Which major telescope was instrumental in observing dim, substellar objects falling on the boundary between large planets and low-mass brown dwarfs?

Answer

Hubble Space Telescope

The Hubble Space Telescope has been key in observing these dim objects, often by searching in active star formation regions for companions in the intermediate mass range.

Which major telescope was instrumental in observing dim, substellar objects falling on the boundary between large planets and low-mass brown dwarfs?
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