The search for the oldest planet is a quest to understand the universe’s timeline, revealing when and how planetary systems first emerged from cosmic dust. Finding objects that formed shortly after the Big Bang offers direct insight into the earliest conditions capable of supporting planet formation. This endeavor is challenging because these ancient stellar systems require sophisticated methods to measure their age across immense spans of space and time. The discovery of these relics shows that planet creation began almost as soon as the first stars ignited.
Identifying the Current Record Holder
The planet currently recognized as the oldest exoplanet is PSR B1620-26 b, nicknamed “Methuselah.” This gas giant, located approximately 12,400 light-years away in Scorpius, is estimated to be about 12.7 billion years old, nearly three times the age of Earth. Its formation occurred about one billion years after the Big Bang, demonstrating that planet-building began very early in the universe.
The planet resides in a triple system, orbiting two central hosts: a rapidly spinning neutron star (pulsar) and a dense white dwarf. This entire system is located just outside the core of Messier 4 (M4), one of the oldest and most densely packed globular clusters in the Milky Way galaxy.
Methods for Dating Ancient Exoplanets
The age of an exoplanet is typically determined by measuring the age of its host star, a process that relies on highly accurate astrophysical modeling. For extremely old systems, such as those in globular clusters, two primary techniques provide the necessary precision. The first method is the main sequence turnoff method, which analyzes the stellar evolution of stars still burning fuel.
In a star cluster, all stars formed at roughly the same time, but their masses dictate their lifespan. More massive stars consume fuel quickly and die sooner. By observing the point on a cluster’s Hertzsprung-Russell diagram where stars deviate from the main sequence—the “turnoff” point—astronomers calculate the maximum mass of stars still alive, which directly correlates to the cluster’s age.
The second method, applicable to PSR B1620-26 b’s system, involves the cooling rate of a white dwarf star. A white dwarf is the dense, hot core left after a star exhausts its nuclear fuel, which slowly cools over billions of years. Since the cooling rate is well understood, measuring a white dwarf’s current temperature and luminosity allows scientists to calculate its “cooling age.”
The age of the record-holding planet was established by estimating the age of the M4 globular cluster (around 12.7 billion years old) and by using the cooling age of the white dwarf. This dual-method approach provides a robust estimate for the planet’s antiquity, assuming it formed alongside its host stars.
Planet Formation in the Early Universe
The existence of PSR B1620-26 b proves that planet formation was possible even when the universe was elementally young. Planets form from a protoplanetary disk of gas and dust, but the material’s composition changes over cosmic time. Astronomers classify all elements heavier than hydrogen and helium as “metals,” which are the building blocks required for rocky cores and dust grains.
The very first stars (Population III) formed from primordial gas containing virtually no metals. These massive, short-lived stars ended their lives in supernovae, scattering the first heavy elements into space. The next generation of stars (Population II) formed from gas clouds slightly enriched with these freshly made metals.
The oldest planets orbit these metal-poor Population II stars, which are common in globular clusters and the galactic halo. Although the metal content is low compared to Sun-like stars (Population I), the presence of the planet suggests that only a small fraction of heavy elements was sufficient to initiate core formation for gas giants. This demonstrates the efficiency of planetary construction shortly after the universe synthesized its first heavy atoms.
Characteristics of the Ancient World
PSR B1620-26 b is a large gas giant, possessing a mass approximately 2.5 times that of Jupiter. It follows a wide, circumbinary orbit around both host stars, completing one revolution roughly every 100 years at a distance of about 23 astronomical units. This distance is slightly greater than the orbital distance of Uranus from the Sun.
The planet’s environment is highly dynamic and gravitationally complex due to the presence of the pulsar, which emits intense radiation, and the super-dense white dwarf. Its survival in this extreme, tightly packed environment within the dense globular cluster M4 is remarkable.
Planets in globular clusters face frequent gravitational encounters that can destabilize or eject them. Scientists believe PSR B1620-26 b may not have formed in its current location but was captured or exchanged into the binary system through a chaotic close encounter. Its continued existence provides an ancient example of how planetary systems can be molded and preserved through violent gravitational interactions.