Stars exist across a wide range of masses, some more massive than the Sun and some less massive. If a star is less massive than roughly 8% of the Sun’s mass, or 80 times the mass of Jupiter, its center is too cool to sustain hydrogen fusion. These stars that lack fusion are known as brown dwarfs. One of the most fundamental questions about brown dwarfs is, what is the lowest mass at which they exist? We have used the most powerful infrared telescope to date, NASA’s James Webb Space Telescope, to search for brown dwarfs at very low masses in a nearby nebula that contains newborn stars. Through these observations, we have discovered brown dwarfs with masses as low as 0.2% of the Sun’s mass (twice the mass of Jupiter), making them the least massive known brown dwarfs. One surprising aspect of these new brown dwarfs is that their atmospheres contain an unidentified hydrocarbon that has not been previously detected in brown dwarfs or planets outside the solar system. It has been previously seen only in the atmospheres of Saturn and Titan. The presence of this hydrocarbon has not been predicted by any theoretical models of brown dwarfs, and its origin remains unknown.