Could life have arrived in our solar system aboard an interstellar meteorite? While meteorites from Mars reach other bodies in our solar system, any solar system meteorites similarly dispatched to other planetary systems hurtle through at 10 kilometres per second or more. They have an insignificant chance of being caught by direct impact or by a planet’s gravitation – so how might they transport seeds of life from one system to another?
You report Edward Belbruno and colleagues at Princeton University showing that the early solar system, when it was in a young, densely packed cluster of similar systems, could throw out catchable rocks at slower relative speeds (6 Sept., p 21). But this applies only to the solar system’s first 10 million years or so, when the terrestrial planets had hardly condensed. Even if these were seeded with life from the start, evolution would barely have progressed: any life forms they dispatched on meteorites would scarcely differ from those they received.
We have addressed, instead, the key problem for spreading life through the galaxy (panspermia), namely how solar system meteorites could deliver seeds of our life to a distant planetary system in formation. Its pre-planetary cloud contains large amounts of solid millimetre-sized grains, which fragment centimetre-sized meteorites explosively on collision. The system’s new protoplanets readily capture some fragments, along with any viable spores.
Primitive life forms that spread through one planetary system evolve adaptively with that system. The environmental diversity and extended timescales allow galaxy-scale evolution to higher levels of genome complexity than is achievable on a single planet.
first published in New Scientist Letters 29 October 2008
Max Wallis and N. Chandra Wickramasinghe, Cardiff Centre for Astrobiology