Our leaky solar system

We've all grown up with the notion that we're a long way from the nearest stars. This isolationist mentality extends to the way we think about all the stuff in our solar system; it's ours, all ours! However, our Sun almost certainly formed together with many other stars from the same cloud of molecular hydrogen, with the usual sprinkling of heavier elements and interstellar dust. Long since dispersed by the inexorable slewing of material as it lumbers around the Galaxy - every 210 million years or so at our placement from the galactic center - our stellar brothers and sisters may be well scattered by now.

There was a time though, some 4-6 billion years ago, when other stars, likely with their complements of dusty planet-forming material, could have swept by. The absolute outermost reaches of our local gravitational well are populated by the cold residue of planet formation, the objects we know as cometary nuclei. This postulated fuzz of stuff is the Oort cloud, and should extend outwards from us to as much as 100,000 astronomical units distance - almost halfway to the closest stars of Alpha Centauri.

An intriguing new work by Levinson et al in this weeks Science re-opens an investigation into the precise origin of the Oort cloud. From what we know about long-period comets, sailing in from this distant population, there appear to be some serious discrepancies about the likely number of objects out there in our deepest reaches. Their basic argument is that there are too many objects in the Oort cloud to be accounted for by their origin around the giant planets - from where they were gravitationally flung outwards during planet formation 4.5-5 billion years ago.

The rather shocking alternative, carefully studied with state-of-the-art computer simulations, is that as much as 90% of the Oort cloud has actually been gravitationally captured from other stars, during close passes billions of years ago. Equally, we've shed plenty of icy lumps that are now stuck with different stars - somewhere along a common path around the galaxy.

With a wave of the wand our solar system is less of an isolated specimen than a separated sibling, carrying along the familial baggage of our stellar birth cluster. This means that some of the long period cometary bodies that spew their guts as they dip towards the Sun are truly alien - born around another star altogether. As a small habitable planet, we get the occasional wash from this material, and the occasional collision. Whatever the chemistry and isotopic mix is of these ancient objects, it's slowly getting mixed into the inner solar system. Remarkably this means that there is a pathway that could conceivably take a molecule formed around a giant planet in another star system and eventually deposit it in your Pina Colada. Anyone for another round?