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Our Solar System Might Have An Extra Planet
But the chances are slim.
You might find this hard to believe, but some astrophysicists think we might have an undiscovered planet in our solar system. This “Planet 9” theory has existed for decades and has divided the astronomy community. Over the years, there have been tantalising glimpses of this mysterious object, if it is there at all. But a recent paper, which looked to calculate the possibilities of Planet 9’s origins, has re-sparked this debate. So, why do astrophysicists think Planet 9 exists? And what does this new research tell us?
So, why do some scientists think we have an extra planet? Well, objects in orbit aren’t isolated from each other; they pull and tug on each other, affecting their orbital paths. As such, you can infer the existence of an unseen planet by its effect on the other’s orbit. In fact, this is how Neptune was discovered. Uranus’s orbit had weird perturbations (changes) that couldn’t be accounted for. Using newtons universal law of gravity and some clever maths, 18th-century astronomers took these perturbations and calculated that they must be caused by a similar-sized planet further out. They were even able to calculate where in the night sky they should be able to find it. Loe and behold, when they looked there, they found Neptune!
Similarly, there are much smaller perturbations in the orbits of Uranus, Neptune and many large Kuiper Belt objects (like the asteroid belt but orbiting way past Pluto) that suggest the existence of an unseen planet. This “Planet 9” is again about the same size and mass as Neptune, but it is orbiting much further out at 400 to 800 AU, compared to Neptune’s 30 AU (AU is the distance from the Sun to Earth). This places it firmly in the Oort Cloud, a spherical cloud of icy debris at the very edge of the solar system that marks the boundary to interstellar space. This far out, very little sunlight would reach Planet 9. So even though we can calculate where in the night sky it should be and try to observe it, we wouldn’t be able to see it, even with modern telescopes, as it is so dim.
That isn’t to say we haven’t had glimpses of it. When trawling through infrared astronomy data from the 80s, researchers found what looked like a possible Planet 9 candidate (read more here). However, the size and location of this candidate didn’t quite match up to the orbital data. This is perplexing, as the infrared data should be able to detect Planet 9 as it is looking for its thermal fingerprint, not its reflected light (in the same way thermal imaging can see in the dark). This led some scientists to postulate that Planet 9 is actually a watermelon size primordial black hole, and there is even some visual data of possible gravitational lensing events to back this claim up (read more here).
Despite these theories and small pieces of evidence, many astrophysicists doubt Planet 9’s existence. Why? Recent studies using more modern telescopes of distant “extreme Kuiper Belt objects” don’t show these same orbital perturbations, which we think they should display if Planet 9 exists.
To try and solve this debate, a team of international researchers calculated the statistical possibility of two origins of Planet 9: solar and extrasolar.
Simulations of how our solar system evolved from birth suggest that long ago, we did have an extra planet about the size of Neptune. But thanks to complex gravitational interactions with Jupiter and Saturn, it was ejected from the solar system. This event took energy out of Jupiter’s and Saturn’s orbits, enabling them to settle in their current stable orbits and, in turn, ensured the Earth stayed in the Sun’s habitable zone for billions of years. This ejected planet was likely about the same size as Planet 9, so this team suggested that rather than being fully ejected, it was sent into the far distant orbit we think Planet 9 is in today. This is the possible solar origin of Planet 9.
So, what about extrasolar? Well, it isn’t just our star system that kicks out entire planets; scientists think this is quite a common occurrence. As such, our galaxy is awash with so-called “rogue planets” meandering through interstellar space. It is possible that Planet 9 is just a rogue planet from another star system that fell into orbit around the Sun later on in the solar system’s evolution.
To calculate the likelihood of both origins, this team ran numerous simulations. They found that one in every 200–300 stars could host an Oort Cloud planet. So, if Planet 9 does exist, it would make it extremely rare. In fact, the team thinks this is an overestimate, as their simulations didn’t consider instabilities in the host star’s birth cluster (the star-forming region that birthed the star) or the planet-stripping potential of nearby passing stars that could knock an Oort cloud planet into interstellar space. Sadly, this fact alone makes Planet 9’s existence very unlikely.
But, this estimates how common Planet 9-like objects are in the galaxy, not how likely it is in our solar system.
So, they ran some more simulations and compiled a lot of data to figure that out. They found that the solar origin possibility of Planet 9 is only 0.5%. This is because this ancient ejected planet was thrown out with such force that it would really struggle to fall back into orbit.
In contrast, they found the extrasolar origin to have a 7% likelihood. However, this is only if the ejected planet and orbital instability events happened after the Sun’s dissolution from its birth star cluster (i.e. moved away from its brother and sister stars and got its own space), and we don’t yet know if that is what happened. If these events happened the other way around, the extrasolar origin chances seem far lower.
So, while this indicates that there is still a chance for Planet 9 to exist, it is vanishingly thin.
But why does this matter? Well, our star system is the only one we know of that host’s life. What’s more, as we survey other star systems and exoplanets, we find that our set-up (rocky planets next to the Sun and gas giants further away) is scarce. If we can understand how our solar system evolved and what key events helped ensure the Earth has been habitable for so long, then we can use this knowledge to search the sky for a genuine Earth 2.0 and help find our cosmic neighbours.
Also, if Planet 9 exists and has an extrasolar origin, as this research suggests, it is an astonishingly unique opportunity to study an exoplanet up close. With modern technology, it would take hundreds of years for a probe to travel to an exoplanet and study it up close, making it basically impossible. But it would only take a few decades to reach Planet 9, allowing us to study and understand the difference between our planets and exoplanets.
So, Planet 9 looks less and less likely, but that doesn’t mean it doesn’t exist. There is still a tantalising possibility of an entire undiscovered world at the very edge of our solar system; that fact alone is mind-blowing! This goes to show just how little we really know about the universe and how much is waiting to be discovered out there.
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