It was once believed that once you got past Pluto, there wasn't much out there at the edge of the solar system. However, a team of astronomers have discovered a bunch of sizable objects in that region of space, called the Inner Oort Cloud, and have data suggesting that many more such objects are out there, including, possibly, a massive planet about the size of 10 Earths.
We know very little about the Inner Oort Cloud, which is why its existence remains theoretical. Located nearly a light-year away from our Sun, it is part of the Oort Cloud, which marks the boundary of our solar system. Spotting objects within this region of space is tricky, because it's so far away, and large planets would appear faint, even when using our most powerful telescopes. However, observations recently discovered a dwarf planet in the region, named 2012 VP113. This planet is the most distant we’ve ever seen, and the second object we’ve discovered in the Inner Oort Cloud region.
The first object discovered in the area was Sedna, another dwarf planet, in 2012. Sedna’s orbit takes it beyond the reaches of the known part of the solar system, but 2012 VP113’s orbit reaches even farther. The discovery of not just one, but two objects in the area suggests that many more are awaiting discovery in that area of space. Astronomers estimate that something like 900 similar objects could be out there. They even believe that a few of these objects could be the size of Mars or Earth, or bigger. These potential planets are very far away, and our current technology has a very hard time detecting them, even if the objects are large. Astronomers believe that one such large planet does exist there, however, because it seems that the orbits of smaller objects in that area are similar, suggesting the existence of a gravitationally-influencing planet that could be 10 times the size of Earth.