Ancient Planet

Earth is four and a half billion years old. On the timescale of the universe, that makes it a youngster. The earliest planets probably were born more than eight billion years earlier — when the universe was only about one billion years old.

One of the oldest planets yet discovered orbits a pair of dead stars. The system is known as PSR B1620-26. It’s in an ancient star cluster that’s more than 12,000 light-years away.

The planet is bigger than Jupiter, the giant of our own solar system. And it has an interesting history.

It probably was born around a Sun-like star. The system zipped through the densely packed center of the star cluster. There, it passed close to a binary — two stars locked in orbit around each other. One member of the system was a neutron star — the corpse of an exploded star. In a complex gravitational dance, the neutron star changed partners. It grabbed the Sun-like star and its planet, and kicked out its original companion.

Later, the planet’s original star reached the end of its life. It puffed up, so some of its gas spilled onto the neutron star. That caused the neutron star to spin faster. Today, it spins about a hundred times a second, beaming a “pulse” of energy in our direction with each turn. Slight changes in the timing of those pulses revealed the planet.

Today, the planet orbits both of these stars — almost 13 billion years after it was born.

More about exoplanets tomorrow.

Script by Damond Benningfield