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NASA Science News for July 10, 2003

Ancient Planet

Some 13 billion years ago in a distant cluster of stars, a planet
formed. Remarkably it's still there, according to data from the
Hubble Space Telescope. 

July 10, 2003: Long before our Sun and Earth ever existed, a
Jupiter-sized planet formed around a sun-like star. Now, almost 13
billion years later, NASA's Hubble Space Telescope has precisely
measured the mass of this farthest and oldest known planet.

The ancient planet has had a remarkable history, because it has wound
up in an unlikely, rough neighborhood. It orbits a peculiar pair of
burned-out stars in the crowded core of a globular star cluster. 

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 2.5 times the mass of Jupiter. Its
very existence provides tantalizing evidence the first planets were
formed rapidly, within a billion years of the Big Bang, leading
astronomers to conclude planets may be very abundant in the universe. 

The planet lies near the core of the ancient globular star cluster
M4, located 5,600 light-years away in the northern-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. Some astronomers have therefore argued that globular
clusters cannot contain planets, because planets are often made of
such elements. This conclusion was seemingly bolstered in 1999 when
Hubble failed to find close-orbiting "hot Jupiter"-type planets
around the stars of the globular cluster 47 Tucanae. Now, it seems
astronomers were just looking in the wrong place, and gas-giant
worlds, orbiting at greater distances from their stars, could be
common in globular clusters. 

"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. 

"This is tremendously encouraging that planets are probably abundant
in globular star clusters," agrees Harvey Richer of the University of
British Columbia (UBC) in Vancouver. He bases this conclusion on the
fact a planet was uncovered in such an unlikely place: orbiting two
captured stars, a helium white dwarf and a rapidly spinning neutron
star, near the crowded core of a globular cluster. In such a place,
fragile planetary systems tend to be ripped apart due to
gravitational interactions with neighboring stars. 

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 was quickly found
through its effect on the clock-like pulsar, as the two stars orbited
each other twice per year. Sometime later, astronomers noticed
further irregularities in the pulsar that implied a third object was
orbiting the others. This new object was suspected to be a planet,
but it also could have been a brown dwarf or a low-mass star. Debate
over its true identity continued through the 1990s. 

Sigurdsson, Richer, and their co-investigators settled the debate by
at last measuring the planet's actual mass through some ingenious
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. Using evolutionary
models computed by Brad Hansen of the University of California, Los
Angeles, the astronomers estimated the white dwarf's mass.

This in turn was compared to the amount of wobble in the pulsar's
signal, allowing the team 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 the precise mass could at last be
known. With a mass of only 2.5 Jupiters, the object is too small to
be a star or brown dwarf and must instead be a planet. The planet is
likely a gas giant without a solid surface like the Earth. 

A 13-billion year old planet orbiting a pair of long-dead stars in a
crowded globular cluster: even for the Hubble Space Telescope, that's
amazing! 

The full team involved in this discovery is composed of Hansen,
Richer, Sigurdsson, Ingrid Stairs, UBC, and Stephen Thorsett,
University of California in Santa Cruz. 
 
Credits & Contacts
Source: A NASA press release 
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

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