Brown dwarfs occupy one of the most intriguing gray areas in astronomy. They form like stars, from collapsing clouds of gas and dust, but never become massive enough to sustain the hydrogen fusion that makes true stars shine. As a result, they gradually cool, dim, and fade, making them extremely difficult to detect.

In a recent SETI Live conversation, Dr. Lauren Sgro of the SETI Institute spoke with U.S. Naval Observatory astronomer Dr. Adam C. Schneider about a major achievement from the citizen science project Backyard Worlds: Planet 9. Through this platform, hundreds of thousands of volunteers have helped identify more than 3,000 motion-confirmed brown dwarf candidates. The discovery is especially striking because astronomers once believed that most nearby brown dwarfs had already been found. Instead, Backyard Worlds revealed that the solar neighborhood still contains many more of these faint objects than expected. The new catalog effectively doubles the known population of nearby brown dwarfs and demonstrates the scientific value of citizen participation.

Brown dwarfs are classified within the extended stellar spectral sequence. Traditional stars range from hot, massive O-type stars to cool, common M dwarfs. Later discoveries showed that even colder objects exist, leading astronomers to add L, T, and Y classes. Most L dwarfs, and all T and Y dwarfs, are substellar brown dwarfs. Y dwarfs are the coldest, with temperatures that can approach room temperature on Earth.

Some brown dwarfs are surprisingly close to us. Schneider noted that the third and fourth closest known systems to the Sun are brown dwarfs: WISE 1049-5319 and WISE 0855-0714, located 6.5 and 7.4 light-years away. They were discovered only in the 2010s because they are so faint, showing how incomplete our map of nearby space remains.

Counting brown dwarfs is important for understanding how stars and substellar objects form. Current models agree fairly well on the formation of ordinary stars but differ sharply when predicting brown dwarf populations. A more complete census helps test which models are correct. Backyard Worlds uses data from NASA’s WISE infrared telescope. Volunteers inspect animated “flipbooks” of the sky, looking for objects that shift position over time. These moving points may be nearby brown dwarfs. The project now combines human inspection with machine learning through initiatives such as Backyard Worlds: Cool Neighbors. Rather than replacing volunteers, artificial intelligence helps direct their attention to the most promising candidates. This collaboration between professionals and citizen scientists continues to expand our understanding of the hidden objects in our cosmic backyard.

For more details, watch the full SETI Live conversation, read the press release and the published paper.