Record Breaker: A planet from the early universe

Astronomers have found the oldest and most distant planet known in the universe.

OLD ORB. Artist’s view of the planet (top) orbiting a pulsar and its white dwarf companion (two brightest dots at lower left) in the globular cluster M4. NASA and G. Bacon/STScI

Residing 7,200 light-years away, the planet weighs 2.5 times as much as Jupiter and formed when the universe was an infant, Steinn Sigurdsson of Pennsylvania State University in State College and his colleagues report in the July 11 Science.

The planet’s very existence suggests that such orbs formed relatively soon after the Big Bang. Moreover, the object’s location–near the crowded core of a star cluster, where planetary systems might easily be ripped apart–indicates that planets might be more abundant than thought.

The planet lies near the center of the globular cluster M4, a dense grouping of stars about 12.5 billion years old. Old stars such as these are metal poor because they formed before subsequent generations of stars had produced heavy elements in abundance. Astronomers have found most extrasolar planets orbiting younger, relatively metal-rich stars. That led astronomers to rate ancient globular clusters as unlikely venues for planets. Indeed, in a 1999 study looking for planets that closely orbit stars in the globular cluster 47 Tucanae, the Hubble Space Telescope failed to find a single one.

But after examining a pair of burned-out stars in M4 with Hubble and a radio telescope, researchers now say that planets may be common in globular clusters after all. They might just be orbiting their parent stars at greater distances than astronomers had looked for in previous studies.

“The conventional wisdom is that high metallicity is required for extrasolar planets to be detected, and we now have a spectacular counterexample,” notes theorist Alan P. Boss of the Carnegie Institution of Washington (D.C.). “Clearly, this discovery opens up a lot of turf for searching for more planetary-mass objects, more turf in both time and space.”

The observations leading to the discovery date to 1988, when astronomers examining M4 discovered a pulsar, a rapidly spinning neutron star that broadcasts radio waves like a beam from a lighthouse. Tiny deviations in the arrival time of the radio waves at Earth indicated that a compact star called a white dwarf is orbiting the pulsar. Further irregularities in the radio signals indicated that yet another body is orbiting the pulsar.

Analyzing Hubble observations of the white dwarf and comparing them with the radio data, Sigurdsson’s team determined the mass of the dwarf and the tilt of its orbit about the pulsar. That in turn revealed that the third body is a Jupiterlike planet that orbits the neutron star–white dwarf duo at about the same distance that Uranus orbits the sun.

Sigurdsson’s team suggests that the planet initially resided at the outskirts of M4 and probably formed much closer than it is now to its parent star. Somehow, the planet survived the frenzy of star birth in what was then a young cluster, and the star and its planet sank toward the center of M4. A billion or so years ago, the pulsar ejected the companion it had then and captured both the star and its planet. The star then evolved into a white dwarf.

“This extrasolar planet sets several records at once: the oldest, the most distant, the lowest metallicity, and the first one to orbit two stars,” says Boss. “Nature continues to astound us.”


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