The companion is an asteroid, named 2010 TK7, and is about 300 metres across. Among the many thousands of asteroids known, most are simply orbiting the sun in a band between the orbits of Mars and Jupiter. However, there is a special class named Trojan asteroids because they orbit around one of the two Lagrangian points which lie 60° ahead of and behind the larger body. in its orbit. Jupiter has a large number of Trojan asteroids, possible as many as exist in the main asteroid belt.
So while Trojans have been associated with Jupiter, Saturn, Neptune and even Mars, none had been known to be associated with the Earth. Astronomers had predicted Earth should have Trojans, but they have been difficult to find because they are relatively small and appear near the Sun from Earth's point of view.
2010 TK7 was initialized discovered by the WISE orbiting infrared telescope which afforded astronomers a different perspective that they usually get from Earth-bound telescopes. Once this object was identified as an interesting candidate, followup observations with Canada-France-Hawaii telescope on Mauna Kea confirmed that 2010 TK7 was indeed the first Earth Trojan.
There are likely to be many more Earth Trojans and now that the first one has been identified the race will be on to discover more.