The instrument used to detect the oldest and most distant objects in the solar system is the Hubble Space Telescope (HST).

Here's why:

* Powerful Optics: Hubble boasts a large mirror and precise instruments that allow it to gather faint light from distant objects.

* Orbiting Above Earth's Atmosphere: Its position in space eliminates the blurring effects of the Earth's atmosphere, providing incredibly sharp images.

* Wide Range of Wavelengths: Hubble can observe in visible, ultraviolet, and near-infrared light, allowing it to study a wide variety of objects.

While other instruments, like ground-based telescopes, have contributed to our understanding of the outer solar system, Hubble's capabilities have been essential in discovering and characterizing the faint, distant objects that hold clues to the early solar system's formation.