The outer edge, where the planet would freeze solid, he said, could extend as far as twice Earth's orbital radius, beyond the orbit of Mars.
For years astronomers have postulated a comet storehouse beyond the orbit of Pluto.
Makemake, dwarf planet orbiting the Sun beyond the orbit of Pluto.
More distant still, beyond the orbit of Neptune, is a band of icy objects called the Kuiper belt.
The pair is part of the Kuiper Belt, a ring of debris beyond the orbit of Neptune.
On Wednesday, the union announced the creation of the term "plutoid" for a dwarf planet beyond the orbit of Neptune.
It had taken mission controllers 10 years and 6bn km to get to this point of cosmic theatre playing, 510m km from Earth, beyond the orbit of Mars.
New Horizons is now on course for its next rendezvous in 2019, with an object called 2014MU69 in the Kuiper belt, which lies beyond the orbit of Neptune.
Comet 67P is thought to have come from what is called the Kuiper belt, a broad band of frozen bodies that begins beyond the orbit of Neptune.
The comet is currently out beyond the orbit of Mars, heading for its closest approach to the sun in August 2015.
Some face was saved last year when the union announced the coining of the term "plutoid" to designate a dwarf planet beyond the orbit of Neptune.
