The new “henge” – which means a circular monument dating to Neolithic and Bronze Ages – is situated about 900m (2,950ft) from the giant stones on Salisbury Plain.
Images show it has two entrances on the north-east and south-west sides and inside the circle is a burial mound on top which appeared much later, Professor Gaffney said.
“You seem to have a large-ditched feature, but it seems to be made of individual scoops rather than just a straight trench,” he said.
“When we looked a bit more closely, we then realised there was a ring of pits about a metre wide going all the way around the edge.
“When you see that as an archaeologist, you just looked at it and thought, ‘that’s a henge monument’ – it’s a timber equivalent to Stonehenge.