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”.
I can't seem to get an answer to this nagging question: if Planet Nine is found and Voyager 1 has left the solar system (despite not reaching the distance of the Sun's outermost planet) is Planet Nine beyond the solar system or has Voyager 1 not really left? (help @plutokiller?)
— ★ Colin Stuart ★ (@skyponderer) December 20, 2017
He got back to me, tweeting:
Voyager has not left the solar system. No one considers the heliopause the edge of the solar system. But it IS the beginning of interstellar space. The edge is where the sun's grav influence is overwhelmed by that of nearby stars, i.e. edge of Oort cloud.
— Mike Brown (@plutokiller) December 20, 2017
He also said:
the whole "left the solar system" was sloppy PR work by JPL that they have been trying to correct ever since. Note all of the recent press releases say "entered interstellar space" instead.
— Mike Brown (@plutokiller) December 20, 2017
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.