##  Chapter 1 Origins of Life on Exoplanets

[1,2,3]Paul B. Rimmer

###### Abstract

Abstract I show that exoplanets can be used to test origins scenarios. Origins
scenarios start with certain initial conditions, proceed via a network of
chemical reactions and, if successful, result in a chemistry that is closer to
a living system than the initial conditions. Exoplanet environments can be
applied to test each of these three aspects of origins scenarios. I show what
tests can be applied to the UV-driven cyanosulfidic scenario and how the
application of some of these tests has already falsified certain versions of
this scenario. Testing initial conditions has replaced certain reactants with
others and has affected the overall chemical network underlying the
cyanosulfidic scenario. The sequence of reactions the scenario invokes provide
a predicted upper limit on the ubiquity of life in the universe that has ample
room for improvement. The outcome of the experiments in different environments
is part of a predicted distribution of biosignature detections that can be
compared to future observed distributions.

###### Abstract

Author Links
http://www.mrao.cam.ac.uk/~pbr27
https://scholar.google.co.uk/citations?user=BBcDP-0AAAAJ

Keywords: origins of life, prebiotic chemistry, exoplanets, Karl Popper, Nancy
Cartwright

###  1.1 Introduction

> “Anybody who thinks they know the solution to this problem (of the origins
> of life) is deluded; but anybody who thinks this is an insoluble problem is
> also deluded.” – Leslie Orgel111Related by Bardi, J. S. in News and Views
> Vol. 3 (The Scripps Research Institute, 2004).

Since the classic experiment of Miller [33], thousands of experiments have
been performed and computational models run, and dozens of scenarios have been
proposed to address the question of how life originated. Some of these
scenarios concentrate on a handful of promising reactions, starting from pure
mixtures with precisely controlled conditions [52]. Others start from
interesting reactions embedded in biological or geochemical systems or both
[11], trying to work backwards to discover the bridge between the two, with
some promising results [28]. Still others are inspired by the myriad compounds
discovered in interstellar space near our galactic center [32], or in the ices
of comets [2]. Some approaches seek to construct a plausible story of how life
arose on Earth [13, 37], while others instead search for the fundamental
principles that govern life [12], if any are to be found.

All scenarios and experiments are joined to early Earth by analogy, though
some analogies are much closer to the real thing (e.g., Milshteyn et al.
[34]). The use of analogy raises a serious problem for origins research and
opens the entire field up to a flood of criticisms, well-summarized in a book
review by William Bains [4]. Underlying these criticisms is a methodological
question: how will we ever know if we found the answer? What hints are there
that we are even on the right track? The central methodological issue in
origins research is testability.

This issue was anticipated early on by Günter Wächtershäuser, who applied the
principle of falsifiability and the link between testability and
informativeness of a theory, following Karl Popper [38], to construct his
scenario and to provide a clear and useful template for the development of
alternative scenarios [59]. Testability is difficult to achieve, however,
since the lab environment is artificial and all geochemical environments yet
discovered on Earth where life can survive have been transformed by the
presence of life.

In this chapter, I propose that exoplanets can act as new laboratories on
which to test origins scenarios. These tests will require an older style of
experiment. Though Miller’s experiment showed up so early on the scene that we
could not understand the results or whether the experimental conditions
themselves were genuinely plausible, Miller’s experimental approach is exactly
what the chemist should return to in the next few years, not blindly, but with
decades of chemical knowledge at our disposal. We can construct more
realistic, more messy, environments within which to perform experiments (see
for example [35]). These experiments will doubtless lead to the discovery of
novel scenarios. Hopefully they will also rule out others.

Applying exoplanet science to test origins hypotheses will require a different
way of thinking about specific scenarios, not just in one environment on
Earth, but displaced to a wide range of diverse extraterrestrial environments,
where the chemistry will be found to succeed or fail, or to reveal an
unexpected new path to explore. This approach can be helped, I find, by a
shift in language.

###  1.2 How to Test Origins Hypotheses

An intermediate goal for origins of life research is the discovery of a
scenario where some local and available chemical and physical conditions lead
to a series of chemical reactions that proceed without the assistance of a
chemist to a protocell: a self-contained self-replicating metabolizing
macromolecular machine with the potential to evolve into something like the
last universal common ancestor (LUCA) via random mutations and natural
selection.

All proposed scenarios are far from this intermediate goal. Nevertheless, any
given scenario can be criticized on the basis of this desired outcome. Maybe a
scenario has implausible initial conditions. Or maybe the proposed sequence of
reactions are unlikely to proceed without help. Finally, since the outcome is
a long way from a protocell, and may look quite different from a functional
protocell, let alone LUCA, it could be argued that the outcome will turn out
to be irrelevant. These three criticisms unique to origins of life are on the
basis of:

  1. 1.

Initial conditions.

  2. 2.

Sequence of reactions

  3. 3.

Outcome

The underlying problem is that when someone tries an experiment in the lab and
it works, this only shows that the experiment will work in the lab. Granted,
if outside the lab the same conditions and sequence of events just happen to
occur, the outcome will be the same. But the chances of all these events
happening in a way identical to a lab and without the chemist will take on the
character of a Rube Goldberg machine. But this requires too much specificity.
Some decisions are made for experimental convenience, some for the sake of
characterizing the product, and some decisions are arbitrary. Many changes
will do nothing to the chemistry. Some changes will result in a different
outcome, but a different outcome is not the same as failure. Some changes will
likely improve the chemistry, and many other changes will likely disrupt the
chemistry [62].

I find it useful to think about these components of a scenario using the
language of ‘natures’ and ‘powers’ (following Cartwright [6]222This language
has a long history in origins science, since natures and powers are closely
related to propensities [7], and propensities were central to the Popperian
cosmology [39], used to great effect by Wächtershäuser[59].). By the nature of
a thing, I mean what a thing tends to do in its immediate environment. This is
a result of the interactions between the powers of the thing itself and its
environment. The nature of a molecule can be ascertained within a lab by
series of experiments across a wide range of physical and chemical conditions,
and the context of that understanding can be broadened by setting up lab
conditions to more closely approximate plausible natural environments, and by
exploring the environmental parameter space to find out what environmental
factors matter for the chemistry.

We can then apply our knowledge of the nature of these molecular species to
model their behavior within a wider chemical context, and can predict both the
intermediate and immediate consequences of that behavior and the interaction
of these molecular species and their environment. Some of these immediate
consequences may be preserved in the environment, and may be observable. For
instance, there has been significant progress toward predicting outcomes of
prebiotic chemical scenarios that can then be tested against observations by
Mars 2020 in Jezero Crater [53].

The criticisms and problems now become tests:

  1. 1.

Does the environment invoked by the scenario provide the necessary initial
conditions for the reactions to take place?

  2. 2.

What is the probability that these reactions will take place in sequence and
unaided in this environment?

  3. 3.

How likely is it that the outcome of these reactions will result in a
protocell? What paths are opened up for chemical progress and what paths are
closed off, either by the environment or by the direct outcome of the reaction
sequence?

Here I will focus on rocky planetary333Planetary here includes some satellites
of other planets, like Enceladus, Europa and Titan. environments. These
environments can be split up into Earth environments and other planetary
environments within our solar system (Mars, Venus, certain moons) and outside
our solar system (exoplanets). The two relevant factors that distinguish
between these groups is resolution and population size.

Earth is the planet where we have by far the most information and best
resolution. This resolution makes clear that Earth is not a single
environment, but many, spanning a variety of temperatures from below zero to
thousands of degrees Celsius, pressures much less than a bar to many GPa, and
a variety of redox conditions from highly oxidizing to highly reducing [16].
Nonetheless, Earth is a sample size of one, and all environments relevant for
prebiotic chemistry have been transformed by life.

Rocky Planetary Bodies in the Solar System. The resolution we have for
planetary bodies in the solar system varies with distance and ease of
investigation [25]. The sample size where we have decent resolution includes a
dozen or so objects. Some of these objects, perhaps all, are and have always
been lifeless (but see [10, 22]). Still, it is challenging to apply statistics
usefully to such a small sample size, and the sample size is biased since all
these rocky bodies are part of the same single solar system.

Exoplanets are the planets we know least about. Most exoplanets we’ve
discovered are inferred from the changing light of their host star, itself a
small fraction of a single pixel on a detector. However, the number of
exoplanets is large and rapidly increasing, spanning many stellar systems
around many different types of stars. The sample is shaped by observational
biases, but new detection techniques will slowly mitigate these.

This chapter will focus on how exoplanets can inform tests of origins
scenarios. In the next section, I will briefly discuss exoplanet detection and
characterization.

###  1.3 Exoplanets as Laboratories

Exoplanets are planets that orbit stars other than the Sun. Thousands of
exoplanets have been discovered444See http://exoplanet.eu/. Exoplanets orbit a
wide variety of stars and their properties span a wide range, e.g. distance
from their host star, their sizes and masses. Exoplanets are primarily
discovered by three different techniques (see Figure 1.1, and see [63] for a
detailed review):

  1. 1.

Transit: Where the star dims as the planet passes in between the star and the
observer.

  2. 2.

Radial Velocity: Where the planet’s gravitational pull on the star causes the
light of the star to shift in frequency

  3. 3.

Direct Imaging: Where the observer blocks the light from the star in order to
observe the light of the planet directly.

Transits determine the ratio of the planet’s radius to the star’s radius.
Where the star’s radius is known, the size of the planet can be determined.
Radial velocity and knowledge of the star’s mass can similarly be used to
obtain the mass of the planet. The combination of these techniques can
constrain the planet’s density, giving us an idea about whether a planet is a
rocky planet or a gas planet [54, 67]. The periodic signal of the transit or
radial velocity, or the measured distance between a star and an orbiting
directly imaged planet gives the planet’s distance from its host star. The
temperature and brightness of the star, often expressed in terms of its
stellar type, can be used to delineate a range of distances from the star
within which liquid water can exist stably on a planet’s surface [26]. We can
call this the habitable zone.

One way the atmosphere of a planet can be characterized is when the planet’s
transit is observed at different wavelengths. The apparent radius of the
planet will change with wavelength depending on the absorption properties of
its atmosphere. This task is much easier to accomplish around small stars,
where the size of the atmosphere divide by the size of the star is larger
[55], and where the habitable zone is nearer to the star, so transits of
planets within the habitable zone are more frequent. We do not yet have the
observational capabilities to observe atmospheres of rocky planets within the
habitable zones even of ultra-cool stars.

In this chapter, I will focus on rocky planets within the habitable zones of
their host stars (see Kane et al. [24] for a catalogue of these planets). But
first, I will briefly say something about the planets whose atmospheres we can
characterize today.

###### Gas Planets

Gas giant planets ($R_{p}\gtrsim 0.5R_{J}$) are by far the easiest planets to
characterize. Intrinsically interesting, gas giants are also relevant for
origins questions in at least two ways. (a) Their composition gives insight
into planet formation history and disk chemistry [40, 30], and (b) physical
processes, such as atmospheric photochemistry, stellar energetic particle
chemistry, and impact-induced chemistry, can be explored on these exoplanets.

Figure 1.1: Schematic of a variety of detection methods, modified from an
infographic from the European Space Agency for the Cheops mission
(https://www.esa.int/ESA_Multimedia/Sets/Exoplanets_infographics/(result_type)/images).

###  1.4 The Scenario

I will discuss the tests listed in Section 1.2 for only one scenario. These
tests can in principle be applied to any other sufficiently specific scenario.

The scenario I discuss is the cyanosulfidic scenario. It began when a 1 km
sized impactor struck the surface of a volcanic island on early Earth, during
the time it had a transient reducing atmosphere [19, 50]. That impact
delivered phosphorus in the form of phosphate (PO_4^2-) [37], and generated a
crater with large amounts of hydrogen cyanide (HCN, [17]). This HCN rained
down and reacted with available Fe(II) to form ferrocyanide ([Fe(CN)_6]^4-).
The volcanism heated the ferrocyanide in the crater causing it to form various
organometallic and other complexes, depending on the local mineralogy and
temperature (see Table 1.1).

Small streams of water flowed over this material, releasing volatile compounds
(see again Table 1.1) and initiated a series of prebiotic chemical reactions
as discussed in detail by Patel [37], generating small sugars that proceeded
to form amino acids, phospholipid precursors and nucleotides, including
pyrimidine ribonucleotides [37], and purine deoxyribonucleotides [65], as
shown in Figure 1.2. Subsequent nitrate chemistry generated non-enzymatic
activating agents [31], that facilitated the oligomerization of these
compounds [29].

The different sequences of reactions are not all compatible with each other,
so different streams must have carried different reactants and intermediate
products. These streams must have intersected at the right time and in the
right sequence for the scenario to succeed. See again Figure 1.2.

I choose this scenario for several reasons. First, it seems to be the most
plausible scenario yet discovered for producing precursors to RNA, DNA,
proteins and membranes in the same local environment. On this point, see
Chapter X by Sutherland. Second, the scenario has been given in sufficient
detail [37, 64, 31, 41, 53, 46] to apply the above tests and to potentially
falsify aspects of the scenario. There may be other geochemical environments
where the same sequence of chemical reactions can occur, e.g. hydrothermal
vents [48], but these alternatives have not yet been worked out in sufficient
detail to apply the above tests.

Finally, I have a pragmatic reason for choosing this scenario. The scenario
depends on starlight, and starlight is the thing about exoplanet systems we
know the most about. For the next few decades, any scenario that uses
starlight will be the most straightforward to connect to exoplanet
observations.

Table 1.1: Organometallic Feedstock Compounds and their Generation

Starting Compound | Transformed Compound | Temperature | Other Species/Conditions
---|---|---|---
(Fe,Ni)_3P |  (Fe,Ni)_3(PO_4)_2$\cdot$8 H_2O | RT111Room Temperature. | Liquid Water
HCN |  {(Na,K)_4,Mg_2,Ca_2}[Fe(CN)_6] | RT |  Fe(II)222Best if Fe(II) is aqueous. The outcome depends on the content of Mg vs Na and K available within the basalt and the presence of surface metal oxides, hydroxides, chlorides, carbonates, etc.
(Na,K)_4[Fe(CN)_6] |  (Na,K)CN | 700∘C |
Mg_2[Fe(CN)_6] |  Mg_3N_2 | 420∘C |
Ca_2[Fe(CN)_6] |  CaNCN | 660∘C |
CaNCN |  CaC_2 | 1000∘C |
(Na,K)CN |  HCN | RT | Liquid Water
Mg_3N_2 |  NH_3 | RT | Liquid Water
CaNCN |  CN_2H_2 | RT | Liquid Water
CaC_2 |  HC_3N | RT | Liquid Water, Cu(II), HCN

††footnotetext: Based on Figure 2 from Patel et al. [37]

Figure 1.2: (Top). First steps of the cyanosulfidic photochemical scenario,
adapted from Sutherland [56] with permission. (Bottom). A series of streams
intersecting each other, running over different organometallic feedstock
compounds. A stream segment $i$ is identified, relevant for Section 1.6.

###  1.5 Initial Conditions

The first test for which exoplanets can act as laboratories is as a test for
initial conditions. What initial conditions are required for the success of
the cyanosulfidic scenario, and what fraction and distribution of exoplanets
meets these conditions? This question can be addressed both in the lab and by
future observations of exoplanet atmospheres. Initial conditions can be split
into two groups: chemical initial conditions and physical initial conditions.

####  1.5.1 Chemical Initial Conditions

The chemical initial conditions for the cyanosulfidic scenario are various,
including cyanoacetylene, cyanamide, and phosphate. I will concentrate on the
two initial chemical conditions that give the cyanosulfidic scenario its name.
The cyanosulfidic scenario depends on both cyanide and sulfide, and I will
discuss the availability of each of these species in turn.

#####  1.5.1.1 Hydrogen Cyanide

Hydrogen cyanide (HCN) can be formed in large quantities when there is
available free nitrogen in a local environment where the C/O ratio is $\geq
1$. HCN can be formed thermochemically, from lightning [9, 3], or in magmas
[48], and can be formed photochemically when thermospheric N_2 is
photodissociated and the resulting atomic nitrogen diffuses downward, where it
interacts with the dissociation products of CH_4 or of CO and H_2 [45]. The
N_2 can be dissociated deeper in the atmosphere by stellar energetic particles
[1].

HCN can also be delivered by smaller impactors, either primordial [36], or
from the destruction of delivered reduced nitrogenous compounds during the
impact [57]. Large impactors carry Fe^0+, which can be oxidized on impact by
H_2O and CO_2, resulting in FeO, CO and H_2. The heat from the impact can
allow these species to further reduce other atmospheric constituents,
resulting in large amounts of CH_4 and NH_3. This transient “warm Titan”
atmosphere can persist for millions of years [66]. Subsequent impacts within
this transient atmosphere produce large amounts of HCN and acetylene (C_2H_2).
The C_2H_2 is in principle detectable in the atmospheres of rocky exoplanets
[46]. The search for acetylene in the atmospheres of young rocky exoplanets
can help us constrain the fraction of planets that have had these atmospheres
which are very useful for prebiotic chemistry [5].

Even without impacts, HCN can be produced in large quantities, but only given
sufficiently reducing local or global environments. The relative ubiquity of
these environments can be constrained by searches for atmospheric
“prebiosignatures’ such as CO, CH_4, HCN, and C_2H_6 on exoplanets [51],
species that are either themselves feedstock molecules for prebiotic chemistry
or are tracers of reduced atmospheres where production of these feedstock
molecules is most conducive.

Figure 1.3: The photochemical upper atmospheric mixing ratio of HCN based on
the local atmospheric C/O ratio. Taken from Rimmer & Rugheimer [45].

#####  1.5.1.2 Sulfite and Sulfide

The cyanosulfidic scenario as presented by Patel et al. begins with H_2S and
HCN in liquid water with phosphate and UV light. At all degassing pressures
with plausible early Earth magma temperature and mantle redox, SO_2 degassing
exceeds H_2S degassing by at least a factor of 2 [18]. In addition, the pKa of
H_2S is 7 and the pKa of SO_2 is 1.81, so for acidic to neutral waters, SO_2
is much more soluble than H_2S. Ranjan et al. [43], first noticed this issue
and further determined that aqueous SO_2 will be converted primarily into
sulfites: HSO_3^- and SO_3^2-. Like sulfides, sulfites can undergo electron
photodetachment under UV light, and the resulting chemistry is similar [64].
Both H_2S and SO_2 are plausible for any volcanic environment on early Earth,
but H_2S will not plausibly be present without SO_2, and not in excess of
SO_2, except in alkaline environments.

####  1.5.2 Physical Initial Conditions

This scenario also requires a range of physical conditions that can be tested
by future experiments and exoplanet observations. Requirements include:

  1. 1.

Surface liquid water

  2. 2.

Dry land

  3. 3.

Sufficient ultraviolet(UV) flux

Other physical conditions such as volcanism and a history of impacts are also
required in the case that they may be required to provide sufficient feedstock
molecules (see the above Section 1.5.1), but are not listed here because there
may be alternative ways to obtain the required chemical initial conditions.

The first requirement is accounted for by the Liquid Water Habitable Zone (see
Section 1.3). The second requirement can be tested by looking at water worlds,
planets that have a sufficiently large mass fraction of water that they have
no dry land. Alternative scenarios may or may not work on water world planets
[27], but the scenario I consider very likely cannot. Water worlds can be
identified by constraining planetary density and by atmospheric observation
(Section 1.3).

###### The Abiogenesis Zone

The involvement of UV light makes possible the delineation of a zone around a
star outside of which this chemistry cannot take place. I call this the
“Abiogenesis Zone” [49]. The Abiogenesis Zone is based on a competition
between the UV photodetachment of electrons from either SO_3^2- or HS^-, and
subsequent reduction of either HCN or glycolonitrile, versus reactions in the
dark that result in inert adducts. By comparing the rates of these reactions,
Rimmer et al. [49] were able to delineate the Abiogenesis Zone for the
quiescent emission of main-sequence stars. I show the Abiogenesis Zone based
on SO_3^2- photodetachment in Figure 1.4.

One implication of Rimmer et al. [49] is that early Earth lies outside the
Abiogenesis Zone with regard to HS^- photodetachment. This result falsifies
the version of the cyanosulfidic scenario from Patel et al. [37], but not the
version of the scenario from Xu et al. [64].

Another implication of Rimmer et al. [49] is that ultracool stars cannot host
planets on which this chemistry will work, based on the quiescent emission of
those stars. This includes the vast majority of temperate rocky planets so far
discovered [24, 20]. Follow-up studies test whether flares may provide
sufficient UV light, testing the hypothesis of Ranjan et al. [42] and the
experimental implications of Rimmer et al. [49] against observations. These
studies indicate that it is likely that the vast majority of these stars’
present activity levels are insufficient for this scenario [23, 15, 21]. The
search for biosignatures on these planets will test this scenario.

Figure 1.4: The abiogenesis zone (orange, with error bars in red) and liquid
water habitable zone (blue) as a function of planetary period and stellar
effective temperature. Outside the liquid water habitable zone, planets with
surface temperatures regulated by CO_2 and H_2O greenhouse gases cannot
sustain liquid water on their surfaces. Outside the abiogenesis zone, the
quiescent UV emission of the star is not intense enough to allow for the
prebiotic photochemical scenario presented in this chapter. Figure taken from
Rimmer et al. [49].

###  1.6 Chances of Success

I now discuss the chances that the sequence of reactions invoked by this
scenario will succeed. To this end, I return to the picture of the
intersecting streams (Figure 1.2). In this figure, there is a stream segment
labeled $i$. The chemistry has a chance of succeeding for each segment,
$c_{i}$. Since, for this scenario, the chemistry must be segregated into
different streams, and the intersection of the streams out of sequence leads
to unwanted chemistry and to a failure for the scenario, we can estimate the
probability that the scenario succeeds in bringing about the origins of life,
where $N$ separate stream crossings are required in the correct sequence to
bring life’s origin about. We don’t yet know whether the entire scenario will
require more and more streams, but we know this scenario at this early stage
requires at least three, probably four. How many more are required will say a
lot about how common or rare life is predicted to be elsewhere in the
universe, given that this scenario is the most plausible scenario for life’s
origins.

We start with a crater full of streams, and the surface of the crater has
organometallic compounds that will react in the water to release cyanide,
cyanamide, acetylene, the prebiotically relevant compounds. First, the stream
we follow must originate in the part of the crater with the right starting
organometallic feedstock, and the reactions to take place in the first segment
must succeed before the intersection with the next stream. This gives us a
leading term of $c_{1}/N$. If a stream intersects another stream, there is a
chance the stream will disrupt the chemistry or will not affect the chemistry,
and the fraction of streams that do not affect the chemistry for segment $i$
is $M_{i}/N$.

The probability for a single stream to succeed is then the product of the
probabilities for each segment, or:

$P_{s}=\dfrac{c_{1}}{N}\prod_{i=2}^{N}\Bigg{[}\Big{(}\dfrac{1}{N-M_{i}}\Big{)}\,c_{i}\bigg{]}.$
(1.1)

If $M_{i}=N-1$, then every stream segment is compatible with any possible
stream. If $M_{i}=1$, then the stream segment is compatible with no other
possible stream. We assume that $M_{i}=1$ for all $i$. If this were not the
case and $M>1$, then components of the chemistry could be collapsed, less
streams would be needed, and the hypothesis can be effectively simplified by
setting $N\rightarrow N-M$. We will also assume that all the $c_{i}$ are the
same value for simplicity.

We can use Eq. (1.1) to estimate the probability that the chemistry will
succeed, $P$, as:

$P=1-\big{(}1-P_{s}\big{)}^{s},$ (1.2)

where $s$ is the number of streams that can flow within the crater until the
reservoir organometallic feedstock is consumed. Given $\tau_{\ell}$ (y), the
timescale over which a certain amount of new organometallic material is
delivered or generated via impact, the timescale for life to originate will be
on the order of:

$\displaystyle\tau$
$\displaystyle=\dfrac{\tau_{\ell}}{1-\big{(}1-P_{s}\big{)}^{s}},$ (1.3)
$\displaystyle P_{s}$
$\displaystyle=\cfrac{c^{N}}{N}\Bigg{(}\dfrac{1}{N-1}\Bigg{)}^{\\!\\!N-1}.$

When $sP_{s}\ll 1$ then $\big{(}1-P_{s}\big{)}^{s}\approx 1-sP_{s}$ and
$N\big{(}N-1\big{)}^{N-1}\approx N^{N}/e$, and Eq. (1.3) simplifies to:

$\tau\approx\dfrac{1}{e}\,\dfrac{\tau_{\ell}}{s}\Bigg{(}\dfrac{N}{c}\Bigg{)}^{N}.$
(1.4)

We will treat $\tau$ as a function of $N$ with $s$ and $\tau_{\ell}$ as
parameters that we can vary. If streams are on average 10 cm across and crater
lakes are on average 1 km2 in area, and each stream consumes the
organometallic feedstock it immediately flows over, then 1 km2 of crater lake
area allows for $\sim 50000$ streams. We also assume that the total area of
crater lakes is the same as the total area of lakes today, which gives us
$4.2\times 10^{6}$ km2 of crater lake area [14], or $2\times 10^{11}$ streams
available during late accretion, over $10^{7}$ years. We will consider 1000
year increments, during which we estimate 420 km2 of new impact-generated
organometallic material is provided for the streams to flow over., and so
$s=2.1\times 10^{7}$.

Figure 1.5 shows that the amount of time for life to originate, the “origins
timescale” depends strongly on the chance that the chemistry in stream segment
$i$ succeeds and the number of streams. If the chemistry is difficult, and
unlikely to succeed through any given segment, so $c_{i}=0.1$ for all $i$,
then if the required number of streams is less than 6, life will arise within
the first 1 My once the conditions are available. If the required number of
streams is 7, it will take on average 100 My years for life to arise. If, as
we assume above, the impact-generation of environments provides a $\sim 100$
My timescale for success, and if the number of streams is greater than 8, and
if this scenario is the most plausible scenario for origins of life, then
Earth is likely the only planet that hosts life within 10 pc.

Things become more optimistic if the probability of chemical success becomes
greater. If $c_{i}=0.5$ for all $i$, then the limit becomes greater than 11
streams before we predict that life is rare on this scenario. Even if the
chemistry always worked out perfectly in every stream segment, such that
$c_{i}=1$ for all $i$, there is still a limit of 13 streams.

All of this is under the assumption of 1000 year timescales with
organometallic feedstock enough for $2.1\times 10^{7}$ new streams per
timescale. Perhaps these estimates are off. The timescale estimate does not
affect the results much at all, simply shifting the lines vertically. The
number of streams matters more, but even increasing or decreasing the number
of available streams, $s$, by $10^{3}$ shifts the line only by one or two
required streams, $N$. This shift by $10^{3}$ is what the dotted lines around
the solid lines illustrate in Figure 1.5.

Calculations like these could be applied to a wide range of scenarios, if the
scenarios are not too vague. These calculations can be supported by real
numbers by simulating sequences of reactions in prebiotic chemical scenarios
and keeping track of outcomes under different environmental conditions. This
would allow us to estimate the values of $c_{i}$, $s$ and $\tau_{\ell}$, or
whatever equivalent values would apply to an alternate scenario, and to test
whether the required number of streams (or other chemical transitions) is
really $N$. The value of $N$ cannot be known until at least the intermediate
problem (see Section 1.2) is solved, but a lower limit can be obtained by the
combination of more realistic experiments and new discoveries of subsequent
chemical steps, as the scenario moves incrementally closer to solving the
intermediate problem.

The resulting probabilities for one scenario can be compared to the
probabilities of others given different global and local environmental
conditions in order to estimate which scenario is more plausible given those
conditions. These probabilities can also be applied to a Bayesian analysis of
biosignatures [8, 61], depending on the relevance of the outcomes of the
scenarios and whether their chemical outcomes are indeed closer to life than
were the initial conditions.

Figure 1.5: The origins timescale (y), the timescale to start life (as defined
in Section 1.6), vs. the number of stream crossings required, for three
different probabilities of success for chemical reactions within a stream
segment, $c_{i}=0.1$ (red), 0.5 (orange) and 1.0 (green). See Figure 1.2 for a
schematic of the stream crossings.

###  1.7 Relevance of the Outcome

Testing the relevance of an origins scenario in terms of its chemical outcome
can proceed in the lab and through future exoplanet observations.

I will say little about how this can proceed in the lab, because this
direction crosses over from chemistry to biochemistry, and biochemistry is
well outside my area of expertise. I can provide one case for how this test
can proceed involving my work with biochemists. Methyl isocyanide is an
effective non-enzymatic activating agent for amino acids, nucleotides and
phospholipids [29]. Methyl isocyanide can be synthesized using nitroprusside,
and nitroprosside is a product of ferrocyanide and nitrites in water
irradiated by a mercury arc lamp. Nitroprusside is not produced from nitrates
instead of nitrites [31]. It turns out that nitrites are not nearly as
plausible as nitrates in early Earth aqueous environments [44]. However, if a
more realistic broad-band light source is used, then ferrocyanide and nitrates
will produce nitroprusside [47], and this pathway forward is not ruled out by
the environment after all.

The successful product of a future observing program to search for exoplanet
biosignatures, here defined as spectroscopic signs of life, will likely be a
statistical distribution of exoplanets that probably host life. The nature of
that distribution as a function of parameters like stellar effective
temperature and activity, the mean density of the planet, and the plausibility
that it had a reducing atmosphere in its past will test and potentially
falsify claims connected with certain origins scenarios.

Not much can be concluded if no life is found, apart from a vague lower limit
on the difficulty for life to arise from non-life on the sample of exoplanets
whose atmospheres were observed. Absence of evidence for life is not evidence
for the absence of life, so applying this test will not be able to provide a
specific answer that is independent of the limits of our observational
capabilities. At most it would weaken the case that life is a phase transition
that will inevitably be crossed within any given temperate environment.

###  1.8 Conclusions

Using the cyanosulfidic scenario as an example, I show how exoplanets can be
used to test origins scenarios. Initial conditions, probability of success and
relevance of outcome can be tested using exoplanet environments. Present
exoplanet observations, and not just of temperate exoplanets, can be applied
to explore new chemical mechanisms and environments more fully in the lab, the
results of which can in turn inform prior probabilities for biosignature
searches. In the more distant future, exoplanet observations themselves can be
used to test particular claims about given scenarios by comparing predicted to
actual distributions of biosignature detections.

These tests will require a large-scale experimental exploration of the
parameter space, and not just for plausible Earth environments (see Walker et
al. [60]). Ideally this effort will be open about what scenarios to explore.
Right now there are no clear ways to falsify most scenarios presently “on the
market”, and so researchers should be free to explore their own direction as
part of a friendly, supportive and tolerant research community, so long as the
underlying chemistry makes sense and the scenario is being explored with an
eye to future testing and potential falsification.

I suspect origins research will not require new physics. But it will require a
means of testing proposed partial solutions. Different terrestrial
environments have been used to perform these tests (e.g. [34]), and other
solar system bodies have just recently come into the picture (e.g., for Mars
[53], and Europa [58]). Exoplanets are a new and largely untapped resource for
this problem, if we are willing to make the effort to connect what we do in
the laboratory to these alien environments in a meaningful way.

###### Acknowledgements

Some of the ideas in this chapter originated from a panel discussion at the
Future of Exoplanet Research symposium sponsored by the Kavli Foundation as
part of the TESS I conference. I’m grateful to Alexander Bird for helpful
feedback on my Chapter’s philosophical claims. I thank Ram Krishnamurthy for
introducing me to the Leslie Orgel quote at the beginning of the introduction.
I am thankful to Longfei Wu, Craig Walton, Oliver Shorttle, John Sutherland,
and many others connected with the Cambridge Universal Life Initiative for
many helpful conversations and new ideas. This work is funded by the Simons
Foundation (SCOL Grant #599634).

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