There may be more planets in our galaxy than previously realised, say astronomers.
By Dr David Whitehouse
BBC News Online science editor
It is a far-reaching conclusion based on the discovery of just one planet in a location where it was not expected.
OLDEST KNOWN PLANET
Planet may be 13bn years old
Is 2.5 times the size of Jupiter
Orbits its two stars every century
Located in M4 globular cluster
In Scorpius; 5,600 light-years away
The new planet formed around a star in a so-called globular cluster - a swarm of ancient stars once thought unlikely to have any planets in orbit about them.
Through a series of remarkable encounters with other stars, the new planet, along with its now dying parent star, have been propelled into a region of space where they have finally been observed.
Their story - along with the finding of planets around nearby, isolated, stars - means that our galaxy, and the wider Universe, is probably teeming with planets.
The remarkable adventures of a star and its planet cover aeons of time.
Over 10 billion years ago, a planet formed around a young star that was one of the first generation of stars to emerge after the Big Bang.
For a long time, it would have led an uneventful existence in the crowded heart of a globular cluster, almost undetectable from the outside.
Then along came a binary star system consisting of a neutron star and a white dwarf. Both are small, compact stars.
In the gravitational tussle that followed, the white dwarf was ejected, and the normal star took its place. The planet found itself orbiting both remaining stars.
Into the open
The expulsion of the white dwarf caused, according to Newton's law, the other stars, and the planet, to recoil in the opposite direction.
Over the next few billion years, the normal star grew old and swelled to become a red giant star, and then a fading white dwarf itself.
Hubble found the parent star
But in its expanding phase, the companion neutron star's gravity pulled material from the aging star's atmosphere on to itself. This caused the neutron star to spin more rapidly.
Eventually, the neutron star was spinning a hundred times a second, and it became a pulsar - a flashing "lighthouse" of radio waves.
Over the next billion years or so, the neutron star left the crowded centre of the cluster to traverse its more sparsely populated outer reaches, and where its radio pulses were detected by telescopes on Earth.
It was the radio observations of the pulsar that first revealed the existence of the small planet within the system.
ANCIENT STAR CONTROVERSY
Ancient stars deficient in heavier elements
These elements make planets, like Earth
Led to theory globular clusters had no planets
Discovery shows planet formation more robust
Suggests galaxy is teeming with other worlds
Now, writing in the journal Science, astronomers, using the Hubble Space Telescope, report the detection of the old white dwarf, thereby completing our understanding of the system's history.
Stein Sigurdsson, of Pennsylvania State University, and colleagues say they never expected a planet to form around such a young star in the first place.
Normally, it would not be possible to see the planet, but thanks to a remarkable series of events, astronomers now realise that stars once thought barren may have planets circling them.