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.
Right: An artist's concept of a planet orbiting two stars--a neutron star and a white dwarf--in the globular cluster M4. The skies of the densely-packed cluster are remarkably starry. See also the animation.
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.
Above: You can see globular cluster M4 yourself. Train a small telescope or binoculars about 1 degree (two moon-widths) west of the bright star Antares in the constellation Scorpius. If you live in the northern hemisphere, look south around 10 p.m. local time to find Scorpius. If you live in the southern hemisphere, look high and southeast.
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.
Oldest Known Planet Identified -- (NASA) NASA's Hubble Space Telescope precisely measured the mass of the oldest known planet in our Milky Way galaxy.
Learn more about the Hubble Space Telescope
M4 Links: Messier Object 4 (SEDS); The Closest Known Globular Cluster (APOD)
Pulsar Planets were discovered by radio astronomer Alexander Wolszczan.