Gravity.

Planet X's gravity is believed to cause the orbits of the planets to tilt, relative to the Sun's equator. Some of the orbits are skewed as much as 90 degrees. Astronomers think that Planet X's immense gravity pulls on its companion planets and Kuiper Belt objects just enough to nudge them off a central rotational axis. Speaking of Kuiper Belt objects...

JPL/NASA

Some bodies similar to Pluto rotate backwards.

At least 5 icy spheres in the Kuiper Belt, a ring of dwarf planets and comets beyond the orbit of Neptune, rotate backwards relative to the majority of other objects. Astronomers credit this phenomenon with the eccentricity of Planet X's orbit and gravitational tug.

Space

Planet X is insanely far away.

If it's out there, Planet X likely has an elongated elliptical orbit that at aphelion (furthest from the Sun) stretches 100 billion miles into space. That's 20 times farther away than Neptune.

JPL/NASA

Most Solar Systems have a super-Earth, so ours very well could, too.

Planet X is thought to be a "super-Earth," or a rocky planet similar to our own except much bigger. Based on the effects of its gravity, it is estimated that Planet X is an icy world 10-20 times more massive than Earth.

JPL/NASA

Super-Earths are the rule, not the exception.

Of the thousands of exoplanets observed since the first was discovered in 1995, many have turned out to be so-called super-Earths; giant, terrestrial worlds many times more massive than our own. In fact, super-Earths may be the most common type of rocky planet or water-world in the Universe.

Planet X may be one such super-Earth. Though this is still theoretical, many scientists think that its likely our Solar System is no exception.

Conditions on Planet X are nothing like here on Earth.

Its composition may be similar to Earth's, but because of its distance from the Sun, Planet X would have wildly different traits than our home world. Scientists have hypothesized that Planet X could be one big, cold, dead rock, containing nothing more than organic compounds and a fleeting atmosphere of noble gases.

But Planet X may be too massive to be rocky, which leaves another tantalizing possibility:

Planet X could be a water world with a dense crust of ice covering a massive global ocean. Other objects in the Solar System, like Jupiter's Moon Europa and Saturn's Enceladus, are believed to have liquid water below their surfaces. What makes this possibility so exciting is that on Europa and Enceladus, the surface is scarred with cracks and cravasses from expanding sub-surface liquid. Enceladus even has cryovolcanos that spew water into space. This means that beneath the sea, there are sources of heat. Heat and water are two ingredients necessary for simple life.

NASA

Planet X could have plate tectonics.

Earth is the only known planet with a dynamic crust. But that doesn't mean others aren't out there. Planet X, with its immense mass, could have a hot, liquid metal core, just like Earth. This raises the possibility of plate tectonics, which, if anything like Earth, could provide a mixture of compounds necessary for the formation of simple life, like on the early Earth. But...

Planet X would be unimaginably cold.

Because it is so far away from the Sun and receives so little light, Planet X would have surface temperatures hundreds of degrees below zero. Currently, the coldest confirmed place in the Solar System is right next door, on our Moon. In 2009, NASA calculated the temperatures in lunar craters to be -397 degrees Fahrenheit. That's only 35 degrees above Absolute Zero, the theoretical point at which all molecular motion stops.

Life on the surface of Planet X is probably impossible, but if there is a sub-surface ocean, all bets are off.

ABC News

How could we directly observe Planet X?

Planet X's extreme distance from the Sun would render it nearly impossible to see, even with the most powerful telescopes. But astronomers have another method of detecting distant objects using a phenomenon called occulation.

It's pretty simple, at least in theory. If astronomers can calculate where they think Planet X may be (using the effects of its gravity on nearby objects), they can wait for Planet X to cross in front of a star or groups of stars, or really any other fixed point in deep space.

This method was used by NASA in 2018 to pinpoint a tiny Kuiper Belt object called Ultima Thule, which lies beyond the orbit of Pluto. As Ultima Thule transited a star, its location, speed, and trajectory were calculated, and NASA was able to steer the New Horizons spacecraft into a flyby of the tiny world.

The images New Horizons sent back are stunning (see above). At 4 billion miles away, Ultima Thule is now the most distant place ever visited by humanity.

EarthSky

Planet X may have formed somewhere else.

It's a compelling idea. Even though there is substantial observiational data supporting the existence of Planet X, one glaring question remains: where did it come from?

Right now, no one knows if Planet X formed along with the rest of our Solar System and its 8 planets, thousands of dwarf planets, hundreds of moons, and trillions of comets and asteroids.

Researches say that additional observation could show that Planet X was a rogue planet that formed somewhere - perhaps in another part of the galaxy - and wandered too close to the Sun, whose gravity snagged Planet X off its interstellar path.

JPL/NASA