PSR B1620-26c

PSR B1620-26c is a planet orbiting the pulsar PSR B1620-26 in the globular cluster Messier 4, about 12,400 light years (117 Em) from Earth in the constellation Scorpius. As the third object found in the system, the planet's official designation is PSR B1620-26c. It has been nicknamed Methuselah, but this name has not yet been accepted by the International Astronomical Union.

PSR B1620-26c orbits a binary pair of stars. One, the pulsar, is a neutron star. The second is a white dwarf with a mass of 0.34 solar mass. These stars orbit each other at a distance of 1 AU about once every half year. The planet, which has a mass of 2.5 Jupiters, is in a much wider orbit, similar to that of Uranus around the Sun, only larger; PSR B1620-26c orbits at a distance of 23 AU (3.4 Tm). Each orbit of the planet takes about 100 years.

The triple system is just outside the core of the globular cluster. The age of the cluster has been estimated to be about 12.7 billion years, and because all stars in a cluster form at about the same time, and planets form together with their host stars, it is likely that PSR B1620-26c is also about 12.7 billion years old. This is much older than any other known planet, and nearly three times as old as Earth. The nickname Methuselah was given as a reference to the long-lived Biblical Methuselah.

Discovery of PSR B1620-26c

The study of the planetary orbit allowed the mass of the white dwarf star to be estimated as well, and theories of the formation of the planet suggested that the white dwarf should be young and hot. On July 10, 2003, the detection of the white dwarf and confirmation of its predicted properties were announced by a team led by Steinn Sigurdsson, using observations from the Hubble Space Telescope. It was at a NASA press briefing that the name Methuselah was introduced, capturing press attention around the world.

Evolutionary history

It is more likely that Methuselah formed in orbit around the star that has now evolved into the white dwarf, and that the star and planet were only later captured into orbit around the neutron star.
Stellar encounters are not very common in the disk of the Milky Way, where our Sun lives, but in the dense core of globular clusters they occur frequently. At some point during the 10 billion years, the neutron star encountered and captured the host star of Methuselah to a tight orbit, probably losing a previous companion star in the process.
About half a billion years ago, the newly captured star began to expand into a red giant (see stellar evolution). But a red giant could not fit inside the tight orbit, and so its surface layers started falling into the neutron star.

The infalling matter produced complex and spectacular effects. For a few hundred million years, the stars formed a low-mass X-ray binary, as the infalling matter was heated to temperatures high enough to glow in X-rays. The infalling matter spun up the neutron star. Even today, the
pulse period is a few milliseconds. Typical pulsar periods for young pulsars are of the order one second, and they decrease with time, so an old pulsar with a faster spin period is direct evidence in favor of this recycling picture.

Mass transfer came to an end when 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. The long-term prospects for PSR B1620-26c are poor, though. The triple system, which is much more massive than a typical isolated star in M4, is slowly drifting down into the core of the cluster, where the density of stars is very high. In a billion years or so, the triple will probably have another close encounter with a nearby star. The most common outcome of such encounters is that the lightest companion is ejected from the multiple star system. If this happens, PSR B1620-26c will most likely be ejected completely from M4, and will spend the rest of its existence wandering alone in interstellar space.

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