In 2004 I wrote a short piece for my local newspaper in which I speculated about the possibility of comet impacts splashing chunks of Earth back into space, and some of the life in those chunks surviving long enough to fertilize other worlds. Here’s that story:
Comets are often compared to a dirty snowball, only not packed so hard. Their density is more like a snowbank. Comets aren’t big enough as a rule for gravity to pull them into a spherical shape, so they’re mostly irregular chunks of dirty snow. On highly elliptical orbits, the comets we see loop way out to the fringes of the Solar System and beyond, then back in close to the Sun. Near the warm center they heat up and blow off huge clouds of dust and water vapor which we are sometimes lucky enough to see. When closest to the Sun, they are venting vast plumes which leave holes on their surface.
We tend to worry about things like comets. All through history we’ve attached abnormal significance to their appearance. Of course, it had to be all about us. Whether portents of the fall of kingdoms or bearers of evil omens, we’ve shown a tendency to be a little self-centered about it. Not that there isn’t evidence of severe damage when Earth is struck by some of the bigger comets. Big ones can hit us so hard that the splash is blown right back into space. The dinosaurs probably went extinct after a big comet strike. We should definitely worry about them.
Something we don’t usually think about is what happens to the Earth material that gets blown into space. There must be tons of Earth ejecta in an expanding cloud throughout the galaxy, given how often and how long comets have been hitting us. The Milky Way has rotated about twenty times since Earth formed, spreading it further. There is evidence of life on Earth going back almost four billion years. It stands to reason that some of it was carried into space in cometary splashback. Most of it would die in such a harsh environment, but some microbes would survive. They would be capable of remaining as dormant spores preserved in Earth dust. Over the eons they would drift through cold space, and a few of them might end up settling onto a new planet capable of stimulating them back to life.
There is a debate whether life started on Earth or was seeded here from elsewhere. The Panspermia theory developed by Chandra Wickramasinghe and the late Sir Fred Hoyle favors the latter. Now Professor Wickramasinghe is one of a few researchers suggesting that life from Earth is also out there seeding other worlds. I like that.
That was then. Now some people at Harvard have suggested a way we can possibly test the Panspermia theory.
Cambridge, MA –
We only have one example of a planet with life: Earth. But within the next generation, it should become possible to detect signs of life on planets orbiting distant stars. If we find alien life, new questions will arise. For example, did that life arise spontaneously? Or could it have spread from elsewhere? If life crossed the vast gulf of interstellar space long ago, how would we tell?
In a nutshell, they think any life we discover would appear in a pattern resembling that formed in an epidemic, if it was spreading via panspermia. It’s fun watching ideas develop.