The sun formed around 4.5 billion years ago, and all the planets began forming soon after. All the planets in our solar system are about 4.5 billion years old, which truly is an immense period of time. However, it is nowhere near as old as the oldest planet ever discovered. The oldest planet ever found in the universe is an exoplanet designated as PSR B1620-26 b at nearly 13-billion years old. It has also been nicknamed Methusalah, after the oldest character from the bible.
Discovery
Unlike most exoplanets discovered, Methusalah does not orbit a typical star. Rather, it is a circumbinary planet, meaning it orbits two different objects. These two objects, however, are not main sequence stars. Rather than stars, Methusalah orbits a white dwarf and a pulsar. Interestingly, Methusalah was one of the first planets ever detected beyond our solar system, although since it does not orbit a main sequence star, it does not technically fall under the definition of an exoplanet. The existence of Methusalah was announced in 1993, and like most planets beyond our solar system discovered at that time, it was found using the radial velocity method. This method looks for any shift in a star’s light caused by the Doppler Effect, which will change the wavelength of light depending on the object’s motion relative to us. In the case of Methusalah, astronomers observed a Doppler shift in the light of the nearby pulsar, and calculations showed that the most likely explanation was that a planet was exerting a gravitational force on the pulsar, causing it to move ever so slightly.
Age
In addition to Methusalah being the oldest planet ever discovered, it is also unique in that it was the first planet ever found to orbit more than one object, and it was also the first planet ever found within a globular cluster of stars. The globular cluster, designated as Messier 4, has an estimated age of 12.7 billion years and is located 12,400 light years away. Globular clusters tend to be quite old, which is due to the abundance of low mass stars that can burn for hundreds of billions of years contained within them. Globular clusters can also contain thousands to millions of individual stars, their mutual gravity holding the structure together for billions of years. Most stars within globular clusters likely formed within the cluster itself, and so any planets that orbit within the globular cluster are likely the same age. Thus, Methusalah is estimated to be around 12.7 billion years old, making it the oldest planet ever found and 2.8 times older than the Earth.
Characteristics
Methusalah has a mass 2.7 times larger than Jupiter, meaning it is likely a gas giant. It orbits at a distance of 2.1 billion miles (3.4 billion kilometres) and takes approximately 100 years to complete a single orbit. Other than its age, mass, and distance, not much more is known about this ancient planet, yet astronomers believe that it may have once orbited a star not so different from our sun. It is unlikely that Methusalah formed in orbit around the white dwarf and pulsar as their gravity is simply not strong enough to pull in enough material for a planet to form. Rather, astronomers believe that Methusalah likely formed around a sun-like star that eventually became a white dwarf. Methusalah is located at a far enough distance that it would have survived the red giant phase of the star just prior to the formation of the white dwarf.
