Astronomers using a telescope that was built using "off-the-shelf" components and mounted on the summit of Haleakala have discovered a planet almost as large as Jupiter orbiting a star about 600 light-years from Earth.

The XO telescope revealed small, regular dips in the light generated by a star in the constellation Corona Borealis, which led to the discovery of the planet.

The 2 percent reductions in output of light from the star were caused by the previously unknown planet as it passed in front of the star. The planet, named XO-1b, revolves around the star in a tight orbit that it completes about every four days.

The Haleakala telescope, constructed from two 200 mm telephoto camera lenses, looks like a large pair of binoculars.

University of Hawai'i astronomer James Heasley, who was part of the team that made the discovery, predicted that small telescopes such as the XO will be used increasingly to identify stars showing telltale signs of planets revolving around them.

Larger telescopes such as those on Mauna Kea on the Big Island can then be used to confirm the existence of newly discovered planets, he said.

In this case, the team confirmed its observations of XO-1b with the Harlan J. Smith Telescope and the Hobby-Eberly Telescope at the University of Texas' McDonald Observatory. The larger telescopes were able to measure the slight wobble that the planet casts upon the image of its parent star, a technique that also allows astronomers to calculate the mass of the planet.

The XO telescope has been making nightly sweeps of the sky to record the brightness of stars, recording tens of thousands of bright stars from September 2003 to September 2005.

Astronomers use a computer program to sift through the thousands of images of stars recorded every two months to hone in on those showing light reductions that might indicate planets are present.

From the hundreds of candidates generated by the computer, astronomers pick a few dozen of the most promising possibilities. Images of those stars are then submitted to the four amateur astronomers to study more carefully.

The star XO-1 was picked as a promising candidate in June 2005, and the amateur astronomers watched it in June and July 2005 to confirm that a planet-sized object was eclipsing the star.

The planet is the first discovered using this program, which is underwritten by a grant from the Origins program of the National Aeronautics and Space Administration.

The research team was led by Peter McCullough of the Space Telescope Science Institute in Baltimore and includes researchers and students from Boston University along with four amateur astronomers from North America and Europe.

The findings on the newly discovered planet have been accepted for publication in the Astrophysical Journal.