Astronomy Topic: Planet 9?

Planet 9: A Theory

Howdy again Blog.

Today we are going to be discussing a theory that explains the unusual orbits of a dozen or so Kuiper Belt Objects (also called Trans-Neptunian Objects).

When I was growing up, there WERE 9 planets, the 9th being Pluto (at that point, it was a smudge). In my lifetime, however, the advancement of telescopes has allowed for many other objects in Pluto’s neighborhood to be discovered (including one larger then Pluto). Astronomers would then come to classify Pluto as a Dwarf Planet, and a part of the Kuiper Belt (a large area filled with icy bodies in distant orbits towards the outskirts of the solar system).

This all seemed well and good for a decade or more, until a whole bunch of KBOs had been identified, and their individual orbits started to be mapped and computed. Advanced software would then make lengthy computations to model the orbits of the objects in the solar system in an effort to study their development and interactivity. That practice of modeling, in and of itself, has been going on for a long time, but the greater the number of actual objects observed, the more “accurate” the simulation becomes.

Albeit, given we don’t understand every little thing about gravity, and interactivity… but we DO get quite a bit of it. We now make space probe flight patterns that exploit the gravity of nearby planets to increase acceleration at a fuel-expense minimum (gravity-assist). It can be said we understand the mechanics of that sort of gravity interactivity fairly well, which leads us to the next point of this progression.

With that in mind, the Astronomers who observed the Kuiper Belt Objects found a few of them that had atypical orbits in contrast to the vast majority of the others. Their orbits all had some common elements though, they were elongated in a similar direction, and had corresponding axial tilts which had clearly been subject to significant gravitational forces at some point. Computers are pretty fucking handy it turns out, and after punching all these orbits into the simulation they asked the computer what could have caused these objects to have the orbits they do.

Initially, I was personally of a mind that it could have been possible that a rogue star had passed through the area nearby our solar system, and the gravity disturbance caused the orbits to be the way they are… but that was also disproven quickly because of the fact that there is no star nearby enough to be a culprit for such a crime. Also, the timeline was wrong, as the orbits of the KBOs had not been perturbed all at once. The passing star theory dies there, as it could not explain several additional observed orbital features, and the axial tilt of these TNOs.

The computer chewed on the data, but gave the Scientists an answer they were not at all expecting. The computation showed a high probability of something large lurking in the deep dark of the outer solar system. This unknown object’s gravity was tugging Kuiper Belt Objects out of their natural orbits. In addition to the projection of a whole new planet, the model also explained a few more unusual KBOs that the Scientists had no previous explanation for.

When the complete data set for all known objects in the Kuiper Belt was combined with the existing model, there were 3 objects that fell into the perpendicular orbits predicted by the simulation. The computer had known these objects were there and why before the scientists did. The data pointed, convincingly, to a big planet somewhere out there. But where?

The software projected that Planet 9 would have an orbit with a perihelion of 200 Astronomical Units and an aphelion of 1,200 AU. Those distances are truly insane, considering our farthest planet, Neptune, is 30 AU away at its longest point (aphelion). Oh, 1 AU is a little over 9 million miles.

Right now, a sky survey done from a space telescope a few years ago did not reveal planet 9 anywhere nearby, which might mean that it’s nearing the distant end of its orbital path (considering it takes a projected 20,000 years to complete 1 orbit of the sun). If it is out there, it is going to be difficult to spot. Another question asked by the scientists was: how big was this thing? Alarmingly, they found the object was AT LEAST 10 times the mass of our planet Earth. So, big. There is a type of planet that scientists have observed in extraterrestrial solar systems, called Super Earths, which are extraordinarily large rocky planets of equivalent mass to the suspected Planet 9.

Theorizing about what sort of planet it might be has, in and of itself, a distinct fork in the logic for existence and formation.

Formation Theory 0: Natural Orbit

The first though was: oh well it must have just formed out there, and we’ll update our models to compensate for that. However, this neglected the severe tilt to the solar plain, as well as the fact that there is not enough material in the outer reaches of the solar system to form a 10 Earth mass planet. This logic train stopped right here, and was abandoned.

Formation Theory 1: Rogue Planet

Since the object has such a severely elliptical orbit, and a 30 degree tilt to the plane of the solar system, there is a possibility the world we now call Planet 9 was a rogue planet. It may have been thrown free of its original parent star eons ago, and was just fortunate enough to pass close enough to the sun to be captured in its current bizarre orbit. There are models that grapple with this possibility, but nothing conclusive.

Formation Theory 2: Flung Giant

If our current model for the size of the accretion disk and the overall substance of the gas cloud that our star formed in is wrong, then maybe there are some processes at work that we do not understand. In extrasolar planetary systems, sometimes large Gas Giants are very close to their stars, in a likely consumption of the inner planets as gravity drew it closer and close as the millennia went by. Possibly?

Now we think about our own solar neighborhood… was its past more violent than we have previously theorized? Astronomers continue to work models that show Planet 9 may have developed as a Gas Giant, and been flung out of the solar system but still captured by the fringes of the sun’s influence. While the variables remain largely unknown, the Gas Giant theory lines up with where something of that mass would have needed to have formed.

Is Planet 9 a mysterious Super Earth caught by the sun’s influence from the emptiness of interstellar space, or a once proud Gas Giant, cast out by gravitational disturbances in the solar system’s formation, exiled to the icy reaches?

Conclusion: The Search

With all the observed evidence and modeling, there is a sense of certainty that Planet 9 is out there, waiting to be found. Now, a “gold rush” of sorts is taking place to scan the furthest reaches of the mysterious planet’s possible orbital path, to try to render the object in photograph and finally verify its existence. Dozens of astronomers are engaged in the search, but the area they need to cover is immense, and the magnitude of Planet 9 is quite dim given how far away it is thought to be.

Now, our society is on the cusp of re-defining how we understand our solar system, and paint a picture of where we might be headed. Albeit, geologic and astronomic time are quite different from human perception, the machinery of the universe runs on math, and it is always calculating the next interaction. There is so much we have yet to fully comprehend, even in the most basic of levels. Our understanding is based on the delicate architecture of guessing, and Planet 9 brings a lot of our theories into collision with reality. Will the planet projected by a computer really be there? Time is the fact that will determine what is out there, if not a planet, than something else very large…

Image credits: Wikipedia

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