Extraterrestrial microbes might have brought life to Earth after travelling through space for millions of years, say scientists.
The theory is based on calculations showing a high likelihood of rock fragments from planets in other star systems landing on Earth long ago.
Some of them could have carried embedded micro-organisms, according to experts writing in the journal Astrobiology.
The research suggests the dormant bugs could easily have survived the long journey through space, despite high levels of cosmic radiation.
Simple life may equally well have travelled from Earth to planets outside the Solar System, the scientists believe.
The process, known as lithopanspermia, could mean the universe is teeming with Earth-like life.
"Our work ... says that lithopanspermia might have been very likely, and it may be the first paper to demonstrate that," said lead researcher Dr Edward Belbruno, from Princeton University in the US.
"If this mechanism is true, it has implications for life in the universe as a whole. This could have happened anywhere."
Large volcanic eruptions, meteorite impacts and collisions with other bodies can cause rocky fragments of planets to fly into space.
When the Solar System was young, and the Sun much closer to its neighbours than it is today, some of this debris could have been exchanged between planets orbiting different stars, say the scientists.
Travelling relatively slowly, there was a good chance of them being "captured" by the gravity of planets they approached.
The researchers ran computer programmes simulating the star cluster in which the Sun was born.
They found that of all the rocky fragments cast off from our Solar System and its closest neighbour, between five and 12 out of 10,000 could have been captured by the other.
During a period of 10 million to 90 million years, anything between 100 trillion and 30 quadrillion objects weighing more than 10 kilogrammes could have been transferred.
Any organisms arriving on Earth would have found a planet already covered in water with conditions suitable for life.
The Sun's birth cluster slowly broke apart when the Solar System was 135 million to 535 million years old.
Earth possessed surface water from when the Solar System was just 288 million years of age, making it likely that the planet was ready to receive alien microbes.
Co-author Dr Amaya Moro-Martin, an astronomer from the Centro de Astrobiologia in Spain, said: "Our study stops when the solid matter is trapped by the second planetary system, but for lithopanspermia to be completed it actually needs to land on a terrestrial planet where life could flourish.
"The study of the probability of landing on a terrestrial planet is work that we now know is worth doing because large quantities of solid material originating from the first planetary system may be trapped by the second planetary system, waiting to land on a terrestrial planet.
"Our study does not prove lithopanspermia actually took place, but it indicates that it is an open possibility."
The research was presented today at the 2012 European Planetary Science Congress in Madrid.
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