By Dr David Whitehouse
BBC News Online science editor
Astronomers may have shown how microbes from Earth could be spread throughout the galaxy taking life to other worlds.
Life could quickly spread throughout the galaxy
Scientists at Armagh Observatory and Cardiff University say bacteria could get into space on rocks blasted off the planet by an asteroid or comet impact.
Their calculations then indicate the microbes would eventually leak out of our Solar System to seed other regions.
The work is reported in two independent papers published in the Monthly Notices of the Royal Astronomical Society.
The implication of the papers is that life could be widespread throughout the galaxy and may not have originated on our planet.
Plenty enough for life
The research advances the case for modern-day panspermia - the controversial idea that life started elsewhere in space and came to Earth when it was young.
Dr Max Wallis and Professor Chandra Wickramasinghe of Cardiff University calculate how debris from Earth, thrown into space as a result of a giant impact, would become incorporated in the frozen outer layers of comets.
Eventually, after hundreds of millions of years, some of these comets would reach the Edgeworth-Kuiper Belt - a region inhabited by small worlds made of rock and ice.
Possible fossil microbes have been identified in Martian meteorites
Because comets gradually leak into interstellar space from this region, some would eventually reach clouds of gas and dust that are new planetary systems in formation.
In these systems, the trapped microbes would be liberated and, if the conditions were right, introduce life on to the surfaces of primitive planets.
Wallis and Wickramasinghe are encouraged in their belief that microbes can survive on such a journey for hundreds of millions, if not billions, of years, by recent discoveries of microbes that have survived for similar periods encased in rock in the Earth.
Their detailed calculations suggest that between a few kilograms and perhaps a tonne of material containing microbes could be passed from our Solar System to others.
They say that one kg of "spore-bearing material is plenty for seeding a new planetary system with life".
In another paper in the same journal, Bill Napier, of Armagh Observatory in Northern Ireland, suggests an alternative way life could travel from Earth out among the stars.
Ejected rocks containing microbes would be eroded when in our Solar System, he says. When they had been reduced to a certain size, the resulting grains of rock would be pushed away by the pressure of sunlight.
Because of this, Napier believes that our Solar System is surrounded by an expanding "biosphere" of dormant microbes preserved inside rock fragments.
In the course of Earth history, Napier says, there must have been many encounters with gas and dust clouds in the process of forming stars during which microbes from Earth will have been included.
The researchers say the implications of their work are obvious and profound.
Wherever it started, life could have spread across the Milky Way on timescales that are short compared with the 10-billion-year estimated age of our galaxy.
This means, they claim, that life must be widespread throughout our star system and that it is unlikely to have originated on Earth.