Astronomers have detected the most ancient planet yet known orbiting a binary system thousands of light-years away. The new discovery indicates that planet formation in the Milky Way may have started sooner and been more widespread than previously believed. A report describing the find appears in the current issue of the journal Science.

In 1988, researchers discovered pulsar PSR B1620-26, a neutron star spinning about 100 times a second, in the 13-billion-year-old globular star cluster known as M4. Scientists soon found a white dwarf nearby and further observations located a third object in orbit around both, which astronomers thought was either a brown dwarf or a low-mass star. The new work, carried out by an international team led by Steinn Sigurdsson of Pennsylvania State University, indicates that the orbiting object is instead a planet with a mass 2.5 times that of Jupiter. Using the Hubble Space Telescope, the team inferred the weight of the planet from its effects on the pulsar, thereby determining that it was too small to be either a star or a brown dwarf. "This is tremendously encouraging that planets are probably abundant in globular star clusters," notes study co-author Harvey Richer of the University of British Columbia.

Because globular clusters have low concentrations of heavy elements--necessary building blocks for planets--scientists believed that they were unsuitable as planet breeding grounds. Indeed, the metal content of M4 is just 5 percent of that of our sun. "Our Hubble measurement offers tantalizing evidence that planet formation processes are quite robust and efficient at making use of a small amount of heavier elements," Sigurdsson notes. "This implies that planet formation happened very early in the universe."

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