[6][7] In 2006, the XO Project, an international team of professional and amateur astronomers, discovered a Jupiter-sized planet, later named XO-1b, orbiting a Sun-like star.

[1] The team, led by Peter R. McCullough of the Space Telescope Science Institute in Baltimore, had four amateur astronomers hailing from North America and Europe.

In that time, McCullough's team of amateur astronomers studied a few dozen stars they had previously identified as promising candidates for extrasolar planets.

McCullough's team then turned to the McDonald Observatory in Texas for information on the object's mass and to confirm it was a planet.

The radial velocity method allowed the team to calculate a precise mass of the planet, which is slightly less than Jupiter's.

Like other known transiting hot Jupiters such as HD 209458 b and TrES-1b, the low density of XO-1b indicates that this planet is a gas giant composed mainly of hydrogen and helium.