Life on exoplanets and the origin of life on Earth

Finding empirical evidence for life on planets outside the Solar System has profound implications for understanding our own origins here on Earth.

Life is everywhere on Earth. It fills the air we breathe in the form of microorganisms and has spread to every nook and cranny imaginable, even deep inside the Earth’s oceanic crust at the bottom of the sea. Life has been around for about four billion years (which is about 90% of life on Earth) and interestingly, all life forms we know of on Earth share the same principles for some of their fundamental functions, indicating that all life forms are related and can be traced back to a universal common ancestor. However, where, when and how life began is still unknown. Similarly, whether it really arose only once, giving rise to the widespread but unique kind of life we ​​see today seems unlikely, but not clear. In short, we do not understand the origin of life and we cannot say if it was a lucky chain of random events that happened under very specific conditions or if it was a logical and natural consequence of standard planetary evolution.

Coordinated efforts to address the origin of life

In recent years, many universities and research institutions have started coordinated efforts, in the form of interdisciplinary networks or research centers, addressing from the perspective of natural sciences where we (and all other life on Earth) come from. A recent example is the new ‘Center for the Origin and Prevalence of Life’ that was established in ETH Zurich.1

One of the main drivers for these centers was the realization that researching the origin(s) of life requires a truly interdisciplinary approach. A single discipline lacks sufficient knowledge and experience, given the complexity of the challenge. A standard example is the question of the formation of complex organic molecules, such as RNA and its precursors, which are considered a vital step towards the formation of life: while the goal of prebiotic chemistry is to reveal through which pathways formation and reaction networks and with what basic ingredients these molecules can be formed, it is crucial to consider the environmental conditions on the early Earth under which these reactions could have taken place. What was the temperature and pressure of Earth’s early atmosphere? What was its composition? What was the pH level of the immediate environment at the surface? Chemists are not normally concerned with these questions, but they are of immediate relevance if one wants to ensure that the correct boundary conditions are used for origin-of-life experiments.

The role of astrophysics

At ETH Zurich, but also in other research centers and networks, astrophysicists are heavily involved in research activities, as they can contribute important pieces of the puzzle. For example, there is the question of the radiation environment of the young Earth: how much radiation did the young Sun provide, and how much ended up on the Earth’s surface, rather than being shielded by the atmosphere? Of particular importance here is the high energy flux provided by the ultraviolet part of the solar spectrum. These photons are a unique and potentially abundant source of energy that can trigger chemical reactions that would not otherwise occur. However, because they are so energetic, photons can also break molecules apart and prevent the formation of more complex compounds. Also, astrophysics comes into play when it comes to understanding the path of formation of planetary systems and the delivery of the fundamental chemical components that were available on the young Earth. In addition to investigating remnants from the formation phase of the Solar System, such as comets and asteroids, revealing the (chemical) composition and physical properties of circumstellar disks that form planets around young stars is an important line of research in this context.

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Beyond the obvious: the role of exoplanet science

While these activities tie directly to the question of the origin of life here on Earth, exoplanet science also has an important role that may not be so obvious at first glance. With more than 5,000two exoplanets (planets that orbit stars other than our Sun) already known at present, the in-depth characterization of these objects is increasingly important –and technologically feasible– in addition to discovering more extrasolar worlds.

Indeed, one of the great hopes for the James Webb Space Telescope (JWST) is that it can help us understand whether small terrestrial exoplanets—objects with similar sizes and masses to Earth—orbiting low-luminosity, but highly active, cool dwarfs . stars can retain an atmosphere despite the strong emission of high-energy radiation from the stars. However, a detailed atmospheric characterization of these objects, that is, generating an overview of the main atmospheric constituents, will likely be beyond the capabilities of JWST. However, in the long term, this is exactly one of the main goals of exoplanet science: to be able to determine the atmospheric composition of dozens of Earth-like exoplanets. The reason is that the atmosphere encodes critical information about the planetary environment and, even more important in the context of origin-of-life research, it may also contain spectral features that indicate the presence of a biosphere on a planet. Therefore, by being able to determine what exoplanet atmospheres are made of, we have an opportunity to identify which planets life can exist on. For example, oxygen, which is produced by plants and algae (ie life), makes up about 21% of Earth’s atmosphere today. For an external observer from a remote point of view, it leaves detectable traces in the form of so-called atmospheric absorption bands. As such, the detection of oxygen in an exoplanet’s atmosphere could hint at the existence of life.

In addition to oxygen, there are also other gases, such as methane or nitrous oxide, that are considered good candidates for so-called atmospheric biosignatures, and various research groups are trying to understand which gases can be added to this list. A critical aspect of this work is to investigate which abiotic processes could also lead to the accumulation of a significant amount of these gases in a planetary atmosphere by mimicking the signal caused by biological activity. In many cases, the simultaneous detection of pairs of biosignatures that effectively point to a strong chemical imbalance, such as having oxygen and methane at the same time, is considered the strongest and most reliable signal to date.

But how does this relate to the origin of life? Currently, our home planet, Earth, is the only place in the Universe known to harbor life. Finding hints of life on another planet orbiting another star could provide the first evidence that, rather than the origin of life, we are actually dealing with the origins of life, and that life could be much more widespread. This could indicate that the pathways and processes leading from non-living matter to living entities are robust and more universally applicable and not limited solely to the specific conditions found on the early Earth. This implication would be particularly important if the star-planet systems, for which such inferences were made, were very different from the Sun-Earth system in terms of fundamental properties such as masses, temperatures, and compositions.

The LIFE mission

It is in this context that the LIFE mission3 it no longer looks like the next (or another) exoplanet space mission. As shown in a series of peer-reviewed scientific articles4,5,6,7,8, LIFE will be able to directly detect dozens of Earth-like exoplanets around different types of host stars, characterize their atmospheric composition, and search for biosignatures. Thus, if done right, the scope of the LIFE mission and its potential scientific heritage expands well beyond astrophysics and touches on one of humanity’s most fundamental questions. Therefore, it is key to join forces with other disciplines to help identify which atmospheric signals to look for and under what circumstances such signals provide strong evidence of biological activity. In addition, comprehensive statistical frameworks that allow researchers to quantify these claims will need to be further developed and validated. More importantly, however, as we increase our understanding of how life could have arisen on Earth, we will be able to begin to formulate hypotheses for which stars and types of planets are most (or least) likely to find evidence of life. alien. life using life as we know it, and that’s all we have, for reference. Work in this direction has already started in some of the collaborations mentioned above, creating an effective bridge between many scientific disciplines. Still, more systematic efforts are needed. The results will feed directly into the LIFE initiative activities to maximize the chances that LIFE can be fundamentally transformative in our understanding of the origins of life.

References

  1. www.copl.ethz.ch
  2. https://exoplanetarchive.ipac.caltech.edu
  3. www.life-space-mission.com
  4. https://arxiv.org/abs/2101.07500
  5. https://arxiv.org/abs/2112.02054
  6. https://arxiv.org/abs/2204.10041
  7. https://arxiv.org/abs/2210.01782
  8. https://arxiv.org/abs/2211.04975

Please note that this article will also appear in the twelfth issue of our quarterly publication.


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