[bryant]

New horizons launched 9.5 years before it reached Pluto, and your average reader who has an interest in Planet 9 will likely know it took New Horizons about that long.

15x means no one alive today will see a mission that reaches the planet, and that's more accessible for most readers per above.

[Retric]

New horizons is hardly the fastest possible probe over the next 30 years.

(9.5 * 15 / 3 = 47.5) + 30 years = 77.7 so some teenagers could live to see a probe reach it even without hypothetical life extension technology.