The universe is an immense expanse of time, estimated to be approximately 13.8 billion years old, a scale that makes our own 4.6-billion-year-old Solar System seem relatively young. The search for the oldest world in existence is a deep dive into the most ancient structures of the cosmos. Astronomers must look far beyond the Sun’s influence, toward the earliest stellar populations, to uncover the planets that formed almost at the dawn of time. Finding these ancient worlds provides insight into the conditions necessary for planet formation when the universe was still in its infancy.
The Oldest Confirmed Exoplanet
The current record-holder for the oldest confirmed exoplanet is a gas giant officially designated PSR B1620-26 b, often informally called “Methuselah” due to its immense age. This world orbits a binary star system located deep within the globular cluster Messier 4 (M4), a dense spherical collection of hundreds of thousands of stars in the constellation Scorpius. The planet’s estimated age is a staggering 12.7 billion years, making it nearly three times older than Earth.
It is an enormous gas giant, possessing a mass about 2.5 times that of Jupiter, and it orbits its two host stars at a distance of approximately 23 astronomical units, comparable to the distance between Uranus and our Sun. The planet’s host system is particularly unusual, consisting of a rapidly spinning neutron star, known as a pulsar, and a cooling white dwarf star. This highly evolved and unique arrangement is a direct consequence of the system’s violent and protracted history within the crowded environment of the globular cluster.
How Scientists Determine Planetary Age
Since exoplanets cannot typically be sampled like rocks in our Solar System, scientists must determine their age indirectly. The most reliable method involves dating the planet’s host star system, based on the assumption that the planet and its star formed at roughly the same time. The age of PSR B1620-26 b was determined primarily by studying the massive globular cluster M4 in which it resides.
Globular clusters are collections of stars that all formed together, allowing astronomers to estimate their age through a technique called the main-sequence turnoff. By observing the color and brightness of the stars, scientists identify the point where the most massive stars have exhausted their core hydrogen fuel and begun to evolve off the main sequence. Since the lifespan of a star is directly related to its mass, this turnoff point serves as a precise clock for the entire cluster.
The age estimate for the Methuselah system was further refined by analyzing its white dwarf companion. White dwarfs are the dense, cooling remnants of dead stars, and their temperature steadily decreases over billions of years. By measuring the white dwarf’s current temperature, astronomers calculate its cooling rate and estimate the time elapsed since it formed. This independently confirmed the ancient age derived from the globular cluster’s turnoff point, providing a highly precise age of 12.7 billion years for the entire triple system.
Placing Ancient Planets in the Cosmic Timeline
The existence of a 12.7-billion-year-old exoplanet fundamentally changes our perspective on when planets began to form throughout the universe. The universe itself is only about 13.8 billion years old, meaning this giant world coalesced within the first billion years after the Big Bang. In contrast, our own Sun and Solar System did not form until approximately 9 billion years later.
The discovery of PSR B1620-26 b challenged previous theories that suggested planet formation, especially for gas giants, required a higher concentration of “metals”—astronomers’ term for any element heavier than hydrogen and helium. Older stellar populations, like those found in globular clusters such as M4, are known to be metal-poor because there had not yet been enough time for previous generations of stars to live and die, scattering heavy elements into space.
Finding a massive gas giant in a metal-poor, first-generation environment implies that planet formation can occur under a wider range of conditions than previously thought. This ancient world demonstrates that the building blocks for planets were available very early in the universe’s history. The formation of this planet suggests that the conditions for planetary systems were established almost immediately.