How Might Life Migrate Through the Universe?
By the time we realized that there was an extrasolar intruder, ‘Oumuamua, named after the Hawaiian word for “scout,” had already passed its closest point to the Sun and was leaving, as fast and stealthily as it had arrived. We are talking about the first sighting, in 2017, of an asteroid from another area of the galaxy, a messenger from distant worlds. What do we know about this dark, probably cigar-shaped shard, which visited our solar system with a trajectory and velocity that allowed it to leave so quickly?
Very little. We know that it was not made of ice, so it must be of the rocky type. It did not ignite like a comet as it approached the Sun. We know that it does not emit electromagnetic radiation. The most powerful radio telescopes have found no trace of it. Its orbit is gravitational, determined by the attraction of the Sun; a small, non-inertial component can be explained by the effect of the pressure of the radiation in our star’s vicinity. We know that its speed, before entering the solar system, was compatible with the characteristic speeds of celestial bodies in the region of the Milky Way, of which our solar system is part. This allows us to exclude the idea that it comes from one of the dozen stars closest to us, as its velocity would have been too high. However, we have identified four more distant stars near which it could have passed in the last million years, with a velocity low enough that it could have originated in one of these star systems.
So, we don’t know exactly where it comes from, if it has already been in our solar system, how many other systems it has visited, or its composition. According to one hypothesis, it could be a fragment of an exoplanet destroyed by tidal effects. In this case it would be an object much rarer than main belt asteroids or objects from the Oort cloud, which formed directly from the original nebula. What is certain is that, on timescales of the order of millions or tens of millions of years, fragments like ‘Oumuamua can bring different star systems into contact. One estimate even predicts that 10,000 extrasolar asteroids cross Neptune’s orbit on a daily basis.
It would be interesting to be able to explore one to see what it was made of. This type of asteroid would seem to be the kind of vector suitable for transporting life, in hibernating form, from one part of the galaxy to another. While a space mission of this kind would be difficult because of the speed at which these fragments are moving, it wouldn’t be impossible, considering that in the future our observational capacity will improve considerably, allowing us to identify these bodies sooner than we were able to identify ‘Oumuamua. Another idea has to do with the possibility that some of these extrasolar objects have become trapped in our solar system after having lost some of their energy in a close encounter with Jupiter; a few candidates have already been identified. This approach would make an exploratory mission much easier to accomplish.
However, even the planets in our own solar system are in communication and exchanging material at a fairly high rate. Not everyone knows that we have about 10 rock samples from Mars here on Earth, even though there has not yet been a mission that brought back material from that planet. The meteorite bombardment on Mars results in fragments that, given its thin atmosphere, can be projected into space. Some of them can reach the Earth, penetrate our atmosphere, and fall like normal meteorites. By comparing the isotopic composition of various meteorites with those measured on Mars during NASA’s robotic missions to the planet, we are able to identify and distinguish Martian meteorites from all the others.
Finally, we should remember that it takes the solar system about 220 million years to revolve around the center of the galaxy. Since it formed 4.5 billion years ago, it has made the full circuit about 20 times. This means that, in the timescale in which life emerged on Earth, the newborn solar system made at least three complete circuits, coming into contact with fragments from distant star systems.
In 2019 I participated in a Breakthrough Discuss conference in Berkeley on “Migration of Life in the Universe.” I was puzzled by the conference theme: We know almost nothing about life in the universe, I thought, so how we could talk about migration of life? But recalling the observation of ‘Oumuamua, I did participate and I am glad I did. I was surprised by the scientific quality of the talks and by the extreme fascination of the topic. Life probably doesn’t need massive, rocky starships to move from one planetary system to another. Considering the minuscule size of bacteria, the smallest living organisms we know, or even viruses, which can live and reproduce inside bacteria, we can also imagine other mechanisms suitable for this kind of transport.
Microscopic ice crystals and dust, for example, containing bacteria and spores capable of withstanding the conditions in space, can spread into space from areas of a planet’s upper atmosphere. When the dimensions become microscopic, the relationship between gravitational force, which is dependent on mass, and the thrust due to stellar radiation, which is dependent on surface area, tips the balance in favor of the latter. It is as if a planet were leaving a trail of perfume behind it. Planetary dust containing hibernating life can be pushed by radiation until it reaches high velocities and moves beyond a given star system, spreading to other systems or nebulae, where it can find suitable conditions to reproduce and evolve. We are used to thinking of space as vast and mostly empty, completely unsuitable for life. Perhaps we should change our minds. Space is less empty than we might think. In reality, the different parts of the galaxy communicate by exchanging material on timescales comparable to those of the appearance of life on our planet.
But how possible is it for life to survive in space? Well, even here, nature surprises us. In fact, we know of various living species that can endure extremely hostile conditions such as those in space: a nearly perfect vacuum, extreme temperatures, and ionizing radiation. Different kinds of lichens, bacteria, and spores are able to survive, losing all of their water and entering into a condition of total inactivity — which can last for extremely long periods — from which they can emerge, once they find themselves in a humid atmosphere again. Tests of this kind have been done on the International Space Station and in various laboratories. Even plankton, made of more complex organisms, shows a capacity to resist these prohibitive conditions.
A truly extraordinary case is that of the tardigrades. These very common micro-animals are about a half a millimeter long and live in water. They have eight legs, a mouth and a digestive system, as well as a simple nerve and brain structure. They are also able to sexually reproduce. They exist in nature in thousands of different versions and have a metabolism with unique characteristics. In order to withstand prolonged drought conditions, their bodies can achieve complete dehydration, losing around 90 percent of their water and curling up into a tiny, barrel-shaped structure. In other words, it’s as if they freeze-dry themselves. Once this process is complete, their metabolism becomes 10,000 times slower. The most amazing thing is that they can stay in this state for decades, only to wake up again within 20 or 30 minutes once exposed to moisture. But there’s more. When in a dehydrated state, they can withstand the vacuum of space as well as pressures higher than normal atmospheric pressures, temperatures near absolute zero or temperatures up to 150°C. Their radiation tolerance threshold is hundreds of times higher than what would be deadly for humans. The secret of their ability to harden is due to a sugar, trehalose, which is also widely used in the food industry. When dried, this sugar replaces the water molecules in the cells, leaving the animal in a kind of vitrified state.
In addition, the tardigrade’s DNA is protected by a protein that reduces radiation damage. Is this information enough to make us assume that these micro-animals come from space? I would say no. Their unusual metabolism is more likely the result of evolutionary adaptation that happened on our planet. In fact, tardigrades are among the very few living beings that have emerged unscathed from all five extinction events that have occurred on Earth. That is why they are the best candidates for a long journey into space aboard a meteorite or a comet. Recently, tardigrades have achieved a bit of media notoriety resulting from the Beresheet mission, a private probe launched by Israel, that crashed on the Moon in early April of 2019. The probe was carrying a colony of these micro-animals, in their dehydrated state. Given their microscopic size, it is likely that they survived the crash and will remain inactive for a long time to come, ready to be reawakened from their hibernation. By replacing the Israeli probe with an asteroid, we have a textbook example of how life might have arrived on Earth.
Or how life could have migrated from Earth to other planets in our galaxy.
So, the problem of the origin of life remains open, even if, step by step, we are making progress toward a solution. In the last decade, increasingly powerful calculation instruments have allowed us to reproduce, starting from the first principles of quantum mechanics, the formation of increasingly large and complex molecular systems, now made up of thousands of atoms. The field of computational biology is growing at a formidable rate; it is now only a matter of computing power.
At the same time we have dramatically developed our ability to decode and manipulate DNA, up to the creation of the first simplified genomic structures, derived from living organisms and able to reproduce. We are now talking about synthetic life, built around human-designed DNA, a field with huge development prospects.
Therefore, it is likely that the creation of the complex molecular structures needed for life or the confirmation of the existence of islands of genomic stability in the evolution of viral and bacterial species are objectives that, in future, will be within our reach. At that point, we will have another tool for understanding how life on Earth developed. Who knows? Perhaps we will discover that aliens are particular biological life forms that have lived with us since the beginning of time; and we were looking for them on Mars or below the icy surface of Jupiter and Saturn’s moons!
Roberto Battiston is a physicist who specializes in the field of experimental fundamental and elementary particle physics, both with particle accelerators and in space. He is the author of several books, including “First Dawn,” from which this article is excerpted.