close to us... as far as we know. In 2022 news broke that one had been discovered 5,000 lightyears away, but this is still extremely far. It’s possible that there are others that are closer, but remain undetected for now… and if one did roam into the solar system, then its mere presence could cause major disruption. It could potentially swallow up planets completely. But again, in the grander scheme of the health of the solar system, the disappearance of planets in this case would really be but one aspect of an exceptionally difficult period in total.

So, what’s the takeaway here? Well, in a sense, we have actually lost planets before… just not literally. For decades after its discovery, Pluto was considered a planet, only to be eventually demoted to a dwarf planet after many more Pluto-like objects were found beyond Neptune. Ceres, too, was once deemed a planet, only to be reclassified twice, first as an asteroid and then again as a dwarf planet. Perhaps the biggest result of a planet actually, physically disappearing, however, would be that humanity would have to grapple with the loss. These worlds have all existed in our skies for billions and billions of years. They’ve been here far, FAR longer than humankind has… and so, for them to one day simply vanish would be at once an extremely sad moment and also a potential trigger for existential crisis. How do you think people would cope if a planet was here today and gone tomorrow?

There are ways in which it could occur via collisions and black holes, although they’re unlikely. There are also theories that it actually has taken place before, in the ancient past. For now, it’s true that the solar system never stands still… but that’s what would happen if it were to start losing planets.