The globular cluster containing the ancient exoplanet PSR B1620-26 b is Messier 4, or M4.
PSR B1620-26 b is famous for its unusual setting: it orbits a pulsar–white-dwarf binary system inside M4. The planet is often nicknamed “Methuselah” because estimates place it at roughly 12.7 billion years old, making it one of the oldest known planets. Its survival through the evolution of two stars makes its history especially remarkable.
M4 is a dense, gravitationally bound globular cluster in Scorpius, approximately 6,000 light-years from Earth. It is also known as NGC 6121 and is the closest globular cluster to the Solar System. Astronomers first identified the pulsar companion in the system and then inferred the planet from timing variations in the pulsar’s regular radio pulses.
The planet is sometimes confused with the pulsar planets around PSR B1257+12, which orbit an isolated millisecond pulsar rather than a binary system in a globular cluster. M4 is the cluster’s designation; PSR B1620-26 identifies the stellar system, and the lowercase b identifies its planet.