Article Posted on : - Nov 19, 2010

This artist’s impression shows HIP 13044 b, an exoplanet orbiting a star that entered our galaxy, the Milky Way, from another galaxy. An ESO photo

PARIS (BNS): In an interesting discovery, a team of European astronomers has traced a Jupiter-like expolanet orbiting a star that entered into our home galaxy Milky Way through ‘galactic cannibalism’.

The extragalactic planet, identified as HIP 13044 b, revolves around its host star HIP 13044 in just about 16.2 days, suggesting that the planet has come very close to the ‘dying’ star. At the closest point in its elliptical orbit, the planet is less than one stellar diameter from the surface of the star (or 0.055 times the Sun-Earth distance).

The star, which lies about 2000 light years from Earth in the constellation of Fornax (the Furnace), is part of the so-called Helmi group 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.

Using the European Southern Observatory's La Silla Observatory in Chile, the astronomers have detected the new planetary system, which they believe, also had other planets, but could possibly have been gobbled up by their parent star.

“This discovery is very exciting. For the first time, astronomers have detected a planetary system in a stellar stream of extragalactic origin,” said Rainer Klement of the Max-Planck-Institut für Astronomie (MPIA), who was responsible for the selection of the target stars for this study.

The planet, 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. Any closer-in planets may not have been so lucky.

“The star is rotating relatively quickly for an horizontal branch star,” Johny Setiawan, a research member said. “One explanation is that HIP 13044 swallowed its inner planets during the red giant phase, which would make the star spin more quickly.”

The star has now contracted again and is burning helium in its core, which may sound the death knell for the giant planet HIP 13044 b.

This could also foretell the demise of our outer planets — such as Jupiter — when the Sun approaches the end of its life.

The extragalactic star also poses some 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.

The results have been published in Science Express on Thursday.