First extra-galactic planet discovered

Posted by Emma Woollacott

Astronomers have for the first time discovered a planet which originated outside our galaxy.

While nearly 500 exoplanets have been discovered over the last 15 years, none outside our Milky Way has been confirmed. Now, though, a planet at least one and a quarter times as massive as Jupiter has been discovered orbiting a star of extragalactic origin.

The star, however, is now within our own galaxy. It's part of the  Helmi stream, a group of stars that originally belonged to a dwarf galaxy that was devoured by our galaxy about six to nine billion years ago.

"This discovery is very exciting," says Rainer Klement of the Max-Planck-Institut für Astronomie. "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 star is known as HIP 13044, and lies about 2,000 light-years from Earth in the southern constellation of Fornax. The  planet, called HIP 13044 b, was detected by ESO's La Silla Observatory in Chile, through the perturbations of the star caused by its gravitational tug.

HIP 13044 b is one of the few exoplanets known to have survived the period when its host star expanded massively after exhausting the hydrogen fuel supply in its core. The star has now contracted again and is burning helium in its core.

It's close to its host star - less than one stellar distance - and orbits in only 16.2 days. Setiawan and his colleagues hypothesise that the planet's orbit might initially have been much larger, but that it moved inwards during the red giant phase.

Any closer-in planets may not have been so lucky. "The star is rotating relatively quickly for an horizontal branch star," says Johny Setiawan, also from MPIA, who led the research. "One explanation is that HIP 13044 swallowed its inner planets during the red giant phase, which would make the star spin more quickly."

Although HIP 13044 b has escaped the fate of these inner planets so far, the star will expand again in the next stage of its evolution. It may therefore be doomed after all.

The star also poses interesting questions about how giant planets form, as it appears to contain very few elements heavier than hydrogen and helium - fewer than any other star known to host planets.

"It is a puzzle for the widely accepted model of planet formation to explain how such a star, which contains hardly any heavy elements at all, could have formed a planet. Planets around stars like this must probably form in a different way," says Setiawan.