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July 21, 2003

Astronomers unravel the strange life and hard times of the farthest and oldest known planet

By Ray Villard, Space Science Telescope Institute

Long before our Sun and Earth existed, a Jupiter-sized planet formed around a sunlike star. Today, 13 billion years later, the planet survives in orbit around a peculiar pair of burned out stars: a helium white dwarf and a rapidly rotating neutron star, or pulsar. Observations with NASA's Hubble Space Telescope have now revealed the odd life story of the farthest and oldest known planet.

A rich starry sky fills the view from an ancient gas giant planet in the core of the globular star cluster M4, as imagined in this artist's concept. The 13-billion-year-old planet orbits a helium white dwarf star and the millisecond pulsar B1620-26, seen at lower left. The globular cluster is deficient in heavier elements for making planets, so the existence of such a world implies that planet formation may have been quite efficient and common in the early universe. Illustration: NASA and G. Bacon (STScI)
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The new Hubble findings close a decade of speculation and debate as to the true nature of this ancient world, which takes a century to complete each orbit. The planet is 3.5 times the mass of Jupiter and is probably, like Jupiter, a gas giant. Its very existence provides tantalizing evidence that the first planets were formed rapidly, within a billion years of the big bang, leading astronomers to conclude that planets may be very abundant in the universe.

The planet lives in an unlikely neighborhood: the ancient globular star cluster M4, which lies 7,200 light-years away in the summer constellation Scorpius. Globular clusters are deficient in heavier elements because they formed so early in the universe that heavier elements had not been cooked up in abundance in the nuclear furnaces of stars. Because planet formation depends on the gravitational binding of heavier elements, some astronomers have argued that globular clusters cannot contain planets.

This conclusion was bolstered in 1999 when Hubble failed to find close-orbiting "hot Jupiter"-type planets around the stars of the globular cluster 47 Tucane. Now, it seems that astronomers were just looking in the wrong place, and that gas-giant worlds at greater distances from their stars could be common in globular clusters.

"This is one more example in a long list of unexpected places where we have been surprised to find planets. It is more evidence that planets must be easy to make, and if they are easy to make they must be common," said Stephen Thorsett, a professor of astronomy and astrophysics at UC Santa Cruz.

"Our Hubble measurement offers tantalizing evidence that planet-formation processes are quite robust and efficient at making use of a small amount of heavier elements. This implies that planet formation happened very early in the universe," said Steinn Sigurdsson of Pennsylvania State University.

Sigurdsson led the team of investigators that described the planet's remarkable existence, orbiting two captured stars near the crowded core of a globular cluster where fragile planetary systems tend to be ripped apart due to gravitational interactions with neighboring stars. A paper describing the discovery was published in the July 11 issue of the journal Science. In addition to Sigurdsson and Thorsett, the authors include Harvey Richer and Ingrid Stairs of the University of British Columbia and Brad Hansen of UCLA.

The story of this planet's discovery began in 1988, when the pulsar, called PSR B1620-26, was discovered in M4. It is a neutron star spinning just under 100 times per second and emitting regular radio pulses like a lighthouse beam. The white dwarf companion star was quickly found through its effect on the clocklike pulsar, as the two stars orbited each other twice per year.

Thorsett has been studying the pulsar's radio emissions since shortly after it was discovered. His analysis in the early 1990s provided the first evidence of a third companion object orbiting the others. This new object was suspected to be a planet, but it could also have been a brown dwarf or a low-mass star. Debate over its true identity continued through the 1990s.

Sigurdsson, Richer, and their coinvestigators settled the debate by at last measuring the planet's actual mass through some ingenious celestial detective work. They had exquisite Hubble data from the mid-1990s, taken to study white dwarfs in M4. Sifting through these observations they were able to detect the white dwarf orbiting the pulsar and measure its color and temperature. This allowed an estimate of the white dwarf's mass to be made. This in turn was compared to the amount of wobble in the pulsar signal, allowing the astronomers to calculate the tilt of the white dwarf's orbit as seen from Earth.

When combined with the radio studies of the wobbling pulsar, this critical piece of evidence told them the tilt of the planet's orbit too, and so its mass could at last be determined. With a mass of only 3.5 Jupiters, the object is too small to be a star or brown dwarf, and must instead be a planet.

"We've been pretty sure the planet was there, and with the Hubble image of the white dwarf we are now able to confirm the basic predictions of our model of the system," Thorsett said. "This greatly increases our confidence in our ideas about how the system formed."

The planet has had a rough road over the last 13 billion years. When it was born it probably orbited its youthful yellow sun at approximately the same distance Uranus is from our Sun. The planet survived blistering ultraviolet radiation, supernova radiation, and shockwaves that must have ravaged the young globular cluster in a furious firestorm of starbirth during its early days.

Around the time multicelled life appeared on Earth, the planet and its star were plunging into the core of M4. In this densely crowded region, the planet and star passed close to an ancient pulsar, formed in a supernova when the cluster was young, that had its own stellar companion. In a slow-motion gravitational dance, the star and planet were captured by the pulsar, whose original companion was ejected from the globular cluster and lost.

The pulsar, star, and planet were themselves flung by gravitational recoil into the less-dense outer regions of the cluster. Eventually, as the star aged, it ballooned to a red giant and spilled matter onto the pulsar. The momentum carried with this matter caused the neutron star to "spin up," dramatically increasing its rotation speed to become a millisecond pulsar. Meanwhile, the planet continued on its leisurely orbit at a distance of about 2 billion miles from the pair.

It is likely that the planet is a gas giant, without a solid surface like the Earth. Because it was formed so early in the life of the universe, it probably doesn't have abundant quantities of elements such as carbon and oxygen. For these reasons, it is very improbable the planet would host life. Even if life arose on, for example, a solid moon orbiting the planet, it is unlikely to have survived the intense X-ray blast that would have accompanied the spin-up of the pulsar.

(Tim Stephens contributed to this story)

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