In the opening scene of Ridley Scott's Prometheus, an alien Engineer is seen seeding the Earth with life — an interesting suggestion as to how life emerged on this planet.
It would now appear, however, that the film got this backwards: A newly discovered gravitational process called "weak transfer" indicates that the Earth was once capable of sending slow-moving, microbe-carrying rocks out of the solar system. As a result, astrobiologists are now wondering if our planet has spawned life elsewhere.
Escape velocity
Proponents of the panspermia hypothesis have spent most of their time trying to understand how an incoming object may have given rise to life on Earth. The basic idea is that a microbe-laden meteorite landed here billions of years ago, resulting in a kind of extraterrestrial genesis.
A fundamental problem with this theory, however, is how such a meteorite could make the journey from a neighboring solar system. According to the lithopanspermia theory, microorganisms may have been ejected into space after a planet suffered a cataclysmic impact with an asteroid, or by virtue of a powerful volcanic eruption. Most scientists don't contest this possibility — but what the pre-existing models have shown is that it is excruciatingly rare for microbe-laden ejecta to escape the gravity well of a solar system.
But according to researchers from Princeton University, the University of Arizona, and the Centro de Astrobiología (CAB) in Spain, a special set of cosmological circumstances can exist in which a slow, small rocky object can easily escape the gravitational pull of its parent star. By factoring realistic velocities down to 50 times slower than previous estimates (about 100 meters per second), and by taking the presence of other large and nearby bodies into account (such as gas giants), the researchers were able to illustrate the effect of "weak transfer."
In such a scenario, a slow-moving planetary fragment wanders into the outer edge of a planet's gravitational pull (what's called the weak stability boundary). Because the planet has only a loose grip on the meteor, it can escape and be propelled back into space, drifting until it is pulled in by another planetary system — including one that's in a completely different solar system.
Swapping rocks
All this said, the planetary configurations required for such an interstellar journey are extremely fleeting. The research indicates that the only time in history when this weak transfer was possible was between 164 million to 288 million years after the formation of the solar system. This was a special time in our solar system's history when our sun was still part of its birth cluster.
According to researcher Amaya Moro-Martín, there are two fundamental requirements for weak transfer: First, the planetary systems involved must contain a massive planet that captures the passing meteor in the weak-gravity boundary (that would be Jupiter). And second, both systems must have low relative velocities (our sun's cluster consisted of 1,000 to 10,000 gravitationally bound stars for hundreds of millions of years). The Earth passes both of these tests.
To prove the viability of their models, the researchers simulated 5 million trajectories between single-star planetary systems (in a cluster with 4,300 stars) and under three different conditions (mostly to do with mass). The resulting data showed that the odds of a star capturing a fragment from another planetary system under these primordial circumstances ranged from five to 15%. That's a stark contrast to previous estimates indicating it would literally be a billion times more unlikely.
And amazingly, the researchers also calculated the amount of solid matter that could have been exchanged between our solar system and our nearest neighbor. They came up with a figure ranging from 100 trillion to 30 quadrillion fragments weighing more than 10 kilograms. Their calculations revealed that some 200 billion rocks from Earth could have been flung away to another star system — rocks that could have contained microbes.
In total, the researchers speculate that roughly 300 million lithopanspermia events could have occurred between our solar system and our closest planetary neighbor.
The fertile Earth?
But did life exist on Earth that long ago?
This is where the researchers' theory gets a bit tenuous — but it's not a ridiculous stretch to suggest that primitive life was in fact present as much as 3.8 billion years ago. Water was already here on Earth at this stage in its geologic history, and the first microorganisms did in fact start to appear shortly after this time (around 3.5 to 3.6 billion years ago).
And mindblowingly enough, the weak transfer window was also open for Earth at this exact time when it too was able to receive interstellar fragments; our planet may very well have been seeded at this time.
As for the journey through space, astrobiologists generally believe that microbes can survive exposure to such harsh conditions — and long enough to make a journey requiring tens of millions of years.
So, assuming that life did exist on Earth at this time, there was a period of about 400 million years when life could have traveled much more easily to another habitable world, and vice versa.
The entire study can be read at Astrobiology.
Top image via. Inset images via Princeton.