'Alien' planet discovered in our Milky Way

European astronomers scanning the Milky Way have discovered a planet near a star of extragalactic origin, implying that it came from outside our own galaxy. 

The finding, which challenges current understanding about how planets are formed and survive, was made in the so-called Helmi stream — a group of stars that originally belonged to a dwarf galaxy that was devoured by the Milky Way in what the astronomers called "an act of galactic cannibalism." 

"As far as I know this is the first time a planet like this has been discovered. It came to our galaxy about 6 to 9 billion years ago, so it's like a visitor," said Johny Setiawan at the Germany's Max Planck Institute for Astronomy, who led the team who made the discovery. 

Over the last 15 years, astronomers have detected nearly 500 planets orbiting stars in our cosmic neighborhood, but none from outside our Milky Way galaxy has yet been confirmed. "This discovery is very exciting," said Rainer Klement, also of the Max Planck Institute. "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 host star is called HIP 13044 and the planet is known as HIP 13044b. It lies around 2,000 light years from Earth in the southern constellation of Fornax, or the Furnace, the scientists said in a study in the journal Science Express. 

It was detected using a high-resolution spectrograph attached to a 2.2-meter telescope at the La Silla European Southern Observatory in Chile. The scientists said the finding raised questions about accepted theories of planet formation since it is the first time a planet has been found around an old star that contains very few elements apart from hydrogen and helium. 

"According to the widely-accepted planet formation model, it is actually not possible to form a planet around such a very metal-poor star," Setiawan said in a telephone interview. His team were also surprised to discover that the planet had gone past what is known as the "red giant phase" of stellar evolution, when stars like the Sun expand up to many times their original size. 

What is unusual is that HIP 13044b has survived that process, when astronomers would normally have expected it to have been swallowed up by its host star as it expanded.