Zero Genie

For over a century, proponents of Panspermia have argued that life is distributed throughout our galaxy by comets, asteroids, space dust, and planetoids. But in recent years, scientists have argued that this type of distribution may go beyond star systems and be intergalactic in scale. Some have even proposed intriguing new mechanisms for how this distribution could take place.

For instance, it is generally argued that meteorite and asteroid impacts are responsible for kicking up the material that would transport microbes to other planets. However, in a recent study, two Harvard astronomers examine the challenges that this would present and suggest another means – Earth-grazing objects that collect microbes from our atmosphere and then get flung into deep-space.

The study, titled “Exporting Terrestrial Life Out of the Solar System with Gravitational Slingshots of Earthgrazing Bodies“, recently appeared in the International Journal of Astrobiology. The study was authored by Amir Siraj (a Harvard undergrad in astronomy) and Abraham Loeb – the Frank B. Baird Jr. Professor of Science and the Chair of the Astronomy Department at Harvard University.

To break it down, there are several versions of panspermia theory. There’s lithopanspermia, the idea that rocks ejected by impacts are responsible for spreading microbes from planet to planet. Then there is the larger variant, where interstellar asteroids and comets are responsible for distributing life between star systems and maybe even galaxies. Siraj summarized it for Universe Today via email:

“Traditional theories of panspermia posit that planetary impacts can accelerate debris out of a planet’s gravitational field, and potentially even out of the host star’s gravitational field. Among other issues, this debris is often quite small in size, providing little shielding from harmful radiation for any potentially enclosed microbes during the debris’ journey through space.”

In addition, the traditional approach to panspermia requires a process that both embeds microbes in rocks but also provides enough energy to eject them from Earth and the Sola3r System. This is no easy task, given that an object needs to be traveling at a velocity of 11.2 km/s (7 mi/s) just to escape Earth’s gravity and 42.1 km/s (26 mi/s) to escape the Solar System.

In contrast, Siraj and Loeb examined whether it would be possible for long-period comets or interstellar objects (such as ‘Oumuamua and C/2019 Q4 Borisov) to spread life. This would consist of these objects entering Earth’s atmosphere, scooping up microbes – which have been detected up to 77 km (48 mi) above the surface – and getting a gravitational slingshot that could send them out of the Solar System.

Compared to objects impacting the surface, Siraj explained, this mechanism offers a number of advantages:

“One advantage of a long-period comet or interstellar object scooping up microbes from high in the Earth’s atmosphere is that they can be quite sizeable (hundreds of meters to several kilometers) and guaranteed to be ejected out of the Solar System by passing so close to Earth. This allows microbes to become trapped in nooks and crannies of the object and gain substantial shielding from harmful radiation so that they might still be alive by the time they encounter another planetary system.”

To evaluate this possibility, Siraj and Loeb evaluated the drag that Earth’s atmosphere would have on an interstellar object, as well as the gravitational slingshot effect. This allowed them to constrain the sizes and energies of objects that could export microbes from Earth’s atmosphere to other planets and planetary systems.

“We then used observed rates of long-period comets and interstellar objects to calibrate the number of times we would expect such a process to have occurred over the time during which life has existed on Earth,” added Siraj. From this, they found that over the course of Earth’s lifetime (4.54 billion years) roughly 1 to 10 long-period comets and 1 to 50 interstellar objects would be to export microbial life from Earth’s atmosphere.

They further estimated that if microbial life existed above an altitude of 100 km (mi) in our atmosphere, then the number of exportation events would increase dramatically to about 105 over the course of Earth’s lifetime. This work builds upon previous research that has shown that interstellar objects may be rather common in our Solar System. As Siraj explains:

“An exciting aspect of this paper is that it provides a concrete process for ejecting large rocks out of the Solar System that are loaded with Earth microbes. The dynamical processes of these rocks then becoming trapped in other planetary systems have been written about previously, so this paper closes the loop, in a sense, for one concrete process by which life could have been transferred from Earth to another planet.”

When the next interstellar object passes through our system, we should naturally wonder, “is this one carrying the seed of life to another star system?” For that matter, we should ask ourselves if this is how life began on Earth, billions of years ago. If interstellar objects are the means through which microbial life is spread, then sending a mission to intercept one and study it more closely should be a top scientific priority in the coming years!

Further Reading: arXiv