Halley's comet and other famous objects in our solar system may in fact have formed in orbit around alien suns far off across the vast gulfs of interstellar space, according to new research.
Comets, Halley's in particular, are old friends of the human race and their regular appearances in the inner solar system are thought to have been noted in humanity's earliest records. But in astronomical terms human intelligence is a very new thing - indeed, so is life on Earth.
According to top international boffins, long long before our home planet had even formed, the Sun and the various stars in our local neighbourhood were much closer together. The accretion discs of dust and space gumble from which all the planets and comets and everything originally formed were almost touching, and matter was routinely passed around among the young and excitable stars.
"When it was young, the Sun shared a lot of spit with its siblings, and we can see that stuff today," says Colorado-based boffin Dr Hal Levison.
Levison and his colleagues believe that the comets forming among the local stellar cloud were then mostly hurled out of their home systems by the newly forming giant planets such as Jupiter and Saturn.
But then the hottest young stars exerted a sort of interstellar blowing effect, blasting most of the dust and gas away and dispersing the tightly clustered proto-suns. Flying comets were then captured by whichever stars they were passing at the time.
Our own Sun's population of comets spend most of their time out in the far-flung Oort Cloud, which reaches halfway to the nearest star. Some, like Halley's, make occasional brief plunges into the inner system where we live.
According to Levison and his colleagues, the Oort cloud has far too much stuff in it to have accumulated merely from the Sun's own original accretion disc. Thus much of it must have been originally from somewhere else.
"If we assume that the Sun's observed proto-planetary disk can be used to estimate the indigenous population of the Oort cloud," says Levison, "we can conclude that more than 90 percent of the observed Oort cloud comets have an extra-solar origin."
The new research is published online by Science Express, here (subscription link).