Duncan Forgan, a research fellow at the University of St. Andrews, recently wrote about one such object called 3753 Cruithne (pronounced krooy-nyuh) on The Conversation, where he's calling it a "second moon."

Here's the best picture we have of this tiny, rocky "moon" that's only about 3 miles across. Because of its small size, astronomers need powerful instruments to see Cruithne. The telescope at Powell Observatory took the image below, where it only shows up as a tiny speck of light.

The space-based instrument NEOWISE took five images of Cruithne that are compiled into a single image below. NEOWISE scours the sky for near-earth objects like asteroid and comets.

The reason the object appears red in this image is because it is radiating light at lower energies than the white stars in the background, James Bauer, a scientist who studies comets at NASA's Jet Propulsion Laboratory, explained to Business Insider in an email.