Globular Cluster M4 - NOAO Ground-Based Image

Three unlikely companions - two burned-out stars and a planet - orbit each other near the crowded core of ancient globular cluster M4, of more than 100,000 stars. 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.