Purest, Most Massive Brown Dwarf Found in the Outermost Region of the Milky Way

In the outermost reaches of our galaxy known as the halo lies a record breaking brown dwarf that have the purest composition and highest mass yet known.

The brown dwarf, described in a paper published in the Monthly Notices of the Royal Astronomical Society, is located 750 light-years away in the constellation of Pisces. Known as SDSS J0104+1535, the brown dwarf is made of gas that is around 250 times purer than the Sun and has a mass equivalent to 90 times that of Jupiter.

"We really didn't expect to see brown dwarfs that are this pure. Having found one though often suggests a much larger hitherto undiscovered population," said lead investigator Dr ZengHua Zhang, from the Institute of Astrophysics in the Canary Island, in a press release. "I'd be very surprised if there aren't many more similar objects out there waiting to be found."

Using European Southern Observatory's Very Large Telescope (VLT), the researchers classified SDSS J0104+1535 as an L-type ultra-subdwarf based on its optical and near-infrared spectrum. Researchers estimate SDSS J0104+1535 to have formed about 10 billion years ago and are now consisting of more than 99.99 percent hydrogen and helium.

The classification scheme used was recently developed by Dr. Zhang. The SDSS J0104+1535 was previously classified as an sdM9.5 subdwarf, but the very blue J - K color of the brown dwarf suggests much lower metallicity compared to normal sdM9.5 subdwarfs. Due to this, the researchers re-classified SDSS J0104+1535 as a usdL1.5 subdwarf.

Brown dwarfs, like the SDSS J0104+1535, are intermediate between planets and fully-pledged stars. They are significantly more massive than planets. However, their mass is not big enough for full nuclear fusion of hydrogen and helium to take place. The size and composition of SDSS J0104+1535 makes it the purest and most massive brown dwarf found to date.

The discovery of SDSS J0104+1535 in the halo of the Milky Way suggests that there are larger undiscovered population of extremely pure brown dwarfs from the Galaxy's ancient past.