'So while, yes, the evidence doesn't exist yet, I thought the bigger point was that he showed us that there are ways to find that evidence.
'I don't think he really has any evidence that suggests it is out there.'
Hal Levison, from the Southwest Research Institute in Boulder, Colorado, said: 'It seems surprising to me that a [solar] companion as small as Neptune could have the effect he sees.
'[But] I know Rodney, and I'm sure he did the calculations right.'
The previous ninth planet, Pluto, is one of the largest of the Kuiper belt dwarf planets, at some 1,400 miles wide.
It got downgraded by the International Astronomical Union in 2006 for failing to meet all the criteria of a 'planet', namely that its mass is not sufficient enough to clear its orbit of surrounding objects.