The planet orbits roughly 483 million kilometres from the stellar duo, about the distance from the asteroid belt to our sun, NASA said in a statement on Thursday.
It completes an orbit around both stars roughly every seven years. The two red dwarf stars are a mere 11 million miles apart, or 14 times the diametre of the moon's orbit around Earth.
The Hubble observations represent the first time such a three-body system has been confirmed using the gravitational microlensing technique.
Gravitational microlensing occurs when the gravity of a foreground star bends and amplifies the light of a background star that momentarily aligns with it. The particular character of the light magnification can reveal clues to the nature of the foreground star and any associated planets.
The objects were discovered in 2007 by an international collaboration of five different groups. These ground-based observations uncovered a star and a planet, but a detailed analysis also revealed a third body that astronomers could not definitively identify.
"The ground-based observations suggested two possible scenarios for the three-body system: a Saturn-mass planet orbiting a close binary star pair or a Saturn-mass and an Earth-mass planet orbiting a single star," explained the paper's first author David Bennett of the NASA Goddard Space Flight Center in Greenbelt, Maryland.
The sharpness of the Hubble images allowed the research team to separate the background source star and the lensing star from their neighbours in the very crowded star field.
The Hubble observations revealed that the starlight from the foreground lens system was too faint to be a single star, but it had the brightness expected for two closely orbiting red dwarf stars, which are fainter and less massive than our sun.
"So, the model with two stars and one planet is the only one consistent with the Hubble data," Bennett said.
NASA's Kepler probe has discovered 10 other planets orbiting tight binary stars, but these are all much closer to their stars than the one studied by Hubble.
The team's results have been accepted for publication in The Astronomical Journal.
