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 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 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 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, State College.
“This is tremendously encouraging that planets are probably
abundant in globular star clusters,” says Harvey Richer of
the University of British Columbia (UBC), Vancouver, Canada.
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 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. 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.
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.
Electronic images and additional information are available
on the Internet at:
