Here's why it's hard to pinpoint a specific planet:

* Pluto's orbit is highly elliptical: Pluto's distance from the Sun varies significantly throughout its orbit. At its farthest point (aphelion), it's around 49 AU from the Sun, and at its closest (perihelion), it's about 29 AU. So, at times, Pluto could be within the range of 5900 million miles.

* The Kuiper Belt: Beyond Neptune lies the Kuiper Belt, a region populated with icy bodies. Many objects in this belt have orbits extending far beyond Pluto's, and some could potentially reach a distance of 5900 million miles from the Sun.

Therefore, it's not possible to definitively say which planet is 5900 million miles away from the sun. It's more likely to be an object in the Kuiper Belt, or perhaps a dwarf planet.