White Dwarf Yields Key to Planet's Mass

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/ESA 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.