There is a mysterious dark, distant, and frigid domain in our Solar System, located far beyond the banded, ice-giant planet Neptune–the farthest known major planet from our Sun. Astronomers have only just begun to explore this strange domain, where a dancing multitude of frozen, icy objects–some large, some small–circle around our Star in the mysterious blackness of interplanetary space, where our Sun shines with only a weak fire, and appears to be just an unusually large star swimming in the perpetual twilight of a cold sky. This region is called the Kuiper belt, and it is the frigid home of the dwarf planet Pluto and its moons–as well as a host of other comet-like objects. In January 2016, astronomers at the California Institute of Technology (Caltech) in Pasadena, California, announced their historic discovery of new evidence indicating the existence of a giant planet tracing a highly elongated orbit in the outer limits of our Solar System. This putative ninth major planet, which the scientists have dubbed “Planet Nine”, sports an impressive mass of approximately ten times that of Earth–and it circles our Star about 20 times farther out on average than does Neptune–which circles our Sun at an average distance of 2.8 billion miles! In fact, the astronomers calculate that it would take this potential new planet between 10,000 and 20,000 years to make just one full circle around our Sun.
The astronomers who made this discovery, Dr. Konstantin Batygin and Dr. Michael Brown, detected the putative planet’s existence using mathematical modeling and supercomputer simulations–but they have not yet observed this possible new addition to our Sun’s family directly.
“This would be a real ninth planet. There have only been two true planets discovered since ancient times, and this would be a third. It’s a pretty substantial chunk of our Solar System that’s still out there to be found, which is pretty exciting,” Dr. Brown commented in a January 20, 2016 Caltech Press Release. Dr. Brown is the Richard and Barbara Rosenberg Professor of Planetary Astronomy at Caltech.
Dr. Brown further noted that the potential ninth major planet–at 5,000 times the mass of poor little Pluto–is large enough for there to be no debate about whether or not it is a true major planet. Unlike the class of smaller objects designated dwarf planets–such as Pluto–Planet Nine clearly would unambiguously gravitationally dominate its neighborhood of our Solar System. Indeed, this brave new world would dominate a region larger than any of the other eight known major planets. As Dr. Brown continued to comment, this fact makes Planet Nine “the most planet-y of the planets in the whole Solar System.”
Lowell Observatory founder, the American astronomer Percival Lowell, speculated a century ago that a mysterious and remote Planet X secretly lurks in the strange, frigid darkness of our Solar System’s outermost fringes–and Planet Nine provides the best fit so far for such an elusive world. Planet Nine, in its elliptical orbit around our Sun, would never get closer than about 200 times the Earth-Sun distance–or 200 astronomical units (AU). That range would place the planet far beyond Pluto, in the strange realm of the Kuiper Belt, where icy bodies tumble around in the deep freeze far, far away from our Star. One AU is equivalent to the separation between Earth and our Sun, which is about 93,000,000 miles.
Dr. Batygin and Dr. Brown inferred the existence of this distant world from the motions of several other Kuiper Belt Objects (KBOs). Alas, the history of similar and earlier scientific speculations about the existence of distant worlds, like Planet Nine, suggest that it might wind up as just another false positive. Indeed, astronomers have speculated for years about the existence of additional large planets inhabiting our outer Solar System, in addition to the four already known: Jupiter, Saturn, Uranus, and Neptune. However, none has yet been confirmed.
“Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer Solar System, we become increasingly convinced that it is out there. For the first time in over 150 years, there is solid evidence that the Solar System’s planetary census is incomplete,” Dr. Batygin commented in the January 20, 2016 Caltech Press Release.
The Historic Hunt For Planet X
The greenish-blue ice-giant planet Uranus–the seventh major planet from our Sun–was discovered completely by accident by the British astronomer William Herschel on March 13, 1781. Herschel was performing a survey of all the stars that were of magnitude 8 or brighter when he noticed an object traveling in front of the stellar backround as time passed. This very clearly indicated that the mysterious object was closer to us than the distant stars. Initially, Herschel thought that he had discovered a comet, but he ultimately came to the realization that this object was a new planet circling our Sun–the very first to be discovered since ancient times. Later, astronomers realized that Uranus had actually been observed as far back as 1690–but it was William Herschel who was the first to determine the true nature of this bright distant world in our night sky.
The German astronomer Johann Gottfried Galle discovered Neptune in 1846, guided by predictions derived from observed perturbations of Uranus’s orbit. In 1906 Perceval Lowell started hunting for the mysterious, hypothetical Planet X, which he predicted would circle our Star beyond Neptune, just as Neptune resides beyond Uranus. Lowell’s calculations led astronomers at Lowell’s namesake observatory in Flagstaff, Arizona, to discover Pluto–but this small, fascinating distant world proved not to be massive enough to be Planet X.
The orbit of each of the known eight major planets of our Sun’s family is slightly disturbed by the gravitational tugs of the other seven planets. Conflicts between what has been observed and that which was expected by astronomers in the early 1900s–with respect to the most distant of the outer planets, Uranus and Neptune–caused widespread suspicion that more planets haunted the outer limits of our Solar System beyond Neptune. However, the chase only resulted in the discovery of little Pluto by the American astronomer Clyde Tombaugh in 1930.
The Kuiper belt was given its name in honor of the Dutch-American astronomer Gerard Kuiper, even though his true role in suggesting its existence has at times been contested. Since the very first Kuiper Belt Object (KBO) was spotted back in 1992, the number of known KBOs has increased into the thousands–and more than 100,000 KBOs that are over 62 miles in diameter are thought to circle our Sun in this remote, frozen region.
In 1999, observed perturbations in the orbits of comets caused some astronomers to propose that a failed star–termed a brown dwarf–haunted the outer fringes of our Solar System. A brown dwarf is the runt of the stellar litter–smaller than a true star, but larger than a planet. These objects–which are really a very pretty pinkish-purple color called “magenta”–probably form the same way as true stars, as a result of the gravitational collapse of a relatively small blob within an enormous, dark molecular cloud composed of gas and dust. However, brown dwarfs are too small for nuclear fusion to light their fires.
The road to the theoretical discovery of Planet Nine was not an easy one. In 2014, a former postdoc of Dr. Brown, Dr. Chad Trujillo, and his colleague Dr. Scott Sheppard published a paper that explained that 13 of the most remote objects in the Kuiper belt are similar in respect to an obscure orbital characteristic. In order to explain that strange similarity, the astronomers proposed the possible presence of a small planet. Dr. Brown thought the planet proposal unlikely, but he became very curious, nevertheless.
Dr. Brown then visited Dr. Batygin down the hall at Caltech, and the two scientists together embarked on a year-and-a-half long study to investigate the remote objects. As an observer and a theorist, the two planetary scientists investigated the situation using entirely different approaches. Dr. Brown, the observational astronomer, stares at the sky seeking to understand everything in the context of that which can be observed. In contrast, Dr. Batygin, the theorist, places himself within the context of dynamics, and tries to determine how things might operate from the viewpoint of physics. Those differences enabled the two astronomers to challenge each other’s ideas and to consider new and different theories. “I would bring in some of these observational aspects, he would come back with arguments from theory, and we would push each other. I don’t think the discovery would have happened without that back and forth. It was perhaps the most fun year of working on a problem in the Solar System that I’ve ever had,” Dr. Brown noted in the January 20, 2016 Caltech Press Release.
Dr. Brown and Dr. Batygin quickly came to the realization that the half dozen most remote objects taken from Dr. Trujillo and Dr. Sheppard’s original collection all travel elliptical orbits around our Sun that point in the same direction in physical space. This observation is especially weird because the outermost points of their orbits travel around the Solar System, but they move 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,” Dr. Brown commented in the same Caltech Press Release. The chances of having that occur are about 1 in 100, he added. But to complicate things even further, the six objects are also all tilted in precisely the same way–they are all pointing about 30 degrees downward in the same direction relative to the plane of the eight known major planets. The probability of that happening is only about 0.007 percent. “Basically, it shouldn’t happen. We thought something else must be shaping these orbits,” Dr. Brown added.
The first explanation for these unusual observations proposes that there are a sufficient number of KBOs–some of which have not yet been discovered–to exert the necessary gravitational effect to keep that subpopulation clustered together. The scientists quickly ruled out this particular explanation when it turned out that such a scenario would demand that the Kuiper belt harbor about 100 times the mass that it has today.
That left the two astronomers with the theory postulating that a hidden, mysterious planet was the true source of their strange observations. In order to test their idea, the scientists ran supercomputer simulations involving a planet in a remote orbit that encircled the orbits of the six KBOs. The hidden planet would act something like an enormous lasso to herd them into their bizarre alignment. Dr. Batygin noted in the Caltech Press Release that, although this concept almost works, it does not reflect the observed eccentricities as precisely as necessary.
Many scientific discoveries are made by accident, and it was by accident that Dr. Batygin and Dr. Brown noticed that if they ran their simulations with a massive planet residing in an anti-aligned orbit, the KBOs in the simulation adopted the precise alignment that is actually observed. An anti-aligned orbit is an orbit in which the planet’s nearest approach to our Star (perihelion) is 180 degrees across from the perihelion of the other objects and known planets–the very remote KBOs created by the scientists in their supercomputer simulations replicated the alignment that is actually being 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,'” Dr. Batygin explained in the January 20, 2016 Caltech Press Release. But through a mechanism known as mean-motion resonance, the anti-aligned orbit of the Ninth Planet actually prevents the KBOs from colliding with it and keeps them aligned. As orbiting objects travel towards each other they exchange energy. The upshot is, for example, that for every four orbits the Ninth Planet makes, a remote KBO might complete nine orbits. They never collide–they cannot. Instead, Planet Nine pushes the orbits of distant KBOs in such a way that their configuration with relation to the planet is preserved.
“Still I was very skeptical. I had never seen anything like this in celestial mechanics,” Dr. Batygin added.
However, as the astronomers investigated additional attributes and consequences of this model, they became less skeptical. “A good theory should not only explain things that you set out to explain, it should hopefully explain things that you didn’t set out to explain and make predictions that are testable,” Dr. Batygin went on to note.
Indeed, the existence of the Ninth Planet helps explain more than just the alignment of the remote KBOs–it also gives an explanation for the mysterious and baffling orbits that a pair of them trace. The first of those objects, named Sedna, was discovered by Dr. Brown in 2003. In a way vastly different from garden-variety KBOs, which get the gravitational boot out by Neptune–only to travel back to it– Sedna never gets very close to Neptune. A second KBO that resembles Sedna, 2012 VP113 was announced by Drs. Trujillo and Sheppard in 2014. Dr. Batygin and Dr. Brown found that the existence of Planet Nine, as it travels in its purported orbit around our Sun, produces Sedna-like bodies by snaring a garden-variety KBO, and then slowly pulling it away into an orbit that is less linked to that of Neptune.
But, perhaps most important of all, the astronomers’ supercomputer simulations also predicted there would be objects in the Kuiper belt traveling on orbits inclined perpendicularly to the plane of the planets. Dr. Batygin kept finding evidence for these in his simulations and then took them to Dr. Brown. “Suddenly I realized there are objects like that. We plotted up the positions of those objects and their orbits, and they matched the simulations exactly. When we found that, my jaw sort of hit the floor,” Dr. Brown recalled in the January 20, 2016 Caltech Press Release. Indeed, over the last three years, observers have spotted a quartet of objects like that.
But where did Planet Nine come from and how did it wind up in the outer limits of our Solar System? Astronomers have thought for years that the early Solar System started out with a quartet of planetary cores that went on to snatch up all of the gas surrounding them–thus forming the four giant gaseous planets of the outer limits. As time went by, collisions and ejections formed the quartet of enormous planets and pushed them out to their current locations. “But there is no reason that there could not have been five cores, rather than four,” Dr. Brown explained in the Caltech Press Release. Therefore, Planet Nine could be the fifth core–and if it had traveled close to Jupiter or Saturn long ago, it may have been booted out into its very remote, eccentric orbit.
Drs. Batygin and Brown continue to refine their supercomputer simulations and learn more about the potential planet’s orbit–as well as its influence on the outer Solar System. Meanwhile, Dr. Brown and his colleagues are searching interplanetary space for Planet Nine. Only the potential planet’s rough orbit is known–not its exact location on that elliptical path.
“I would love to find it,” Dr. Brown said in the Caltech Press Release. He added that “I’d also be perfectly happy if someone else found it. That is why we’re publishing this paper. We hope that other people are going to get inspired and start searching.”
Dr. Brown, who is well known for the role he played in the demotion of Pluto from a major planet to a dwarf planet, further noted that “All those people who are mad that Pluto is no longer a planet can be thrilled to know that there is a real planet out there still to be found. Now we can go and find this planet and make the Solar System have nine planets once again.”