Backyard Worlds: Planet 9 is a NASA-funded citizen science project which is part of the Zooniverse web portal.

[1] It aims to discover new brown dwarfs, faint objects that are less massive than stars, some of which might be among the nearest neighbors of the Solar System, and might conceivably detect the hypothesized Planet Nine.

The project's principal investigator is Marc Kuchner, an astrophysicist at NASA's Goddard Space Flight Center.

This hypothetical new planet would be located so far from the Sun that it would reflect only a very small amount of visible light, rendering it too faint to be detected in most astronomical surveys conducted to date.

[5] Due to the effects of proper motion and parallax, Planet Nine would appear to move in a distinctive way between images taken of the same patch of sky at different times.

Citizen scientists accessing the website search through a flip book-style animation of specially-processed mid-infrared images captured by WISE known as unWISE coadds,[6] taken with filters at the wavelengths of 3.4 and 4.6 micrometers.

The coadded unWISE images permits fainter objects to be detected than previous processing of WISE imagery allowed.

[9] In July 2018 an update on the project's blog stated that in total 42 brown dwarfs had been spectroscopically confirmed from a list of 879 candidates.

[16] At the 235th meeting of the American Astronomical Society in January 2020 a summary of the current status of the project was presented and this included 1503 L, T and Y dwarf candidates.

[17] In June 2017, it was announced that Backyard Worlds had made its first official discovery: a brown dwarf designated WISEA 1101+5400, of spectral type T5.5 and located 34 parsecs (111 light years) from Earth.

[18][7] In October 2018, a participant in the project discovered LSPM J0207+3331 – the oldest and coldest white dwarf known to host a circumstellar disk, despite being 3 billion years old.

[19][20] At the 235th meeting of the American Astronomical Society in January 2020 the discovery of the wide brown dwarf binary W2150AB was presented by Jacqueline Faherty.

[21][22] The discovery of WISE J0830+2837, the first Y-dwarf discovered by volunteers was also presented at the 235th meeting by project scientist Daniella Bardalez Gagliuffi.

This estimated temperature would place it between the majority of the Y-dwarf population so far identified and WISE 0855−0714, the coldest object of this type known.

[24] These high-proper motion objects display unique colors and near-infrared spectra that do not fully match current models.

The models producing the best matches to the spectra imply the brown dwarfs have [Fe/H] ≤ -1, meaning they have extremely sub-solar metallicity, containing far lower amounts of elements heavier than hydrogen or helium compared to the Sun.

The extremely low metallicity implies these brown dwarfs are very old, approximately 10 billion years, as the galaxy at this time would have featured lower quantities of heavy elements.

95 had Spitzer mid-infrared colors consistent with being a cold brown dwarf, with 75 of these having their proper motion confirmed by comparison to their position in WISE images.