PSR B1620−26 b

In 1988 radio astronomers found a pulsar in the globular cluster M4, a dense ball of ancient stars in the constellation Scorpius. A pulsar is the spinning core of a star that exploded, and this one turns about 90 times a second, sweeping a beam of radio waves past Earth on every turn. Its pulses arrive with clockwork regularity, and that is what gave away its secrets.

Found by a ticking clock

The pulsar has a companion: the pulses arrive slightly early and then slightly late as the two swing around each other every 191 days. But in 1993 two teams noticed a slower drift on top of that, the sign of a third body much farther out. Years of further timing showed it had to be far lighter than any star. It might be a planet, but the timing alone could not say how heavy it was.

The answer came in 2003. Steinn Sigurdsson and his colleagues used the Hubble Space Telescope to find the pulsar’s inner companion, a faint white dwarf with about a third of the Sun’s mass. Knowing its mass let them work out how the inner pair is tilted, and from that the mass of the outer body: about 2.5 times Jupiter’s, give or take one, on an orbit about 23 AU from the pair. It is a giant planet, taking about a century to go round once.

Older than almost everything

M4 is one of the oldest objects in the Milky Way. In 2002 a team measured how far its white dwarfs have cooled and found that the cluster is about 12.7 billion years old, almost as old as the universe. If the planet formed with the cluster, it is nearly three times as old as Earth. That is why it was nicknamed the Methuselah planet, after the longest-lived man in the Bible, and why NASA announced it as the oldest planet known.

Its age is inferred rather than measured: it rests on the planet having formed together with the cluster’s stars.

A planet where planets were not expected

M4’s stars contain only about 5% as much of the heavier elements as the Sun, and planets were thought to need those elements to form. A search for close-in giant planets in another old cluster, 47 Tucanae, had found none, which seemed to support the idea. The Methuselah planet suggested that planets could form very early, even from such poor material.

How it ended up around a pulsar is a story of a crowded cluster. In the explanation offered by the 2003 team, the planet formed around an ordinary Sun-like star. In the dense core of M4, that star and its planet passed close to the pulsar and its original companion, and in the encounter the pulsar captured them. The star later swelled, shed its outer layers and became the white dwarf seen today. Another proposal has the planet forming much later, in a disc of gas around the pair, which would make it far younger.