In the past, scientists have speculated about the presence of additional planets in our solar system, and various suspicions have been explored and discredited throughout the years. Newer discoveries relating to the presence of objects farther from the sun resulted in the demotion of Pluto to a dwarf planet when Eris, another similarly sized body, was found in the Kuiper belt.
While exploring the Kuiper belt, Mike Brown and others observed numerous smaller bodies. Discovered in 2014, an object named 2012 VP113 has an orbit that suggests because of the way it is aligned with a number of other objects orbiting the sun, that there may be a distant, large planet to explain the alignment of them. There is an approximately 0.007 percent chance that this kind of positioning would occur randomly, which means there is a high chance that some other object, namely Planet Nine, is accountable for it.
In January, Konstantin Batygin and Brown announced that they believe gravitational resonance is responsible for this, which would place Planet Nine on the opposite side of the sun. Suspected to be 10 times as massive and 4 times as large as Earth, its elongated orbit would allow it to remain approximately 200 or more times further away from the sun than the Earth in its orbit. Additionally because of this, a single orbit would take Planet Nine between 10,000 to 20,000 years, which would help account for why scientists have yet to have observed it.
More recently, the region of space astronomers are searching for Planet Nine in has grown smaller. As well, Agnès Fienga and her colleagues at the Côte d’Azur Observatory in France believe that Saturn’s orbit has been visibly affected by the presence of Planet Nine. The Cassini spacecraft is currently observing Saturn, and Fienga noted that slight perturbations in its orbit could potentially be explained by this large, distant body. Astronomers Matthew Holman and Matthew Payne from the Harvard-Smithsonian Center are currently analyzing the Cassini data to see if they can learn more about potential orbits.
As of April 20, Planet Nine’s location has been narrowed down to “two stripes in the sky.” Data seems to point towards the constellation Cetus, which despite it’s large region is much smaller than what Fienga pointed to. As it so happens, the Dark Energy Survey is already monitoring this area for a project on the acceleration of the universe, which means that some data is present to work with. Astronomers like Gerdes, a cosmologist at the University of Michigan who is a part of the DES team, are “dropping everything to work my hardest to do this search in our data” and are “be[ing] pulled along by the excitement.”
For more information, here is the original paper by Konstantin Batygin and Michael Brown: http://iopscience.iop.org/article/10.3847/0004-6256/151/2/22
Aron, Jacob. “Hints That ‘Planet Nine’ May Exist on Edge of Our Solar System.” New Scientist. Daily News, 20 Jan. 2016. Web.
Batygin, Konstantin, and Michael E. Brown. “Evidence For A Distant Giant Planet In The Solar System.” The Astronomical Journal 151.2 (2016): 22. The American Astronomical Society. Web.
Hall, Shannon. “We are closing in on possible whereabouts of Planet Nine.” New Scientist. Daily News, 20 Apr. 2016. Web.