Giant extra-galactic planet discovered
Astronomers have for the first time spotted a planet that originated outside our galaxy.
To date about 500 planets have been discovered within our galaxy, but this is the first known to have been born elsewhere.
The gas planet, at least 25pc heavier than Jupiter – or 400 times heavier than Earth – orbits a star that started life in a dwarf galaxy.
Known as HIP 13044b, the hydrogen and helium planet sits in a solar system belonging to a group of stars called the Helmi stream, some 2,000 light years away from Earth.
Between six and nine billion years ago the Helmi merged with the Milky Way in an act of "galactic cannibalism". It is now in a southern constellation of the Milky Way called the Fornax, or Furnace.
Because of the vast distance, astronomers at the European Southern Observatory (ESO) in La Silla, Chile, could not detect the planet visibly, using the 2.2 metre diameter telescope.
Instead, they inferred its existence from tiny telltale wobbles of the star. These are caused by the gravitational tug of its large orbiting companion, which the astronomers detected with a high-resolution spectrograph attached to the telescope.
Rainer Klemen, of the Max-Planck Institute for Astronomy in Heidelberg, Germany, said: "This discovery is very exciting.
"For the first time, astronomers have detected a planetary system in a stellar stream of extragalactic origin. Because of the great distances involved, there are no confirmed detections of planets in other galaxies. But this cosmic merger has brought an extragalactic planet within our reach."
The planet is orbiting a star which is approaching the end of its life, having exhausted its hydrogen fuel and gone through a stage of massive expansion – called the red giant phase – in which it probably consumed the inner planets in its solar system. It has now contracted again and is burning helium in its core.
The findings have been published in the journal Science Express.