Something has been bugging me for a while. In recent years astronomers have realised there might be another planet in our solar system. For now it is called Planet Nine and we think it is there because we can see its gravity lining up smaller solar system objects. It would be on an orbit that takes between 10,000 and 20,000 years to complete.

That means it is far further out than the current position of the Voyager 1 probe – humanity’s most distant emissary. In 2012, an achievement by Voyager 1 was greeted with much fanfare. Measurements of the solar wind indicated it had crossed over the heliopause – the boundary between the Sun’s magnetic bubble (the heliosphere) and interstellar space. It was lauded as the first human-made object to leave the solar system. But how can something have left the solar system if it hasn’t reached the orbit of the Sun’s outermost planet?

This question had been rumbling around in my head for a while and I ask it directly in a book I have coming out in March 2018 (The Universe in Bite-Sized Chunks). But today I decided to search out a firm answer. I sent out this tweet, copying in Mike Brown – the discoverer of the dwarf planet Eris and self-styled “Pluto Killer”.

He got back to me, tweeting:

He also said:

So there you have it. Voyager 1 has not left the solar system, but it has crossed over the heliopause into interstellar space. Even NASA’s Jet Propulsion Laboratory (JPL) has realised the error of its ways.