Our leaky galaxy

A while back I wrote about the leakiness of our solar system - how significant amounts of material in the outer regions around the Sun could have really come from other stellar systems. Then I got on a bit of a jag about planets in other galaxies and the impending collision (well, one has to take a long-term view) with the Andromeda galaxy. Then lo and behold, like a fabulous new ice-cream flavor, someone turns up a new planet in the Milkyway that almost certainly came from another galaxy.

HIP 13044b is a giant world a little more massive than Jupiter orbiting a red giant star every 16 days, reported by Setiawan et al. in Science. Detected via the tell-tale velocity wobble of its host planet it might not garner much attention except for the fact that this system is part of the so-called Helmi star stream, some 2000 light years from us. The Helmi stream is the remnant of a dwarf galaxy that dove into the Milkyway sometime between 6 and 9 billion years ago, becoming shredded by galactic tidal forces.

Clearly the original host galaxy for this system was pretty puny, and one might argue that it was never truly distinct from the Milkyway - merely a part of its halo entourage, like some loose hair or slightly wayward limbs that would inevitably get subsumed. Nonetheless, it has some quite striking implications. The host star has an extremely low heavy element content - about a hundredth that of the Sun. This is the least element rich star yet found to host any type of planet. So the fact that it has at least one chunky world orbiting it is very interesting since lower heavy element abundance in stars correlates with a lower probability of giant planets. It's also an old and doddery star, well past its prime and has swollen in size as a result - possibly engulfing even shorter orbit planets, and possibly shedding some element-rich outer atmosphere.

Taken altogether it's another tick mark for planet formation being both a universal phenomenon and apparently a quite efficient one. As we learn more about the stellar populations in our galaxy we see just how messy they really are. Satellite galaxies can dive through the galactic plane, not just shedding their stars but also pulling out bona-fide Milkyway residents in their gravitational wakes. There may well be stars and planets that formed snug in our galactic disk that are now arcing over us, dragged tens of thousands of light years away from home. Remarkably, the opportunity to study planets with truly extragalactic origins may be all around.