Three unlikely companions - two burned-out stars and a planet - orbit each other near the crowded core of an ancient globular cluster of more than 100,000 stars. Only one companion, however, is visible in the images. In the image at right, taken by NASA's Hubble Space Telescope, the white arrow points to a burned-out white dwarf star. Radio astronomers discovered the white dwarf and the other burned-out star - a rapidly spinning neutron star, called a pulsar - a decade ago. The third companion's identity was a mystery. Was it a planet or a brown dwarf? The object was too small and too dim to image.
Hubble observations of the dim white dwarf helped astronomers to precisely measure the mass of the mystery object (2.5 times larger than the mass of Jupiter), confirming that it is a planet. In fact, it is the farthest and oldest known planet. Hubble's Wide Field and Planetary Camera 2 resolved individual stars near M4's densely packed core [right] and pinpointed the white dwarf.
The image at left, taken by the National Optical Astronomy Observatory, shows the companions' home, a 13-billion-year-old cluster called M4. The green box marks the location of Hubble's close-up view.
The Hubble observations of the white dwarf held the key to discovering the identity of the third companion. Astronomers used Hubble to measure the white dwarf's color and temperature. By knowing those physical properties, astronomers then calculated the white dwarf's age and mass. They then compared that information to the amount of wobble in the pulsar signal, which allowed astronomers to calculate the tilt of the white dwarf's orbit as seen from Earth. That critical piece of evidence, when combined with the radio studies of the wobbling pulsar, allowed astronomers to determine the tilt of the planet's orbit and subsequently its mass.
The cluster is located 5,600 light-years away in the summer constellation Scorpius. The Hubble image was taken in April 1996.