An exoplanet orbiting a star that entered our Milky Way from another galaxy has been detected by astronomers.  This Jupiter-like planet is unusual because it is orbiting a star nearing the end of its life and could be about to be engulfed by it.   Over the last 15 years, astronomers have detected nearly 500 planets orbiting stars in our cosmic neighborhood, but none outside our Milky Way has been confirmed.

The planet has a minimum mass 1.25 times that of Jupiter and is part of the so-called Helmi stream — a group of stars that originally belonged to a dwarf galaxy that was devoured by our galaxy, the Milky Way, in an act of galactic cannibalism about six to nine billion years ago. 

The star is known as HIP 13044, and it lies about 2000 light-years from Earth in the southern constellation of Fornax (the Furnace). The astronomers detected the planet, called HIP 13044 b, by looking for the tiny telltale wobbles of the star caused by the gravitational tug of an orbiting companion. For these precise observations, the team used the high-resolution spectrograph FEROS attached to the 2.2-metre MPG/ESO telescope at ESO's La Silla Observatory in Chile.

HIP 13044 b is also 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 red giant phase of stellar evolution. The star has now contracted again and is burning helium in its core. Until now, these so-called horizontal branch stars have remained largely uncharted territory for planet-hunters.

"This discovery is very exciting," says Rainer Klement of the Max-Planck-Institut für Astronomie (MPIA), who was responsible for the selection of the target stars for this study. "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."

HIP 13044 b is near to its host star. At the closest point in its elliptical orbit, it is less than one stellar diameter from the surface of the star (or 0.055 times the Sun-Earth distance). It completes an orbit in only 16.2 days.   The researchers hypothesize that
HIP 13044 b's orbit might initially have been much larger, but that it moved inwards during the red giant phase.

Artist’s impression of the slightly elliptical orbit of HIP 13044 b, an exoplanet around a star that entered our galaxy, the Milky Way, from another galaxy. HIP 13044 b is particularly unusual, as it is orbiting a star nearing the end of its life and could be about to be engulfed by it, giving clues about the fate of our own planetary system in the distant future.Credit: ESO/L. Calçada

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. HIP 13044 b may therefore be about to be engulfed by the star, meaning that it is doomed after all. This could also foretell the demise of our outer planets, such as Jupiter, when the Sun approaches the end of its life.

HIP 13044 is visible as an unremarkable moderate brightness star at the exact centre of the picture - HIP 13044 b widefield image
CLICK IMAGE ABOVE FOR LARGER SIZE.   This visible light wide-field image of the region around the exoplanet HIP 13044 b was created from photographs forming part of the Digitized Sky Survey 2. The field of view is approximately three degrees across.   HIP 13044 is visible as an unremarkable, moderate brightness star at the exact centre of the picture. The planet HIP 13044 b is much too faint to be seen in this image. The cross shapes and halos around brighter stars are artifacts and are not real.  Credit: ESO and Digitized Sky Survey 2. Acknowledgment: Davide De Martin

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," adds Setiawan.

Citation: Johny Setiawan, Rainer J. Klement, Thomas Henning, Hans-Walter Rix, Boyke Rochau, Jens Rodmann, and Tim Schulze-Hartung, 'A Giant Planet Around a Metal-Poor Star of Extragalactic Origin', Science Nov 18, 2010 DOI:10.1126/science.1193342