The sun formed about 4.5 billion years ago, and all the planets began to form soon after. All the planets in our solar system are about 4.5 billion years old, which really is an immense amount of time. However, it is not as old as the oldest planet ever discovered. The oldest planet ever found in the universe is an exoplanet designated PSR B1620-26 b that is almost 13 billion years old. He has also received the nickname of Methuselah, in honor of the oldest character in the Bible.
discovery
Hubble image of Messier 4, the globular cluster that contains Methuselah. Image credit: NASA/ESA
Unlike most discovered exoplanets, Methuselah 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. Instead of stars, Methuselah orbits a white dwarf and a pulsar. Interestingly, Methuselah was one of the first planets detected beyond our solar system, although since it does not orbit a main-sequence star, it does not technically fall within the definition of an exoplanet. Methuselah’s existence was announced in 1993 and, like most planets beyond our solar system discovered at the time, it was found using the radial velocity method. This method looks for any changes in a star’s light caused by the Doppler effect, which will change the wavelength of the light based on the object’s motion relative to us. In the case of Methuselah, 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 slightly.
age
Enlarged Messier 4 image showing the approximate location of Methuselah. Image credit: NASA/ESA
In addition to being the oldest planet ever discovered, Methuselah is also unique in that it was the first planet found to orbit more than one object, and it was also the first planet found inside of a globular cluster of stars. The globular cluster, designated 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 same cluster, so any planets orbiting within the globular cluster are likely to be the same age. Methuselah is thus estimated to be about 12.7 billion years old, making it the oldest planet ever found and 2.8 times older than Earth.
Characteristics
Representation of Methuselah together with a pulsar and a white dwarf in the background. Image credit: NASA
Methuselah has a mass 2.7 times that of Jupiter, meaning it is likely a gas giant. It orbits at a distance of 2.1 billion miles (3.4 billion kilometers) and takes approximately 100 years to complete a single orbit. Other than its age, mass and distance, not much else is known about this ancient planet, but astronomers believe it may have once orbited a star not so different from our sun. Methuselah is unlikely to have formed in orbit around the white dwarf and pulsar, as its gravity simply isn’t strong enough to attract enough material for a planet to form. Rather, astronomers believe that Methuselah likely formed around a sun-like star that eventually became a white dwarf. Methuselah is at a far enough distance that it would have survived the star’s red giant phase just prior to white dwarf formation.
Aidan Remple October 16, 2022 in Science