The object, known as SDSS J0104+1535, is a member of the so-called halo - the outermost reaches - of our galaxy, made up of the most ancient stars.
It is located 750 light years away in the constellation of Pisces and is made of gas that is around 250 times purer than the Sun and consists of more than 99.99 per cent hydrogen and helium, researchers said.
The dwarf is estimated to have formed about 10 billion years ago, measurements also suggest it has a mass equivalent to 90 times that of Jupiter, making it the most massive brown dwarf found to date.
"We really did not expect to see brown dwarfs that are this pure. Having found one though often suggests a much larger hitherto undiscovered population - I would be very surprised if there are not many more similar objects out there waiting to be found," said ZengHua Zhang of the Institute of Astrophysics in the Canary Islands, a Spanish archipelago off the coast of northwestern Africa.
Brown dwarfs are intermediate between planets and fully-fledged stars. Their mass is too small for full nuclear fusion of hydrogen to helium (with a consequent release of energy) to take place, but they are usually significantly more massive than planets, researchers said.