The Milky Way galaxy is an immense, spiraling collection of hundreds of billions of stars spanning over 100,000 light-years. The search for exoplanets, or planets outside our Solar System, has revealed worlds ranging from the very young to the truly ancient. Pinpointing the absolute oldest planet is challenging because its age must be deduced from the faint light and gravitational influences of its parent stars, which are often billions of light-years away. Discovering a planet that formed shortly after the galaxy itself began to coalesce provides a unique window into the earliest era of planet formation, confirming that planetary bodies existed long before the Sun or Earth.
The Oldest Known Planet in the Milky Way
The current title for the oldest known exoplanet belongs to PSR B1620-26 b, a gas giant with an estimated age of approximately 12.7 billion years. This makes the planet nearly three times older than Earth, which formed about 4.5 billion years ago. Due to its extreme longevity, the planet has earned the unofficial nickname “Methuselah,” after the biblical figure known for his long lifespan.
The planet is massive, weighing about two and a half times the mass of Jupiter. It exists in a highly unusual and gravitationally complex environment, orbiting two stellar remnants in a circumbinary configuration. These host stars are a rapidly spinning neutron star, known as a pulsar, and a dense, cooling star called a white dwarf. The entire triple system is situated far from the galactic center, residing in one of the Milky Way’s oldest structures.
The Ancient Environment of Globular Clusters
PSR B1620-26 b was discovered within the globular cluster Messier 4 (M4), a dense, spherical collection of stars in the constellation Scorpius. Globular clusters are relics of the early universe, being among the first large structures to form in the Milky Way. The age of M4 is estimated to be around 12.7 billion years, which sets the age for all the stars and planets contained within it.
These ancient stellar cities are characterized by extremely low metallicity, which is the astronomical term for the concentration of elements heavier than hydrogen and helium. Early in the universe, only the lightest elements were available, as heavier elements are forged through stellar fusion and supernova explosions. The existence of a giant planet like PSR B1620-26 b in such a metal-poor environment challenged earlier theories that suggested high metallicity was a prerequisite for forming gas giants.
The dense conditions inside a globular cluster suggest that PSR B1620-26 b did not necessarily form around the stars it orbits today. Gravitational interactions within the cluster’s crowded core are powerful enough to strip a planet from its original star and allow it to be captured by another system. This dynamic history means the planet likely survived multiple violent stellar events, including the supernova that created its current pulsar companion. The planet’s survival underscores the resilience of planetary systems even in the galaxy’s most tumultuous environments.
How Astronomers Determine Planetary Age
Determining the age of an exoplanet relies almost entirely on calculating the age of its host star, since planets and their stars form simultaneously from the same cloud of gas and dust. For stars in globular clusters, astronomers use a technique centered on the Hertzsprung-Russell (H-R) diagram, which plots a star’s temperature against its luminosity. All stars in a cluster are assumed to have the same age, so their position on the diagram reveals their evolutionary stage.
Massive stars burn through their nuclear fuel quickly and evolve off the main sequence first, transforming into red giants. The point on the H-R diagram where stars begin to leave the main sequence is called the main-sequence turnoff. By comparing the position of this turnoff point with theoretical models of stellar evolution, scientists can accurately estimate the age of the entire cluster.
In the case of PSR B1620-26 b’s system, a second, specific method was available due to the presence of a white dwarf. A white dwarf is the dense, leftover core of a star that has exhausted its nuclear fuel and slowly cools over billions of years. By measuring the temperature and luminosity of the white dwarf companion, astronomers can determine how long it has been cooling, which acts as a highly reliable cosmic clock for the system’s overall age. This precise methodology confirmed the system’s formation roughly 12.7 billion years ago, solidifying the planet’s status as the oldest yet found.
Contextualizing Cosmic Time: Comparing Ages
The 12.7-billion-year age of PSR B1620-26 b places its formation just over a billion years after the Big Bang, which occurred approximately 13.8 billion years ago. This means the planet was created during the universe’s infancy, at a time when the Milky Way itself was still taking shape. Its ancient existence highlights that planet formation began very early in the cosmic timeline.
To put this in perspective, our own Solar System, including the Sun and Earth, is a relative newcomer, having formed only about 4.6 billion years ago. When PSR B1620-26 b was already a fully formed gas giant, the material that would eventually become our Sun and planets was still an unformed cloud of gas and dust. The contrast in ages demonstrates that ancient, long-lived worlds are a confirmed part of our galactic neighborhood.