The XO Project is an international team of amateur and professional astronomers tasked with identifying extrasolar planets. They are led by Peter R. McCullough of the Space Telescope Science Institute.^[1] It is primarily funded by NASA's Origins Program and the Director's Discretionary Fund of the Space Telescope Science Institute.^[2][3]

Preliminary identification of possible star candidates starts at the Haleakala telescope in Hawaii by a team of professional astronomers. Once they identify a star that dims slightly from time to time (the transit method), the information is forwarded to a team of amateur astronomers who then investigate for additional evidence suggesting this dimming is caused by a transiting planet. Once enough data is collected, it is forwarded to the University of Texas McDonald Observatory to confirm the presence of a transiting planet by a second team of professional astronomers.^[2]

McCullough and his team employed a relatively inexpensive telescope called the XO Telescope, made from commercial equipment, to search for extrasolar planets. The construction of the one-of-a-kind telescope cost $60,000 for the hardware, and much more than that for the associated software.^[4] The telescope consists of two 200-millimeter telephoto camera lenses, and resembles binoculars in shape. It is similar to the TrES survey telescope. It stands on the summit of the Haleakalā volcano and 3,054 m (10,000 foot) in Hawaii.^[1] Their first discovery of a Jupiter-sized planet orbiting a Sun-like star 600 light-years from Earth in the constellation Corona Borealis—XO-1b—was reported May 16, 2006 on Newswise.

In 2016 three similar double telescopes were operating, two in Spain and one in Utah.^[5]

The XO telescope has discovered six objects so far, five are hot Jupiter planets and one, XO-3b, may be a brown dwarf.

• List of extrasolar planets

A subset of XO light curves are available at the NASA Exoplanet Archive.