Everywhere we look, we find planets. That was the lesson of yesterday’s historic announcement that a habitable planet orbits Proxima Centauri. And it’s the lesson of every exoplanet survey astronomers have conducted. A few decades ago, it was still possible to wonder whether other stars play host to round, rocky worlds. Not anymore.
But even amidst this multitude of newly discovered planets, we can’t be sure that worlds with rich biospheres like ours are common. It might be fairly easy for nature to manufacture microbes, but complex life seems to need nurturing.
Earth’s history suggests that even under the best of circumstances, it takes time for large, big-brained organisms to evolve. And not every planet can count on a 4-billion-year run of relatively life-friendly conditions—not in a universe where a neighboring star could explode at any time. Some terrestrial worlds will only have a billion years of habitability, or a few hundred million years, and that might not be enough time to ramp up rich, diverse ecosystems with interesting alien creatures.
But what if we could give them a head start?
A new paper by Claudius Gros, a systems theorist at Goethe University in Frankfurt, suggests that future humans could—and more interestingly, should—send “Genesis missions” to planets with limited habitability windows. With our current technical constraints, it’s hard to imagine sending a Noah’s-Ark-style probe housing plants and animals across cosmic distances. But it looks increasingly plausible that we will soon be able to beam small, light-weight spacecraft to distant stars.
If so, we might be able to stash tiny cell-synthesizing machines on these probes. Once they arrive at a target planet, they could knit together single-celled organisms that would be sealed into dissolvable capsules, and dropped onto the planet at regular intervals. The capsules could seed the planets with a specific mix of microbes, resembling those that existed on Earth just before the Cambrian explosion—the most creative evolutionary period in Earth’s history, when most categories of complex animal life emerged.
On Earth, evolution spent most of its time—billions of years—getting single-celled organisms up to pre-Cambrian levels of complexity. A successful probe would allow a planet to skip those steps, and move quickly toward a lush biosphere capable of spawning intelligent life. We could send these probes to thousands of local planets, and wait a few hundred million years. By that time, our corner of the galaxy might be the envy of the cosmos.
Earlier this week, I asked Gros about the details—and the philosophical implications—of Genesis missions. Our conversation has been condensed and edited for clarity.
Ross Andersen: In your paper, you say that a Genesis mission wouldn't be for human benefit. Why not?
Claudius Gros: Because the time spans involved are too large. The probe itself will take at least a few hundred to a few thousands years to arrive. For a science mission that would be an extraordinary long planning horizon. And even after the probe arrived it would take 10 to 100 million years for enough oxygen to accumulate in the planet’s atmosphere for it to be habitable for us.
Andersen: And so, why do this at all?
Gros: If we take the stance that a rational action is one which benefits ourselves, or society, or humanity, then there is no explicit rationale for a Genesis mission. For me, personally, it is a question of aesthetics. Life is something beautiful, and giving life the possibility to blossom elsewhere in the universe would be wonderful.
Andersen: Complex life took billions of years to evolve on Earth. How would you compress that process?
Gros: The idea is to give the planet the optimal starting conditions. On Earth, evolution actually spent most of its time developing the internal molecular machinery of cells. That is not always appreciated. The subsequent task to control a body plan is minor in this respect. Compression would hence be achieved by bypassing about 4 billion years of unicellular evolution.
Andersen: Your paper describes a strategy where you would synthesize a mix of planet-seeding microbes in flight, a mix that would then recreate this very specific moment in Earth's evolutionary history, right before the Cambrian explosion. Why that particular moment?
Gros: It is the best we can do. The pre-Cambrian is the moment in the history of life when unicellular organisms were ready to take the next step and develop into complex life forms. It would not be possible to bring current complex life forms to an exoplanet for two reasons. First, it would require an ark, not a tiny robotic probe. And second, it would require atmospheric oxygen, which is not expected to be present on a planet devoid of life.
Andersen: Can you walk me through what would happen after the probe arrived at the planet?
Gros: The probe would stay in orbit. First an in-depth study of the planet would take place. The seeding would be a continuous process, with micro-capsules full of microbes being dropped at regular intervals. That could be continued for a few centuries.
Andersen: Is there a barren planet or moon in our solar system that you could test this mission on?
Gros: Not really. Venus is way too hot. And any remnants of life existing on Mars would be of high value to scientists, such that we wouldn’t want to supersede Marian life with terrestrial life. The same holds for potential life in the oceans below the ice of Europa.
Andersen: How many planets in our galaxy could you conceivably send these probes to?
Gros: In the galaxy as a whole there would be many. Restricting ourself to our cosmic neighborhood and to travel times not exceeding ten thousand years, there should be should be several hundred candidate planets.
Andersen: What are the ethical issues involved here? For instance, what if one of these probes landed on a planet that already had a functioning biosphere, and the probe species became invasive?
Gros: A Genesis mission is meant to give life the possibility to flourish, not to destroy it, so if the robe detected an existing biosphere, it would not deploy the microbes. Of course, we might not be able to assess status of a pre-existing microbial biosphere from orbit. A misjudgment on that question could then lead to a faulty go-ahead decision. In this case indigenous and terrestrial life forms would clash. We can, however, be confident that the presence of complex life could be detected reliably from orbit.
Andersen: Do you worry that sending out a bunch of probes like this would create a kind of cosmic monoculture, instead of allowing diverse biospheres to evolve?
Gros: No. The key idea of the Genesis probe would be target only transiently habitable planets. On these planets evolution would not have the time to develop complex life forms on its own.
Andersen: Is it unethical to kickstart a dynamic biosphere on a planet that will only be habitable for a short time?
Gros: This is a touchy issue. There are people who believe that it is unethical to give birth to children because they will die. I don’t think that way. Death is a part of life. And in any case, if the planet had, say, 500 million years that would be quite an extended time span.
Andersen: So it shouldn’t be a requirement of this experiment that whatever complex life you create has some chance of getting off the planet before it ceases to be habitable?
Gros: No, it is not a requirement—it would however be a welcome continuation of the cycle.
Andersen: Your paper is largely concerned with the feasibility of this sort of mission, but do you think humans will actually attempt a large-scale galactic seeding project like this?
Gros: Maybe not humanity as a whole—humans can hardly agree on anything, after all. But some humans will do this. I have found that people are enthralled by the idea that we may eventually give something back to life on this larger cosmic scale.