Astronomers have long wondered whether life on Earth might have had origins somewhere else in the cosmos. The idea that the key ingredients for life, or even simple organisms, might be carried through space by non-animate objects—a concept known as panspermia—has been proposed as one way such “seeding” of life on Earth might have occurred.

But what if it could also happen in reverse? In other words, what if life were one day found on an alien moon, which could later be traced back to Earth?

This is the very idea presented in a new study that explores the provocative idea that Jupiter’s frozen moon Europa may not only be hospitable, but that life from Earth could be thriving there.

In the new study, published in the International Journal of Astrobiology, astrophysicist Zaza Osmanov of the Free University of Tbilisi explored whether Earthly bacteria might have been able to escape our planet by traveling on microscopic dust particles, which, after escaping Earth’s gravity, could have drifted through space until they reached Europa.

There, a vast subsurface ocean is believed to exist beneath the moon’s icy crust, meaning that any simple organisms capable of escaping from Earth on tiny spacefaring rafts of dust might have found their way to a new home, and one that could support life.

Panspermia, but in Reverse

The long-debated idea of panspermia already presents the general idea for how this might work. However, Osmanov’s study investigates this idea in reverse order: not only might life on Earth have originated from elsewhere, but our planet could also hypothetically be the source of similar “seeding” events that might occur on alien worlds.

Based on mathematical models, Osmanov calculated how bacteria-rich dust grains could be launched into space, resulting from collisions with incoming dust in Earth’s upper atmosphere. Upon escaping the planet’s gravity, some of these bacteria might be able to sail along toward distant locales like Europa with the help of pressure generated by solar radiation—essentially the same concept as a solar light sail spacecraft, albeit much smaller.

With the help of solar radiation and occasional gravitational interactions, Osmanov theorizes that some particles might eventually be “steered” by such natural processes over time, eventually making their way to Jupiter.

Tiny Earthling Alien Invaders

Based on Osmanov’s estimates, there could potentially be as many as hundreds of millions of Earth-originating dust particles that might make their way to Europa every second. That’s a significant number, and although he estimates that the majority would be destroyed on impact, a small fraction could arrive at extremely shallow angles, which he believes may be able to survive landing on Europa’s surface.

However, even if a few bacterial invaders managed to survive, the next challenge they would face would be to reach the moon’s ocean that astronomers believe to be concealed beneath its icy exterior. Based on past studies, tidal forces generated by the massive host planet Jupiter offer a solution: such events are believed to likely cause fractures in Europa’s icy shell, which lead to the creation of entry points where surface materials might be capable of migrating downward.

Adding to this, Osmanov also points to instances where ice melting may also provide a means for the movement of material from the surface down into the moon’s hidden ocean. Over periods of thousands of years, some of these tiny bacterial invaders might make their way past the harsh surface environment and down into the moon’s far more hospitable subsurface ocean.

Osmanov estimates that there may be considerable numbers of Earth-derived particles that could reach Europa’s ocean over geological timescales. This could mean that terrestrial particles, Earthly microbes that have made the journey, may have had time to establish themselves as microbial colonists—if, of course, current presumptions about the moon’s conditions for supporting life are correct.

While the theory remains speculative, and Osmanov concedes that it cannot be tested directly at present, it suggests something fascinating: that the eventual discovery of life on an alien world could have ancestral links to our own planetary home.

Osmanov’s recent study, “Earth as a potential source of life for Europa’s subsurface ocean,” appeared in the journal International Journal of Astrobiology.

Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.

Future interstellar travelers who hope to reach distant star systems will likely need better propulsion technologies than the chemical rockets humans currently rely on for our exploration of space.

Among the range of proposed alternatives, one potential candidate involves what are called solar light sails, a theoretical propulsion system that relies on pressure generated from solar radiation against a large surface to move a spacecraft.

Such concepts have been proposed since the 1980s, some of which combine the idea of using sunlight as a power source with the implementation of lasers, which could propel such futuristic starships to speeds our best current technologies cannot attain.

However, not all spacecraft propulsion experts are necessarily sold on the idea of solar light sails just yet. According to new findings detailed in a paper by researchers Jiaze Li and Chao Shen of the Harbin Institute of Technology in China, this novel form of propulsion may have a few hurdles to cross before it can attain the near-light-speeds many experts had hoped for.

A Tale of Three Forces

According to Shen and Li’s recent research, a key problem arises once a solar light sail spacecraft begins nearing the speed of light: drag forces resulting from the very source of energy used to propel the devices could place significant limitations on their maximum attainable speeds.

A combination of different forces can be expected to occur as sunlight strikes a light sail, the authors say, which includes incident light, specular reflection, and diffuse scattering.

Incident light describes the physical momentum generated by light energy in the form of photons as they strike the sail, whereas specular reflection describes the momentum produced as photons reflect off the sail. Finally, diffuse scattering is a separate form of momentum generated by photons, which, rather than bouncing away upon impact, is actually absorbed by the sail, the energy from which is thereafter released in random directions.

New Limitations on Solar Light Sails

Based on these primary forces, the authors identify that once a light sail begins reaching relativistic speeds, it also becomes subjected to intense Doppler effect—the phenomenon that occurs as changes in the frequency of waves result as their source and an object or observer’s distance changes in relation to it.

In the case of solar light sails, the more the frequency of light waves striking the sail decreases, the less the amount of thrust generated by the three forces the paper identifies. In simpler terms, the faster a solar light sail spacecraft attempts to travel, the harder it becomes for it to accelerate.

Relativistic Light Aberration

At approximately 75% of the speed of light, the researchers say that an additional hindrance emerges, as what physicists call relativistic light aberration takes over. This phenomenon, first described by Einstein more than a century ago, involves relativistic corrections of light sources for observers who move at velocities approaching light speed.

In the case of our prospective future solar light sailors, this would mean that the weakest of the three forces impacting the sail (diffuse scattering) would begin to impose an active drag once the sail reaches approximately three-quarters of light speed. While the overall force of lasers helping to propel the light sail would remain positive at this stage in the system’s acceleration, the authors note an inevitable decline in efficiency.

The authors do concede that while their paper examines the radiative dynamics of solar light sail propulsion, it does not account for other factors that may include drag resulting from encounters with materials like gases, dust, or other physical sources (i.e., non-radiative sources), which could also impact overall speeds.

Additional factors worthy of consideration, but which the authors do not directly address in their paper, include the melting point of any materials used for solar light sail propulsion, which could be heated to extreme temperatures due to combined exposure to solar radiation, laser energy, and friction from movement.

Fundamentally, the team reports that their paper “provides a rigorous and in-depth theoretical foundation for lightsail dynamics,” which illustrates that while this future propulsion concept has its merits, we may still be far from seeing its realization for practical space travel.

By outlining the factors that contribute to constraints on light sail propulsion, Li and Shen aim to improve our overall understanding of this prospective future technology, thereby helping engineers to realize its potential for use in space exploration at some point in the future.

The team’s paper, “Relativistic Lightsail Propulsion Dynamics,” appeared on the preprint arXiv.org server.

Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.