Here's why:

* Eris's orbital period is incredibly long: It takes approximately 557 Earth years for Eris to complete one orbit around the Sun.

* Eris was discovered relatively recently: Since its discovery in 2005, less than two decades have passed. Therefore, it has not had enough time to complete even a fraction of its orbit.

While we've only seen a tiny piece of Eris's journey around the sun, astronomers can use its current position and speed to calculate its entire orbit.