Alternative Explanation For KBO Orbits Doesn't Need Planet 9

We've been looking for planet 9 but haven't found it yet. Perhaps it isn't there? New ideas for what may be causing strange KBO orbits in outer solar system.

2812 Views | Published on 12th Jun, 2018

Hello Space Fans and welcome to another edition of Space Fan News. In this episode the story this week comes from my alma mater, the University of Colorado at Boulder with a story that has many good and interesting things about it: it’s from CU, there may not actually be a Planet 9 that’s causing the strange outer solar system orbits, and the major work behind this study was done by a CU undergrad - that’s right, and undergraduate student is shedding light on a difficult problem.

OK, most of have know for a while about the theory that what might be causing the observed orbits of these strange objects in the outer solar system called ‘detached objects’. The orbits were so strange that normal planetary motions and theories of solar system formation could not explain why they orbited the Sun the way they did.

So, astronomers Konstanin Batygin and Mike Brown proposed, after trying to rule out every other possible explanation, proposed that maybe there is an as yet unseen planet affecting the orbital dynamics of these bodies.

This idea made a lot of sense, we found the planet Neptune this way, we saw the effects this planet had on the other planets, namely Uranus, in our solar system and was able to figure out that there must be a planet there and sure enough, after lots of observing and looking for it, Neptune was found in 1846.

So doing the same kind of math, Batygin and Brown determined that if the orbits observed by these strange objects, among them the minor planet known as Sedna, then the planet must be really far out, around 800 astronomical units or so, be really big, about 2-4 times the radius of Earth and about 10 times more massive, and orbit the Sun once every 10 to 20 thousand years or so.

OK, so this was a big news and everybody has been trying to find it ever since. And no one has found it yet.

As I said, Batygin and Brown (whom I’ve had in hangouts before) have said that they knew this claim would require extraordinary evidence so they worked really hard to eliminate every other possible explanation before saying this might be due to a planet.

Well they didn’t work hard enough because last week at the summer meeting of the American Astronomical Society, the CU Boulder team announced another possibility. The collective gravity of all the strange little objects floating around past the orbit of Neptune, when you add them all up, could explain the strange orbits seen by Sedna and the other objects plotted by astronomers.

The outer solar system is a really strange place gravitationally, there a lots of orbits of lots of things that when you plot them accurately, don’t make any sense. Sedna for example, a little smaller than Pluto, takes 11 thousand years to go around the Sun once.
It’s orbit is also circle-shaped and doesn’t go anywhere near big planets like Jupiter of Neptune. So, how did Sedna get there? It remains a mystery and hence the name: detached objects, the closest they ever get to the Sun is so far away that they aren’t affected by the gravity of the larger planets, the appear detached from the solar system.

So the outer solar system has lots of things circling the Sun in really strange orbits that can’t easily be explained, not even with another planet out there.

But if you add up all the gravitational effects of all the detached objects out there, some really interesting things happen that appear to explain these orbits. An undergradate student working in the Dept of Planetary and Atmospheric Sciences at CU, a place I spent way too much time in, wrote some code that calculated the orbits of of the minor planets, such as Pluto, icy moons and a variety of space debris.

According to the simulations, the orbits of the icy objects beyond Neptune circle like the hands of a clock, in a very Copernican way. Some of those orbits, like asteroids moved faster and in tandem, flying in formation. Other objects, big ones like Sedna, move much more slowly.

So what happens when the fast ones, overtake the slow ones? It’s a very interesting gravitational problem, one that hasn’t been really considered before. When this happens, the smaller objects begin to pile up on one side of the sun. These orbits crash into some of the bigger bodies causing a minor planet like Sedna, which once had a normal, ellipsoidal orbit, into a detached, circular one.

This simulation was confirmed by predicting another strange thing about detached objects, the bigger a detached object is, the further away its orbit becomes from the sun.

Another interesting idea that came out of this: as space debris interacts in the outer solar system, their orbits tighten up and widen in a repeating way. This cycle could have the effect of altering comet orbits towards the inner solar system including the Earth, on a predictable timescale.

So let this story be an example to all of you looking for a career in science. Many people ask if you need a PhD to do astronomy and the answer is a resound NO!

It may come to pass that we will ultimately find Planet 9, but given how hard everyone is looking, it really should have been found by now, it was not hypothesized to be a small thing after all. This research provides another explanation for observations and I’d love to hear what Batygin and Brown have to say, but they haven’t responded to my emails yet. I’m sure they are quite busy now but I will try to keep you posted.

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