Methuselah (planet)

PSR B1620-26 b, nicknamed Methuselah, is an extrasolar planet whose discovery was announced on July 10, 2003. It is approximately 5,600 light years from Earth, located near the heart of the globular cluster M4 (see Messier object) in the constellation Scorpius.

Methuselah has a mass twice that of Jupiter, and is estimated to be about 12.7 billion years old, believed to be by far the oldest planet yet discovered. The nickname Methuselah is a reference to the long-lived Biblical Methuselah.

The planet's orbit is about 100 years long. It travels around two old stars (a rapidly rotating neutron star known as the pulsar PSR B1620-26, and its companion white dwarf star) at a distance that is likely similar to Uranus's distance from the Sun.

Evolutionary history

It is believed that all stars in a globular cluster form in a relatively short time. As planets form at the same time as the stars they orbit, the age of Methuselah is close to the age of the host cluster M4 (12.7 billion years). Methuselah was formed in orbit around the star that has now evolved to a white dwarf, as it is unlikely that a planet would have survived the supernova explosion that created the pulsar.

Stellar encounters are not very common, but in the dense core of globular clusters they are possible. At some point during the 10 billion years, the pulsar encountered and captured the host star of Methuselah to a tight orbit. In a few billion years, the star tried expanding into a red giant (see stellar evolution). A red giant could not fit inside the tight orbit, and therefore it's surface layers started falling into the neutron star.

The infalling matter produced complex and spectacular effects, and for a few hundred million years, the stars formed a low-mass X-ray binary. The infalling matter spun up the neutron star. Even today, the pulse period is a few milliseconds. Typical pulsar period is of the order one second, and increases with time.

All things come to an end, the mass transfer is no exception. The surface layers of the mass-losing star were depleted, and the core slowly shrunk to a white dwarf. Now the stars peacefully orbit around each other. Unless close encounters with other stars happen, they will continue on their orbits for billions of years.