exoplanet-584

This artist's impression shows HIP 13044 b, an exoplanet orbiting a star that entered our galaxy, the Milky Way, from another galaxy. ((L. Calçada/ESO) )

Astronomers say they've found the first known planet to orbit a star that began its life outside our own Milky Way galaxy. 

In the last 15 years, almost 500 planets have been detected orbiting stars in our galaxy. But until now, no planet has been discovered that came from another galaxy.

The discovery of a giant planet around the star HIP 13044 changes all that, a team of astronomers claims.

The star is actually now part of our galaxy, but scientists say it didn't start out that way. HIP 13044 was part of a group of stars that originally belonged to a dwarf galaxy that the Milky Way absorbed six to nine billion years ago in an act of galactic cannibalism.

"For the first time, astronomers have detected a planetary system in a stellar stream of extragalactic origin," says Rainer Klement of the Max Planck Institute for Astronomy.

"Because of the great distances involved, there are no confirmed detections of planets in other galaxies," he said in a statement. "But this cosmic merger has brought an extragalactic planet within our reach."

The planet — named HIP 13044 b — lies about 2,000 light years from Earth in the southern constellation of Fornax. Its mass is at least 1.25 times that of Jupiter's and takes only 16.2 days to complete an orbit.

It was discovered by looking for the tiny wobbles produced when an orbiting object creates a pull on a star's gravity. Astronomers used a high-resolution spectrograph attached to the 2.2-metre MPG/ESO telescope at the La Silla Observatory in Chile.

The planet's host star is near the end of its life, which makes it remarkable in that the planet managed to survive. Stars undergo massive expansion in their final years, turning into red giants. HIP 13044 has now contracted again.  

"This discovery is particularly intriguing when we consider the distant future of our own planetary system, as the sun is also expected to become a red giant in about five billion years," said Johny Setiawan, also of the Max Planck Institute.

The research appears in Thursday's edition of Science Express.