Planet 9 from Outer Space!
May 9, 2016
From the man who killed Pluto comes another tale of darkness, mystery and heretofore-uncharted territory in the outer reaches of our solar system...
...It’s "Planet 9 From Outer Space!" Maybe...
California Institute of Technology planetary scientist Mike Brown, who led the charge to strip Pluto of its identity as a planet, now aims to name a new planet the ninth in our solar system.
Brown and fellow Caltech planetary scientist Konstantin Batygin recently presented evidence that another planet may orbit the sun, further away even than Pluto. Their findings, recently published in the "Astronomical Journal," describe the properties of and reason for the hypothetical new planet, which Batygin and Brown have dubbed, “Planet Nine.”
According to Batygin and Brown’s calculations, Planet Nine would be about 10 times as massive as Earth, or around two-thirds the mass of Neptune and Uranus—the planets it would probably be most similar to.
The striking feature, however, of Planet Nine is how far it is from the sun. Astronomers define the Earth’s distance from the sun, roughly 93 million miles, as one astronomical unit (AU). Neptune orbits at an average of 30 AU from the sun; Pluto, between 30 and 50 AU. At its closest approach, Planet Nine would be 200 AU from the sun and could travel as far out as 600 or even 1200 AU at its furthest point.
To put that in perspective, it takes the sun’s light 8 minutes and 20 seconds to reach Earth. To reach Planet Nine the sun’s light would have to travel for one day, four hours and 40 minutes at it’s closest. If Planet Nine goes as far out as 1200 AU, travelling at light speed for a week wouldn't quite get you there. Batygin and Brown calculate that it would take Planet Nine about 15,000 years to do a single lap around the sun.
If those orbital characteristics are correct, Planet Nine would be unlike any other planet we’ve discovered so far, and it begs a number of questions about the formation and structure of our solar system.
The first question would be, if this planet is so vastly different from anything we’ve seen before, why do we think it’s even there? The answer goes back to 2003 when Brown discovered a dwarf planet called Sedna.
Sedna is a similarly distant object ranging between 76 and 950 AU in its orbit around the sun. Astronomers had visual evidence of Sedna so they could confirm that it’s there, how fast it’s moving and what its orbit looks like. What they couldn’t quite figure out was how it got there.
Sedna’s oval-shaped or elliptical orbit and its distance from the sun are weird. It’s a little too far out to be considered part of the Kuiper Belt—a distant ring of icy and rocky objects where Pluto and its fellow dwarf planets live—and similarly too far out to really be waylaid by Neptune’s gravity. It is also too close to the sun to really be considered part of the Oort Cloud—a hypothetical area almost a light year from the sun where icy comets reside.
Brown and his colleagues at the time hypothesized that Sedna’s odd position might be due to a time when the solar system was young, and our sun was closer to a few other stars. During that time, gravity from one of the other stars could have skewed Sedna’s orbit to where it is today.
But over the next 10 years, more objects like Sedna began to show up: small chunks of rock and ice with an elliptical orbit very far from the sun. Oddly enough, all of these objects reach their perihelion, or the point at which they are closest to the sun, in a relatively compact cluster close to the plane where the other planets orbit the sun.
Brown thought this clustering might be explained by coincidence or by an ancient star passing by. In fact, the probability of all six of those objects having such similar perihelions and orbits angled so similarly is about 0.007-percent. One of his colleagues on the Sedna discovery also published a paper predicting that a large planet might be responsible.
Brown brought the data to Batygin, an expert in astronomical computation and they ran simulation after simulation for more than a year until they came to their description of Planet Nine. The mass and orbit would cause Sedna and its cousins to orbit the way they do. Planet Nine’s description has the added bonus of accounting for the orbits of several other objects that run perpendicular to the planets. The paths of these newly discovered objects match perfectly what Brown and Batygin’s simulations predict.
The math is there, but now the question becomes, is the planet there too?
No one has seen Planet Nine, and the only way to prove it’s there would be to see it with a telescope and take a picture. There are several telescopes on Earth, and the Hubble in space that could theoretically detect it. It also helps that scientists know about how big it is and how it moves so they have a pretty good idea of what it looks like.
It seems like a simple job, but given Planet Nine’s distance from the sun, its relatively small size and the truly gargantuan plot of space in which it could currently be travelling, finding Planet Nine is an unenviable job.
Imagine you have someone paint a “Where’s Waldo” puzzle on your bedroom ceiling. All the little people are normal sized, but the puzzled stretches the width of your bedroom. Now imagine lying on your bed, searching for Waldo, only you have to look through a straw while you do it. How long would it take you to find him?
Waldo is a pretty unique-looking character but even if you knew roughly where to look, finding him like that would be a pretty heroic task.
Finding Planet Nine would be similar to this scenario, except the astronomers looking for him would need to sort through billions of little cartoon people and instead of a straw to look through, they would have something more like a needle.
That’s not even to mention all the quasi-Waldo’s—hatless Waldo, glassesless Waldo, fat Waldo, green-striped Waldo, etc.— that you find in a Where’s Waldo puzzle. Astronomers have no reason to expect to find other, similar planets to Planet Nine out there, but until recently they had no reason to expect Planet Nine out there either. So even if they do find something like Planet Nine out there, they need to check and double check to make sure they actually did find Waldo.
Brown and Batygin’s research proposes a range in which Planet 9 might be travelling, but that range encompasses a mammoth expanse of space. New research, however, is already narrowing down the range where the planet might be hiding.
Astronomer Agnes Fienga and her colleagues at the Cote d’Azur Observatory in southern France are using gravitational breadcrumbs to predict where Planet 9 might be, just like Brown and Batygin. Unlike Brown and Batygin, however, Fienga is not looking at how Planet 9 affects Sedna and other small rocks in its neighborhood. She is watching the planets on the interior of the solar system.
Any two objects with mass have a gravitational attraction to each other, and the bigger they are, the greater the attraction is. Even when the two objects are very far apart, the attraction is still there, it’s just very small. Fienga is looking for the tiniest clues in the way the interior planets move to see whether there is any indication of something pulling them where they shouldn’t be going.
Because Planet 9, in theory, is so far away from everything else, you would need very sensitive instruments and most likely need to be pretty close to the planets themselves to see those forces. Luckily, those instruments exist and are already in place in the form of space probes NASA and other agencies have sent out to explore the solar system. The Cassini probe, which is currently investigating Saturn and its moons, is one such instrument in perfect position to help.
Fienga and her colleagues developed a system called INPOP to collect more than 150,000 measurements of planetary movement from all over the solar system, stretching back more than a decade, and it tells scientists whether their proposed orbits can work with the data.
Imagine the correct orbit of Planet 9 as a long “down” word in a crossword puzzle. Brown and Batygin’s calculations told us that the answer is a “famous landmark,” with 16 letters. The category eliminates a gigantic number of possibilities and the set length eliminates even more.
That said, without more information getting the correct answer is just blind luck. It could be St. Peter’s Basilica or the Golden Gate Bridge or the Great Wall of China, but you have no way of knowing which.
What INPOP does is start to fill in those “across” clues so that you can start trying out your world landmarks and eliminating ones that don’t work. For example say INPOP data finds the third-to-last letter is “i.” St. Peter’s Basilica is still possible, as is Great Wall of China, but Golden Gate Bridge no longer works.
INPOP does not tell scientists exactly where Planet 9 is but it begins to narrow down the possibilities. Fienga and her colleagues published their work in the journal "Astronomy and Astrophysics."
Still, there is a lot of space to scan before scientists find Planet 9, if it is even out there. Discovery of another planet, however, would fundamentally change how we look at our solar system.
— Daniel Lane
Daniel Lane covers science, medicine, engineering and the environment in North Carolina.