A team of astronomers using ESO's 2.2 metre telescope at the La Silla Observatory in Chile have detected a hot-jupiter planet like no other, orbiting a star of extragalactic origin that now finds itself in our own Milky Way.

The 1.3 Jupiter mass planet orbits its star, HIP 13044, at a distance of just 0.12 astronomical units (1 AU is equal to the separation of the Sun and the Earth). Residing 2,200 light years away in the constellation of Fornax, the system is part of the Helmi Stream, the remains of an ancient galactic merger between a dwarf galaxy and the Milky Way billions of years ago. Furthermore, the star has already passed through the red giant phase of stellar evolution to burn helium in its core as a 'horizontal branch' star, providing insight into planetary evolution around a dying star.

"HIP 13044 is in a unique evolutionary state not yet studied in exoplanet surveys," Rainer Klement of the Max Planck Institute of Astronomy (MPIA) told participants of a press conference where the results were announced yesterday. "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 merger would not have disrupted the planet from around its star, explained Klement, because planetary orbits are only disturbed by encounters with another star on a similar small scale – on a galactic scale the stars are spaced with enough distance for the forces of a galactic merger to be negligible on the planet-star system.

The planet was detected via the radial velocity method – using the high-resolution spectrograph FEROS (Fibre-fed Extended Range Optical Spectrograph) attached to the 2.2-metre telescope at ESO’s La Silla Observatory in Chile – by looking for tiny wobbles in the star's orbit as the planet makes it presence known by invoking a small gravitational tug. A clear signal was seen in the data, indicating a planet with at least 1.3 times the mass of Jupiter orbiting its host star once every 16.2 days. The observations are also backed up by independent groups such as the SuperWASP team.

The scientists estimate that HIP 13044 was around the same size of our Sun before it evolved into a red giant, and so will provide a natural laboratory for studying how our planetary system might evolve when the Sun exhausts its fuel supply in about five billion years time. "If a star has a mass of more than 3-5 solar masses, there would be no chance for the outer Solar System planets to survive," says Johny Setiawan, also from MPIA. "But HIP 13044 is a bit less than one solar mass, so the influence of the star is not powerful enough to destroy the outer planets, although it could have already swallowed some inner planets."

But HIP 13044b isn't out of the water yet, so to speak, for its star will swell again in the next stage of its evolution to consume its remaining planet, likely foretelling a similar fate for Jupiter and Saturn in our own Solar System.

The star also throws into questions theories of planetary formation. Most stars hosting planets are metal rich, whereas HIP 13044 contains very few elements heavier than hydrogen and helium – it is the lowest metallicity star known to host planets, no less. “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,” says Setiawan. “Planets around stars like this must probably form in a different way.”

Klement adds that there would be no chance to find rocky exoplanets like the Earth, Venus or Mars, around a low metal star since metals are needed for the core accretion model to work.

Although tentative claims have been made for the detection of extragalactic exoplanets via gravitational microlensing, in which the planet passing in front of an even more distant star leads to a subtle flash, this method relies on a single event and the chance alignment of a distant star, planetary system and observer here on Earth, and so remains impossible to confirm.

The results of Setiawan and colleagues' study are presented in the current issue of Science Express.