Astronomers have discovered the oldest known planet, a primeval world 12.7 billion years old that will force them to reconsider how and when planets form. The discovery raises the prospect that life may have begun far sooner than most scientists ever imagined.
A leading planet-formation expert not involved in the work called the discovery mind-boggling.
The ancient world is well more than twice the age of Earth and all other known planets. It is nearly as old as the universe itself. And it has had an incredibly wild ride through time.
The world formed when the universe was just a billion years old, researchers said at a NASA press conference today. It began its travels around a fairly normal star much like our Sun. The next 10 billion years were fairly routine.
Then the planet was booted from its stellar orbit and captured by the gravity of another star that was well into its death throes.
That's where astronomers found the planet, in a controversial search that began a decade ago.
An early Earth?
The planet is at least as big as Jupiter and almost surely gaseous. It would not harbor life as we know it.
And because it orbits a dying star, any other planets in the system would not receive the sort of life-giving heat and light provided by the Sun.
But since the object's initial eons were spent around Sun-like star, astronomers said it's possible it had a neighbor somewhat like Earth, a place where life might have found opportunity at a time when the Sun wasn't even a glimmer in eyes of the cosmos.
Steinn Sigurdsson, a Penn State University researcher who led the work, said the Jupiter-like planet formed in a nearly circular orbit, somewhere between two and eight times as far from a star as Earth is from the Sun. That orbit would have been favorable to the development of an Earth-like planet, a so-called "terrestrial."
"This [orbit] is wide enough that a terrestrial planet could have comfortably fit in the habitable zone -- if [the terrestrial] formed in the first place," Sigurdsson told SPACE.com. "It certainly makes it more likely that planets capable of hosting life could have formed earlier than hitherto thought. Possibly much earlier and much more commonly."
Any Earth-like planet that might have developed inside the orbit of the gas giant -- and there is no evidence that one did -- would have been destroyed during a later bout with gravitational chaos.
The planet's chaotic history
The planet was forged from gas around a newborn yellowish star in an ancient "globular" star cluster called M4, 7,200 light-years away and within our Milky Way Galaxy. Its existence was first suspected in 1992.
Recent Hubble Space Telescope observations allowed Sigurdsson and his colleagues to devise a remarkable tale of the planet's presumed journey.
About two or three billion years ago, as the yellow star and its planet were plunging into the crowded core of M4, they passed near a collapsed, dense and dying neutron star, an object that resulted from some previous explosion of a very massive star.
The neutron star had an orbiting companion star. The gravitational tug-of-war that ensued booted the neutron star's companion into space. But the neutron star, a weighty competitor, captured the yellow sun-like star and its planet.
The sun-like star aged, bloating into a red giant (our Sun will do the same one day). The red giant's gas flowed onto the neutron star, energizing it. The neutron star spun faster. Today, it rotates on its axis 100 times every second and is known as a pulsar.
Meanwhile, the red giant's fuel was exhausted and it turned into a cool, fairly dim white dwarf.
The newfound planet now orbits both the white dwarf and the neutron star.
"We probably would never have found this planet if it had just stayed with its original star," Sigurdsson said. "Its history put it in the right place; the interactions helped us see it."
End to controversy
Researchers discovered the pulsar, named PSR B1620-26, and its companion white dwarf star in the late 1980s. The planet was first suspected in 1992, but some astronomers thought the data showed a third, distant orbiting star, rather than a close-in planet.
"By 1999 we knew the object was low-mass, with a significant probability it was a planet," Sigurdsson said.
Ending a decade of controversy, the new Hubble observations of the neutron star's white dwarf companion, and how the objects wobble in space due to the gravitational tugs, allowed Sigurdsson's team measure the trajectory and mass of the suspected planet. They confirmed that it is indeed not large enough to be anything else. The data, along with knowledge of the stars' ages, also allowed them to trace the planet's remarkable history.
The planet has not been seen or imaged directly, so final proof of its existence awaits further study.
It is estimated to be 2.5 times the mass of Jupiter. Its heft might equal Jupiter's, Sigurdsson said, and it is definitely no lighter. There's a slight chance it is a few times Jupiter's mass.
The planet's orbit is about 100 years long. It goes around the two old stars at a distance that is most likely similar to Uranus' distance from the Sun.
At 7,200 light-years from Earth, it is not just the oldest known planet but also the farthest.
The results will be detailed tomorrow in the journal Science.
The planet exists in an unlikely place. Astronomers assumed the gravitational interactions in a globular cluster -- M4 contains 100,000 tightly packed stars -- would rip planetary systems apart.
"This is tremendously encouraging that planets are probably abundant in globular star clusters," said study team member Harvey Richer of the University of British Columbia.
But its the objects apparent history that has astronomers reeling.
"The fact that this system managed to form a gas-giant planet 12.7 billion years ago certainly boggles the minds of those of us who are used to having a hard time going back just 4.5 billion years in time," said Alan Boss, a leading planet-formation theorist at the Carnegie Institution of Washington.
Boss, who was not involved in the discovery, said the tortured history of the system implies there is no life there now. And back when it formed, there would have been less rock -- the stuff of a terrestrial planet -- because heavier elements form with subsequent generations of stars.
But there would have been some rock, and Boss agrees that the discovery suggests life might have had a chance.
"If there were gas giants around at 12.7 billion years ago, I would think that there could be a few terrestrial-like planets too," Boss said in an e-mail interview. "Presumably some of them [would have] experienced a more gentle history than this poor world, and so some might have experienced some sort of flirtation with life, if not something much more serious."
Fresh view of planet formation
More certain, Boss said, is that the primeval planet supports his own radical model of planet formation.
Building on the controversial work of others, Boss last year proposed that all the giant planets in our solar system formed via direct collapse of material in a disk of leftover stuff that circled the newborn star.
The model, "wild" even by Boss' reckoning, is called "disk instability."
The standard model of planet formation -- which all experts agree no longer explains all of reality -- held that gas giant planets formed after a rocky core larger than Earth had first assembled. The massive rocky core then attracted lots of gas.
But this so-called "core accretion" model takes considerable time, and there has to be a lot of rocky material available.
The newfound planet may have formed too quickly for core accretion to be responsible.
Boss points out that the early universe was composed mostly of gas. Stars in the M4 cluster have just 1/20th the amount of rocky or metal material that exists in the Sun, he said. That implies a slow rate of growth for a rocky core and probably the lack of sufficient mass to build a Jupiter-like planet through core accretion.
"Disk instability would seem to be the only hope in a system composed almost entirely of hydrogen and helium," he said.
Earth-sized pulsar planets, too
The newfound planet is not the first found orbiting a pulsar.
Another trio of planets -- all about the size of Earth -- orbit the pulsar PSR 1257+12. They were confirmed to exist just two months ago and, prior to that, their presence had long been suspect.
The Earth-sized pulsar planets, found by Alex Wolszczan (also of Penn State), are a different breed, however. They probably formed after the stellar explosion that created the dying star they go around, Boss said. So they are almost surely devoid of life and never could have supported any.
The International Astronomical Union lists Wolszczan's worlds in its catalogue of extrasolar planets. But other planet hunters choose not to list them, partly because their formation and environment is so different, and partly because they are not considered habitable and so are not as important in the search for life.
The newfound pulsar planet, having presumably formed around a normal star, is unlikely to be ignored. Its discovery brings the total of known extrasolar planets, counting Wolszczan's, to 121.
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