I was driving north on I-25 near Berthoud. I was admiring the sun setting behind Longs Peak, so this occurred within a few minutes of sunset. I looked to the north west about 15 - 20 degrees of Longs Peak when I noticed a small silver dot in the sky, within about a minute it grew, about triple it's original size. Picture a pencil eraser about 20 feet away, that's about how big this silver object grew. I noticed the silver color, there appeared to be no reflection from the sunset.It was a sphere, and it was present for about 3 minutes, when it shrunk in size and disappeared.A couple of minutes later I noticed an airplane, about 15 degrees further north from where I saw the sphere, It's contrail was visible but the plane was very very small. The plane was about one quarter the size of the sphere at it's largest.I watched the sky for a few minutes longer, while trying to still drive, but did not see this again.Could this simply have been a refraction of a reflection of the sunlight off of the plane? I don't know. I'm curious.Never seen it or heard of anything like it.