Astronomers have been discovering planets outside of our solar system – or exoplanets – at a steady rate in recent years. The number has now topped 500 and with earth-bound detection improving all the time and the Kepler mission out hunting with the largest camera ever sent into space, the rate is not likely to slow down anytime soon. Among these discoveries are some extraordinary finds like the first "potentially habitable" exoplanet, but what's different about this latest discovery is not the Earth-like qualities of the planet, it's the fact that it originated from outside the Milky Way – which makes it an extragalactic exoplanet.
HIP 13044 b, as the giant exoplanet is known, has a mass of at least 1.25 times the mass of Jupiter (and Jupiter is 2.5 times the mass of all the other planets in our Solar System put together). It is part of a star system located about 2000 light-years from Earth that is believed to have been "swallowed" by the Milky Way. In other words, HIP 13044 b originated in another galaxy.
The planet was discovered with the radial velocity method using the the high-resolution spectrograph FEROS at the 2.2 m MPG/ESO telescope at ESO's La Silla observatory in Chile.
"This is an exciting discovery," says Rainer Klement of the Max Planck Institute for Astronomy (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."
The exoplanet is also unusual because it appears to have survived the "red giant" stage of it's parent star. This is the process that sun-like stars undergo towards the end of their evolutionary cycle – when the hydrogen fuel in the star's core runs out it causes more reactions and a rapid expansion of the outer layers. Because the star in its expanded state has a lower surface temperature, the light output shifts towards red in the visible spectrum – it becomes a red giant.
This is what astronomers believe will happen to our Sun in around 5 billion years, at which point it will engulf all the inner planets up to (and probably including) the Earth.
The fact that HIP 13044 b has survived its parent stars' red giant stage gives astronomers a fascinating glimpse of the possible future of our own solar system.
Research leader Johny Setiawan and his colleagues hypothesize that HIP 13044 b' initially had a much larger orbit, with the planet moving inwards during the star's Red Giant phase.
While this might be comforting if we all lived on Jupiter, it seems that inner planets in this system also migrated inwards as the star became a red dwarf – they didn't make it.
"HIP 13044 is rotating relatively quickly for a star of this particular type," says Setiawan. "One explanation is that HIP 13044 swallowed its inner planets during the Red Giant phase, which would make the star spin more quickly."
But don't book your ticket to Jupiter just yet – further expansion in the next stage of the stars evolution could spell the end for HIP 13044 b.
Further research by Setiawan and his colleagues will focus on finding more exoplanets orbiting similar stars.
Johny Setiawan, Rainer J. Klement, Thomas Henning, Hans-Walter Rix, Boyke Rochau and Tim Schulze-Hartung (all from the Max Planck Institute for Astronomy) and Jens Rodmann (European Space Agency) make up the research team. The paper entitled "A Giant Planet Around a Metal-poor Star of Extragalactic Origin" is published in Science Express.
