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)
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
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)
Return to Front Page