The planet, more than twice the size of Jupiter, orbits two stars, a pulsar and a white dwarf that linked together about a billion years ago. The system is in the constellation Scorpius within a globular cluster called M4 that contains stars that formed billions of years before the sun and its planets.
"All of the stars in this cluster are about the same age, so the presumption is that the planet is that age also," Harvey Richer, an astronomer at the University of British Columbia in Vancouver, Canada, said Thursday at a NASA news conference.
The pulsar, a rapidly spinning star, was discovered in M4 about 15 years ago. Astronomers shortly afterward found that it was gravitationally bound to a white dwarf, the remnants of an ancient, sunlike star that had exhausted its hydrogen and helium fuel. There was suspicion that yet another body was orbiting nearby, but the planet was not discovered until astronomers studied data from the Hubble Space Telescope.
Alan Boss, an astronomer at the Carnegie Institution in Washington, said finding such an ancient planet is a "startling revelation" because it means that planets could have formed within a billion years after the big bang, far earlier than most theories have stated.
"This means that 13 billion years ago, life could have arisen and then died out," said Boss. "This has immense implications."
Astronomers in recent years have found 107 other extra-solar planets — planets outside of the solar system — but all of those are about the same age or just slightly older than the sun, 4.5 billion years.
It was thought that planets could not form until there had been at least one generation of stars after the Big Bang because the planet building requires heavier elements, such as carbon, silicate and iron. These elements, called "metals" by astronomers, are thought to have formed during the life cycle of the early stars, when hydrogen and helium were burned in fusion fires.
The sun is a third-generation star, but the M4 stars are believed to be in the first generation after the Big Bang, some 14 billion years ago.
Boss said the solar system has about 30 times as many heavier elements as M4.
Harvey said the discovery suggests that astronomers should now search for planets in the more ancient star fields, which includes systems like the M4 globular cluster.
"The door is open now to start looking in the metal-poor clusters," he said.
Steinn Sigurdsson, a professor of astronomy at Pennsylvania State University, said that based on orbital measurements and other data, astronomers can infer a history for the M4 planet.
He said it is believed the planet formed about a sunlike star near the edge of the globular cluster. Over time, the star and its planet were gravitationally captured by the pulsar, which was then a neutron star with another star as a companion. As the sunlike star was sucked into the mix, the companion star was ejected from the group. This left the sunlike star and neutron star bound to each other while the planet orbited both.
Eventually, the sunlike star burned up its fuel, bloomed into a red giant and then collapsed into a white dwarf. The neutron star, with its greater density, sucked in material from the collapsing star. This caused the neutron star to start spinning at 100 times a second and emitting radio signals, turning into a pulsar. It was the clocklike pulsing of these radio signals, picked up by radio telescopes, that led to other observations and the discovery of the complex.
Sigurdsson said there were enough heavy elements in the M4 complex to have formed some terrestrial planets, like Earth and Mars, in orbit of the sunlike star. He said it is theoretically possible that life could have formed on those planets some 12.5 billion years ago.
But when the sunlike star was pulled into orbit of the neutron star, any planets near the sun would have been destroyed. Only the gaseous planet, orbiting some two billion miles out, would have survived.
"Over a billion years ago, any near-in planet would have been wiped out," said Sigurdsson. "But it could have been stable for 10 billion years before," plenty of time for intelligent life to have formed.
If there was intelligent life on such a planet, he said, it was destroyed as the parent sun was pulled toward the neutron star.
"They would have seen it coming," Sigurdsson said of creatures that may then have been living on that planet.