U.S. and Canadian astronomers have discovered the oldest known planet orbiting two stars in the constellation Scorpius. It is very different from Earth and its sisters circling our sun and hints of a new and possibly abundant class of planets out there.
Earth and the solar system are relatively young, just 4.5 billion years old, compared to the estimated 14 billion year age of the universe. But astronomers have discovered a planet nearly as old as the cosmos.
Pennsylvania State University researcher Steinn Sigurdsson says it is 13 billion years old, formed when the universe was still in its infancy.
"What we think we found is an example of the first generation of planets formed in the universe," he said. "We think this planet formed with its star 13 billion years ago when the galaxy was very young, just in the process of forming and formed with the very earliest generation of stars that formed in our galaxy, much, much earlier than the sun."
The planet is a gas giant like Jupiter, but 2.5 times more massive. It is in the constellation Scorpius 5,600 light years away and so far out from the pair of stars it circles that its year equals 100 of ours.
The planet is not visible from Earth, but the scientists found it indirectly by observing one of the stars of the pair with a radio telescope on the ground and the orbiting Hubble Space Telescope. This star is located in a cluster of stars and is a rapidly spinning collapsed star called a pulsar, sending out pulses of light to us as it turns. The physics of the light beam led the scientists to infer it was part of a pair of burned out stars that orbit each other. They later concluded a third body was involved, the planet that swings distantly around them.
"This identification is a stunning revelation," said Alan Boss, an expert on planet formation not involved in the research. He says the planet's 13 billion-year age makes it unique among the 100 or so planets discovered much closer to our solar system in recent years.
"They are presumably stars that are no older than our own sun," he said. "But in this case we have a star where it presumably could have formed not only a gas giant planet but also a habitable planet where life could have arisen and died out long ago, well before we came along to the galactic party."
The cluster of stars the planet inhabits is so old that it lacks the heavier elements like iron that formed in abundance within stars later, elements that make up Earth and its sister planets. Some astronomers have argued that such so-called globular star clusters cannot contain planets as a result, and Hubble research in 1999 backed up that view. It could not locate any close to their stars in distant clusters, but it appears it looked in the wrong place.
One of the other co-discoverers of the planet, University of British Columbia astronomer Harvey Richer, says it suggests that planets began forming rapidly after the universe began. This could mean they are very abundant, but he is cautious about this notion.
"We ought not to extrapolate from an example of one," he said. "It would be nice to say the universe is full of very old planets. It very well may be, but I think that we have to do at this stage is at globular clusters in other metal-poor environments to really see, are there lots of planets there?"
The finding supports a theory of an alternative planet forming mechanism. Those in our solar system and presumably many others are thought to have formed over eons as clumps of matter collected from the gravitational pull of dust particles and collided with other clumps. But Alan Boss says without rocky elements, gas alone appears to be able to amass into a planet.
"I really must congratulate my colleagues here for, with one single identification, solving perhaps some of problems in the theory of planet formation," he said.
