Evidence of a Real Ninth Planet discovered

Evidence has been found of a giant planet tracing an elongated orbit in the outer solar system, nicknamed Planet Nine, has a mass ~10X that of Earth and orbits ~20X farther from the sun on average than Neptune (which orbits the sun at an average distance of 2.8 billion miles). In fact, it would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun. Konstantin Batygin and Mike Brown, discovered the planet’s existence through mathematical modeling and computer simulations but have not yet observed the object directly.

“This would be a real ninth planet,” says Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy. “There have only been 2 true planets discovered since ancient times, and this would be a third.”

Brown notes that the putative 9th planet – at 5000X mass of Pluto – is sufficiently large that there should be no debate about whether it is a true planet. Unlike the class of smaller objects now known as dwarf planets, Planet Nine gravitationally dominates its neighborhood of the solar system. Planet Nine helps explain a number of mysterious features of the field of icy objects and debris beyond Neptune known as the Kuiper Belt.

The road to the theoretical discovery was not straightforward. In 2014, it was noted 13 of the most distant objects in the Kuiper Belt are similar with respect to an obscure orbital feature. To explain that similarity, they suggested the possible presence of a small planet. Batygin and Brown realized that the 6 most distant objects from Trujillo and Shepherd’s original collection all follow elliptical orbits that point in the same direction in physical space. That is particularly surprising because the outermost points of their orbits move around the solar system, and they travel at different rates.

“It’s almost like having six hands on a clock all moving at different rates, and when you happen to look up, they’re all in exactly the same place,” says Brown. The odds of having that happen are something like 1 in 100, he says. But on top of that, the orbits of the 6 objects are also all tilted in the same way — pointing about 30 degrees downward in the same direction relative to the plane of the eight known planets. The probability of that happening is about 0.007%. “Basically it shouldn’t happen randomly,” Brown says. “So we thought something else must be shaping these orbits.”

Effectively by accident, Batygin and Brown noticed that if they ran their simulations with a massive planet in an anti-aligned orbit – an orbit in which the planet’s closest approach to the sun, or perihelion, is 180 degrees across from the perihelion of all the other objects and known planets – the distant Kuiper Belt objects in the simulation assumed the alignment that is actually observed.

“Your natural response is ‘This orbital geometry can’t be right. This can’t be stable over the long term because, after all, this would cause the planet and these objects to meet and eventually collide,'” says Batygin. But through a mechanism called mean-motion resonance, the anti-aligned orbit of the ninth planet actually prevents the Kuiper Belt objects from colliding with it and keeps them aligned. As orbiting objects approach each other they exchange energy. So, for example, for every four orbits Planet Nine makes, a distant Kuiper Belt object might complete nine orbits. They never collide. Instead, like a parent maintaining the arc of a child on a swing with periodic pushes, Planet Nine nudges the orbits of distant Kuiper Belt objects such that their configuration with relation to the planet is preserved.

And indeed Planet Nine’s existence helps explain more than just the alignment of the distant Kuiper Belt objects. It also provides an explanation for the mysterious orbits that two of them trace. The first of those objects, Sedna, was discovered by Brown in 2003. Unlike standard-variety Kuiper Belt objects, which get gravitationally “kicked out” by Neptune and then return back to it, Sedna never gets very close to Neptune. A second object like Sedna, known as 2012 VP113, was announced by Trujillo and Shepherd in 2014. Batygin and Brown found that the presence of Planet Nine in its proposed orbit naturally produces Sedna-like objects by taking a standard Kuiper Belt object and slowly pulling it away into an orbit less connected to Neptune.

But the real kicker for the researchers was the fact that their simulations also predicted that there would be objects in the Kuiper Belt on orbits inclined perpendicularly to the plane of the planets. Batygin kept finding evidence for these in his simulations and took them to Brown. “Suddenly I realized there are objects like that,” recalls Brown. In the last 3 years, observers have identified four objects tracing orbits roughly along one perpendicular line from Neptune and one object along another. “We plotted up the positions of those objects and their orbits, and they matched the simulations exactly,” says Brown. “When we found that, my jaw sort of hit the floor.”

“When the simulation aligned the distant Kuiper Belt objects and created objects like Sedna, we thought this is kind of awesome – you kill two birds with one stone,” says Batygin. “But with the existence of the planet also explaining these perpendicular orbits, not only do you kill two birds, you also take down a bird that you didn’t realize was sitting in a nearby tree.”

Where did Planet Nine come from and how did it end up in the outer solar system? Scientists have long believed that the early solar system began with 4 planetary cores that went on to grab all of the gas around them, forming the four gas planets – Jupiter, Saturn, Uranus, and Neptune. Over time, collisions and ejections shaped them and moved them out to their present locations. “But there is no reason that there could not have been 5 cores, rather than 4,” says Brown. Planet Nine could represent that fifth core, and if it got too close to Jupiter or Saturn, it could have been ejected into its distant, eccentric orbit.

Only the planet’s rough orbit is known, not the precise location of the planet on that elliptical path. If the planet happens to be close to its perihelion, Brown says, astronomers should be able to spot it in images captured by previous surveys. If it is in the most distant part of its orbit, the world’s largest telescopes eg W. M. Keck Observatory and Subaru Telescope, all on Mauna Kea in Hawaii – will be needed to see it. If, however, Planet Nine is now located anywhere in between, many telescopes have a shot at finding it.

This ninth planet that seems like such an oddball to us would actually make our solar system more similar to the other planetary systems that astronomers are finding around other stars. First, most of the planets around other sunlike stars have no single orbital range – that is, some orbit extremely close to their host stars while others follow exceptionally distant orbits. Second, the most common planets around other stars range between 1 and 10 Earth-masses.

“One of the most startling discoveries about other planetary systems has been that the most common type of planet out there has a mass between that of Earth and that of Neptune,” says Batygin. “Until now, we’ve thought that the solar system was lacking in this most common type of planet. Maybe we’re more normal after all.” http://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523