How Do You Find a Planet?

Recently, scientists have announced the potential existence of another planet in our solar system out past Pluto. If true, this would bring our planet count back up to nine, after the demotion of Pluto to a dwarf planet. Planet 9 is theorized to be a gas giant, larger than Earth, exist out in the Kuiper belt, and have a very long orbit around the sun, some 200 years. This is a very cool discovery, but how do you find a planet in the solar system? You’d think they’d all be found by now.

Well, the scientists first got hint of a large planet out in the Kuiper belt by observing the dwarf planet Sedna which is in the belt. Sedna was found in 2013 and the scientists noticed it’s orbit was odd. But what does “odd” mean exactly? First lets talk about gravity and orbital motion. In space, gravity is the primary force that drives the motion of planets, stars, and asteroids. In the simplest case, we can consider just two bodies, one large main body that is fixed and one smaller that orbits around the main body. For example the Sun and the Earth. This is called 2-body orbital motion. It assume that the main body is so much larger that we can neglect the mass and thus gravity of the smaller body. In 2-body motion, the smaller body follows a constant and nice elliptical orbit around the main body.

In reality though there are lot of bodies out there, and every object exerts gravity on every other object. Just in our solar system there are 8 large planets. So those planets are constantly tugging on each other even though the sun is the one controlling the main motion (which is why everything orbits the sun). Jupiter is the largest planet and it has a noticible effect on everything in the solar system. Jupiter may be partly responsible for the existance of the human species actually. Jupiter’s strong gravitational pull may have acted like a asteroid shield of sorts and altered the trajectory of space rocks that would have hit Earth in the early days of life on this planet. So the planets, and especially Jupiter cause the Earth’s orbit around the sun to be changed slowly over time. This is still happening today. But the changes are too small for us to notice, but we can measure the deviation from the 2-body orbit with scientific instruments.

The scientists noticed something similar with Sedna. After watching it and calculating its position over time, we can determine the orbit it follows. We can use computers to account for the perturbing effects of Jupiter and the other planets on Sedna. The asteroids and other objects in the Kuiper belt are all too small compared to the planets to have a significant effect on Sedna’s orbit. These days we’re pretty good at calculating orbits if we know all the major bodies involved. They can see where Sedna is at some later time to confirm that the calculated orbit is correct . Now what the scientists probably saw was that Sedna was not moving according to the calculated orbit, which means there’s a large body with a strong gravity that wasn’t accounted for. The deviation form the calculated orbit was probably very small, maybe a few kilometers, which in space is minuscule but enough to suggest there’s something else out there. So the scientists spend some time and ran more computer simulations using the measured position data of Sedna. They probably created many different bodies in the Kuiper belt and calculated their effect on Sednas orbit looking for one that would match the actual data. The result seems to be a large gas giant about 10 times bigger than Earth in a large egg-shaped orbit around the sun out past Pluto. Now comes the challenge of trying to find planet 9.

What will come next will likely be close examination of the scientist’s data to double check their work. If others agree, then they will likely take measurements of other objects in the Kuiper belt over time and see how the behave and if planet 9 could be responsible for any oddities. Finally, after watching enough Kuiper belt objects, we’ll have a good guess of where planet 9 could be. Then it becomes a game of cat a mouse as we turn all our telescopes towards a small patch of sky and try to find one plants in the blackness of space. It’ll be a long process since finding planets or stars requires a lot of data and you’re looking (or the computer is looking) for very small changes in light from one image to the next, on a pixel by pixel basis. But if we find it, it’s be the first planet since Pluto was found in 1930.


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