The body could help shed light on how planets form and resolve a decades-old mystery about where such objects were hiding.
The discovery is the first time scientists have ever seen one of the objects, despite them having been predicted for more than 70 years.
They are thought to be an important missing-link in the search to understand how we went from the initial clumps of dust and ice that formed and the planets they would eventually become today.
The object was found in the Kuiper Belt, a collecton of small objects floating beyond Neptune. The most famous of those objects is Pluto, but there are a number of bodies there.
They are thought to be remnants of the early solar system. Because they are so distant and mostly unaffected by radiation and the bigger planets, they are still largely as they would have been in those days, potentially allowing scientists a way of looking back in time to how the solar system might have looked before planets formed.
Scientists had long predicted that objects of this size – a kilometre to several kilometres – were hovering out there, but they had not previously been seen. Now astronomers from National Astronomical Observatory of Japan have managed to spot one using a technique called occultation, where stars are watched until something passes in front of them and causes the light from it to dip.
The discovery suggests that there might be many of the objects than had previously been thought. It also indicates that that the objects that will eventually go onto be planets first form into kilometre-sized clumps before merging together to create the worlds that surround us today.
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