Centauri Dreams

Jim Benford’s study of ‘lurkers’ — possibly ancient probes that may have been placed here by extraterrestrial civilizations to monitor our planet’s development — breaks into two parts. The first, published Friday, considered stars passing near our Sun in the lifetime of the Solar System. Today Dr. Benford looks at the Drake Equation and sets about modifying it to include the lurker possibility. Along the way, he develops a quantitative way to compare conventional SETI with the strategy called SETA — the search for extraterrestrial artifacts. Both articles draw on recently published work, the first in JBIS, the second in Astrobiology. The potential of SETA and the areas it offers advantages over traditional SETI argue for close observation of a number of targets close to home.

by James Benford

Introduction

“To think in a disciplined way about what we may now be able to observe astronomically is a serious form of science.” –Freeman Dyson

I propose a version of the Drake Equation for Lurkers on near-Earth objects. By using it, one can compare a Search for Extraterrestrial Artifacts (SETA) strategy of exploring for artifacts to the conventional listening-to-stars SETI strategy, which has thus far found no artificial signals of technological origin. In contrast, SETA offers a new perspective, a new opportunity: discovering past and present visits to the near-Earth vicinity by ET space probes.

SETA is a proposition about our local region in the solar system. SETA is falsifiable in its specific domain: ET probes to investigate Earth would locate on the nearest objects down to a specified resolution. SETI, on the other hand, is about messages sent from distant stars. For example, one can falsify a proposition such as “Are signals being sent to Earth at this moment within 100 ly?” But there is the region beyond 100 ly and beyond 1000 ly, etc. So SETI is falsifiable only within larger and larger domains. Of course other factors can also weaken falsification: our sensitivity might be inadequate, duty cycle might be small, and of course frequency coverage will always be incomplete.

Rose and Wright pointed out the energy efficiency of an inscribed physical artifact vs. an EM signal, because the artifact has persistence and the EM signal has to be transmitted indefinitely (Rose & Wright, 2004). Here I point out that artifacts are not only energy efficient, but increase the chance of contact. Rose and Wright did not explore where to locate the artifact so it would be identified; here I suggest there are attractive locations near Earth where they might be readily observable.

In a recent paper, I introduced the term ‘Lurker’: an unknown and unnoticed observing probe from an extraterrestrial civilization, which may well be dead, but if not, could respond to an intentional signal. And/or it may not, depending on unknown alien motivations (Benford, 2019). Lurkers include self-replicating probes, based on von Neumann’s theory of self-replicating machines, which is why they are often called von Neumann probes (Von Neumann, 1966). Recently concepts have appeared for self-replicating probes that could be built in the near future (Borgue & Hein, 2020).

Another pioneering work on this concept was of course famously developed in “2001 A Space Odyssey” (Clarke, 1968). A ‘solarcentric’ Search for Extraterrestrial Artifacts advocated by Robert A. Freitas, who coined the term SETA for it in the 1980s (Freitas & Valdes, 1985). There are also the papers from the mid-1990s by Arkhipov (Arkhipov , 1995, 1998a, 1998b). Scot Stride has shown that autonomous instrument platforms (i.e. robotic observatories) to search for anomalous energy signatures can be designed and assembled using commercial off-the-shelf hardware and software. That provides an economical, flexible and robust path toward collecting reliable data (Stride 2001a and 2001b). Further analysis has appeared recently (Haqq-Misra & Kopparapu, 2012, Lingam & Loeb, 2018, Cirkovic et al. 2019, Shostak, 2020).

Near-Earth objects could provide an ideal way to watch our world from a secure natural object (Benford, 2019). They are attractive locations for extraterrestrial intelligence to locate a platform to observe Earth while not being easily seen.

2. Drake Equations

2.1 The Standard Drake Equation estimates the number of radiating civilizations that are detectable, NC , as the product of the rate of creation of such radiating civilizations (Drake, 1965),

This modified Drake Equation is:

I replace the usual Drake Equation symbol for time over which they radiate L, with T_R. And I also multiply by:

f_R = fraction that actually do radiate signals that might be observable at Earth. That is, they radiate with the intention of trying to communicate. Leakage radiation is unintentional, but comes in two types: radar, which has no message, and broadcasts, which come from many incoherent sources which cancel out, such as TV.

These parameters are listed in Table 1:

Table 1: Drake Equation Parameters. Subscripts are italicized letters in definitions

2.2 A Drake Equation for Alien Artifacts An equivalent to the Drake equation for the number of Lurkers in our solar system, N_L, can similarly be expressed as the rate of creation of radiating civilizations, times the fraction that also develop interstellar probe technology f_ip, times the sojourn that Lurkers would be in the solar system, T_L:

f_ip = fraction that also develop interstellar probe technology and launch them

T_L = time that Lurkers could reside in the solar system

(Note that for such civilizations, f_C =1; a civilization with the capability to build such probes surely can build interstellar transmitters.)

Then a Drake equation for alien artifacts is

The new parameters are listed in Table 2:

Table 2 Drake Alien Artifact Equation Parameters

In the ratio of equations 1 and 2, of the number of Lurkers in our solar system to the number of radiating civilizations, most terms, in the first bracket, cancel so:

This initial result is that the ratio of civilizations sending probes that are now resident in our solar system to the number sending messages is the product of two ratios: A ratio of motives:

the fraction that also develop interstellar probe technology and launch them, divided by fraction that only radiate, so f_ip/f_R < 1,

and a ratio of times:

the time Lurkers are present in the solar system/ the time ET civilizations release electromagnetic signals. Surely a civilization with the capability to build such probes can build interstellar transmitters, so I will argue that T_L/T_R > 1.

Our own civilization has been capable of radiating for about 50 years, including message-free Cold War radar transmissions and inadvertent leakage radiation has been emitted for a long time (Quast, 2018). Intentional messages have also been sent but are difficult to detect with Earth–scale receiver systems (Billingham and Benford, 2014). We cannot yet build interstellar probes capable of traveling to and decelerating into a star system and conducting operations there. But that may be possible in the next century. If so, relatively soon we will be capable of both radiating to the stars and sending probes to explore nearby star systems.

However, equation 4 does not take account of the space volumes that the two groups operate in.

2.3 Space Volume Factor

Another factor must be included: Equation 4 must be modified for V_L, the volume over which Lurkers can travel, and its corresponding range R_L vs. V_B, the volume over which Beacons can transmit and be plausibly detected, and its corresponding range R_B. Lurker probes traveling at a small fraction of the speed of light should be compared to the transmissions from an interstellar Beacon propagating at the speed of light. That means that the volumes from which signals can be detected from Beacons is much larger than the volume over which Lurker could travel.

For example, assume that interstellar probes could operate at ~10% c, the speed of light, as contemporary concepts of fusion rockets are designed for. An example: for the Icarus Firefly magnetically confined Z-pinch concept at 4.7% c, traveling 10 ly would take about two centuries (Freeland & Lamontagne, 2015). Starshot, which is a flyby probe concept, at 0.2 c takes more than 20 years to arrive at the Centauri system. Assuming the attention span of the civilization is measured in centuries, a rough estimate of the distance over which probes will be launched is tens of lightyears. (The signal from the probe reporting back to its origin would travel at the speed of light, of course.) If it is possible for probes to move close to c, then the beacon volume to probe volume would be close to unity.

In contrast, the electromagnetic waves of an interstellar Beacon, be it light, millimeter-wave or microwave, propagate ~20 times faster, at the speed of light. For example, we can estimate the range over which a Beacon would be used to be hundreds of light years. By that I again mean that the attention span of a civilization might be measured in centuries.

I define the volumes and ranges in Table 3:

Table 3 Space Volume Factor Parameters

Therefore equation 3 must be multiplied by the ratio of these 2 volumes, V_L/V_B:

As volume scales as the cube of the distance to them, R_L/R_B:

This is a ‘Success Ratio’ of searching for artifacts compared to listening to stars. It allows us to quantitatively evaluate their relative merits. Although the volume ratio would argue that long-range Beacons will be much more likely to be detected than probes that come to observe Earth, the time ratio tends to mitigate that advantage.

2.4 Decision Tree Parameters

The ratio of the number of lurkers to the number of radiating civilizations can be estimated using the three factors in equation 5, which have the following’s sizes:

So the ‘Success Ratio’, Eq. 5, will depend on choices for these parameters.

The key parameters making up these factors can be divided into objective and subjective components, where ‘objective’ means it can be quantified or at least estimated and ‘subjective’ means it’s a matter of opinion. Here is a Table of the parameters:

Table 4: Objective and subjective SETA Parameters and determining factors

The issues determining the objective parameters are listed; subjective parameters are a matter of taste and underlying assumptions.

By making choices among the objective and subjective parameters, one constructs a decision tree: A set of parameter choices leads to a conclusion about the success ratio for SETA and SETI strategies, as embodied in equation 6. Because ET civilizations will vary enormously in motivations, we can expect a variety of outcomes for the Success Ratio.

2.41 Estimates of T_R, time that ETI Beacons radiate

In the literature, estimates of T_R fall between a hundred and 100 million years, a very wide range. Michael Shermer estimated T_R by averaging the lifespans of 60 Earth civilizations, getting 420 years, (Shermer, 2002). Using 28 civilizations since the Roman Empire, he gives ~300 years for “modern” civilizations. But Shermer’s number for the lifetime of societies is not relevant if new societies arise to replace old ones.  In that case, one should take the summation of existence times for all the technological cultures on a planet. Note that the longest operating institution still existing on Earth is the Catholic Church, ~2,000 years. We’ll take the times to be 300-10,000 years, an order of magnitude range.

2.42 Estimates of T_L, time Lurkers could reside in the solar system

A key point is that Lurkers will still be discoverable even though dead for a long time. That’s not true of an EM transmission, which is simply passing through at the speed of light. That fact weighs to the advantage of the Lurker search strategy.

The time that Lurkers would be in the solar system, T_L, will be limited by the lifetime of the orbits they are in, which provides an upper bound. The Moon, Earth Trojans and co-orbitals of Earth lifetimes are:

The Moon

Our Moon is thought to have formed about 4.5 billion years ago. For T_L we use the time that life became evident in our atmosphere, 0.65 10⁹ < t₁ < 2.5 10⁹ years.

Earth Trojans

There may be many objects in the Earth Trojan region (Malhotra, 2019). Their lifetime in Trojan orbits is likely to be on the order of billions of years, and some objects there may be primordial, meaning that they are as old as the Solar System, because of their very stable orbits about the Lagrange Points (?uk et al., 2012, Dvorak et al., 2012, Marzari & Scholl, 2013, Zhou et al. 2019). Orbital calculations show that the most stable orbits reside at inclinations < 0° to the ecliptic; there they may survive the age of the solar system, ~2.5 Gyr.

Earth Co-orbitals

Morais and Morbidelli, estimate lifetimes to run between 1 thousand and 1 million years (Morais and Morbidelli, 2002). With a mean lifetime of 0.33 million years. Morbidelli says that no further studies have been done on their approach (A. Morbidelli, personal communication).

3. Scenarios for Success Ratio Estimates

Here we show several scenarios, some of which show that the two strategies, SETA and SETI, are competitive.

Scenario 1: Choosing via relative costs at equal ranges:

Assume that:

1) The ratio of fractions of ET civilizations would be proportional to the cost of interstellar probes vs. Beacons. The cost of interstellar probes will be substantially more than the cost of interstellar Beacons. Stated differently, Beacons will have substantially longer range for a fixed cost.

2) R_L and R_C are equal.

If we take as an example a Beacon at 100 ly and a Lurker probe launched from 100 ly, then R_L and R_C in Eq. 5 cancel out. For Beacons that have a range of 100 ly the cost is of order $1 billion. This is from extrapolations, based on current cost scaling and costs (Benford, 2010, Billingham and Benford, 2014). The Firefly interstellar fusion rocket has an estimated cost of $60 billion. Two thirds of that cost is fuel to accelerate and decelerate (A. Lamontagne, personal communication). Therefore the cost ratio is ~100 in favor of Beacons. If cost is the deciding factor, then f_P/f_R = 1/100 and Eq. 5 reduces to

Next, one chooses an orbital location for the Lurker: Our Moon is thought to have formed about 4.5 billion years ago, long before life appeared. So we use the time life became evident in our atmosphere, 0.65 10⁹ < T_L < 2.5 10⁹ years.

Next, one guesses the transmit time of the Beacon: estimates of civilization radiating times T_C vary from ~300 -10⁵ years. Here the ‘dash’ means the range of credible values:

So for these parameter choices, a Lurker search is much more likely to be successful. Note, however, that if we assume the Beacon civilization is at 100 ly, and the probe-building civilization is at 10 ly, a factor of 1/1,000 reduces the ratio to 0.1 to 100.

Scenario 2: What if cost doesn’t matter? That would be at variance with all we know of economics on Earth, but is a hypothetical we could consider. If cost doesn’t matter, then a civilization wanting to investigate the life of Earth or whether civilization was here could build probes to investigate the ecosystem, visible in spectra of our atmosphere, and also build Beacons to broadcast to us. In such a case, f_P/f_R = 1, and, as we’re talking about a single civilization, R_L/R_C = 1. Consequently the Success Ratio N_L/N_C = T_L/T_C, which would surely be >>1. Again, lurker strategy is likely to be more successful. In this scenario, the time ratio is the important factor.

Scenario 3: Early spacefaring civilizations: A civilization such as ours, which is presently capable of only interplanetary speeds, cannot build interstellar probes as envisioned by some of our starship concepts. Starships are centuries into our future and will always be more expensive than Beacons. They could be only a radiating society and might build Beacons. In this case the success ratio N_L/N_C = 0, and a listen-only strategy is appropriate.

Scenario 4: Supercivilizations capable of fast interstellar flight: The opposite extreme from scenario 3 is a civilization where starships can travel at a large fraction of the speed of light. In this case, Beacons, although still cheaper, would serve to reveal our civilization only if we respond by sending a message back to them. At about the same time their probes would be arriving and could be reporting the existence of our civilization. This could’ve occurred over geological time frames, so in this case N_L/N_C >>1, and we would expect to find dead Lurkers on the nearby objects described in 2.42.1, and we would expect to find dead Lurkers on the nearby objects described in 2.42.

Scenario 5: Lurkers in Co-orbitals and short radiating time: Instead of a Trojan or the Moon, we choose one of the co-orbitals, which have a mean lifetime T_L ~0.33 million years. 1) For T_R , choose the 300-year lifetime estimate of Shermer for the Beacon to radiate. Then T_L/T_R = 1,000. 2) Let’s assume that starship probes are launched from a civilization 10 ly away. (A probe such as Firefly, traveling at 0.2c and decelerating into our solar system, would take 50 years to come 10 ly.) 2) Assume the Beacon civilization is at 100 ly, and the probe-building civilization is at 10 ly. So R_L/R_B = 0.1. 3) Further, again assume that the willingness of civilization to undertake the expense would be determined by economics. A continuous Beacon at hundred light-years would cost about $1 billion and a Firefly probe is estimated to cost $60 billion (M. Lamontagne, personal communication), so f_P/f_R = 0.01. Therefore the Success Ratio, eq. 5, is:

For this case listening-to-stars has a higher success ratio. But if one assumes that the radiating civilization also develops interstellar probes, f_R~f_p, the two strategies have a roughly equal success ratio:

So one’s assumptions of the parameters in the Table determine the answer.

Scenario 6: Lurkers in Co-orbitals and long radiating time: If we use the band of estimates in the literature for co-orbital lifetime, ~10⁵ years, and estimates of civilization radiating times T_C vary from 10² – 10⁵, then T_L/T_R varies from 1 to 1,000. For the previous 100 ly/10 ly distance ratio, Eq. 5 then gives a Drake Equation ratio of

And the listening strategy will be preferred.

It is clear from these scenarios that 1) the two strategies, SETA and SETI, are competitive, 2) the Moon and the Earth Trojans have a greater probability of success than the co-orbitals.

5. Research for Finding Alien Artifacts

I advocate a sequence of tasks:

• We have had the Lunar Reconnaissance Orbiter in low orbit around the Moon since 2009. It has taken about 2 million images at high sub-meter resolution (M. Revine, personal communication). We can see where Neil Armstrong walked! The vast majority of the photos have not been inspected by the human eye. Searching these millions of photographs for alien artifacts would require an automatic processing system. Development of such an AI is a low-cost initial activity for finding alien artifacts on the Moon, as well as Earth Trojans or the co-orbitals (Davies & Wagner, 2011, Lesinkowski et al., 2020). Note the recent AI analysis of 2 million images from LRO which revealed rockfalls over many regions of the Moon (Bickel et al., 2020).
• Conduct passive SETI observations of these nearer-Earth objects in the microwave, infrared and optical.
• Use active planetary radar to investigate the properties of these objects
• Conduct active simultaneous planetary radar ‘painting’ and SETI listening of these objects.
• Launch robotic probes to conduct inspections, take samples of Earth Trojans and the co-orbitals. The low delta-V, 3-5 km/sec, make this an attractive early option, is well within present capability (Stacey & Conners, 2009, Venigalla et al., 2019). China plans a mission to co-orbital 2016 HR 3 in the middle of this decade (Zxiaojing, et al., 2019).

6. Conclusion

Clearly looking for alien artifacts in the region of the solar system near Earth is a credible alternative approach, a strategy of ETI archeology. The formulation given here is a way of discussing the SETA strategy and comparing it to SETI.

The listening-to-stars strategy that SETI researchers have been following for over 50 years, is now being pursued very vigorously by Breakthrough Listen. What has SETI learned so far about life in the universe? Only that there is no intelligent life broadcasting signals toward Earth at the time we’ve listened, within the sensitivity levels, duty cycles and frequencies we have observed. If the ongoing SETI listening program continues to not hear a signal, the case for looking for Lurkers will grow ever stronger.

The SETA strategy was not pursued after it was suggested in the 1980’s, because listening to stars is easier and observing technologies and spacecraft were not sufficiently developed to pursue it. But now SETA is more attractive:

• Close inspection of bodies in these regions can now be done with 21st Century observatories and spacecraft.
• The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.
• The Moon and the Earth Trojans have a greater probability of success than the co-orbitals.
• There are differences in detection in the two strategies: in the artifact case we should listen to those objects and image them in the optical or radar from Earth or send probes to visit them. In SETI, we can only listen.
• SETA is a concept that can be falsified, a fundamental requirement for a science. SETA can be falsified or verified in practice by precisely specifying what one is looking for. For example, the statement “No artificial objects larger than 1 m exist on the surface of the Earth Trojan” can be verified by observing that object at that resolution. Smaller objects wouldn’t be resolved. If we conduct the efforts described in Section 5, and don’t find artifacts, the SETA concept is disproven for the near-Earth region, where it is most credible. If we find them, it’s verified.

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Jim Benford is continuing his research into the still nascent field known as SETA, the Search for Extraterrestrial Artifacts. A plasma physicist and CEO of Microwave Sciences, as well as a frequent Centauri Dreams contributor, Benford became intrigued with recent discoveries about Earth co-orbital objects — there is even a known Earth Trojan — and their possibilities in a SETI context. If we accept the possibility that an extraterrestrial civilization may at some point in Earth’s 4.5 billion year history have visited the Solar System, where might we find evidence of it? Two papers grew out of this, one in Astrobiology, the other in the Journal of the British Interplanetary Society (citations below). In the first of two posts here, Jim explains where his work has led him and goes through the thinking behind these recent contributions.

by James Benford

Part 1: How Many Alien Probes Could Have Come From Stars Passing By Earth?

1. Searching for Extraterrestrial Artifacts

Alien astronomy at our present technical level may have detected our biosphere many millions of years ago. The Great Oxidation Event occurred around 2.4 billion years ago; it was a rise in oxygen as a waste product due to organisms in the ocean carrying out photosynthesis. Long-lived robotic probes could have been sent to observe Earth long ago. I will call such a probe a “Lurker,” a hidden, unknown and unnoticed observing probe, likely robotic. They could be sent here by civilizations on planets as their stars pass nearby.

Long-lived alien societies may do this to gather science for the larger communicating societies in our Galaxy. The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.

Here, in part 1, I estimate how many such probes could have come here. This is explained in detail in [1].

In Part 2, titled ‘A Drake Equation for Alien Artifacts’, I propose a version of the Drake Equation to include searching for alien artifacts that may be located on Moon, Earth Trojans and co-orbital objects [1]. I compare a Search for Extraterrestrial Artifacts (SETA) strategy of exploring near Earth for artifacts to the conventional listening-to-stars SETI strategy.

1.1 Observing Earth

From Figure 1, the time over which our biosphere has been observable from great distances, perhaps thousands of light years, due to oxygen in the atmosphere, is a very long time, measured in the billions of years [7,8]. The first oxidation event occurred about 2 .5 billion years ago and the second, largest oxidation event about 0.65 billion years ago, so 0.65 10⁹ < T_L <2.5 10⁹ years.

An ET civilization that passes nearby can see there’s an ecosystem here, due to the out-of-equilibrium atmosphere. They could send interstellar probes to investigate.

Figure 1. History of Oxygen content of Earth’s atmosphere is observable from great distances. Dashed line is present value. Horizontal axis is in millions of years before present. (Wikipedia Commons)

2. How Often Do Stars Pass By Our Sun?

It is not widely known that stars pass close to our solar system. The most recent encounter was Scholz’s Star, which came 0.82 light-years from the Sun about 70,000 years ago [3]. A star is expected to pass through the Oort Cloud every 100,000 years or so, as Scholz’s Star did, shown in Figure 2.

Bailer-Jones et al. showed that the number of stars passing within a given distance R, N_S (R), scales as the square of that distance [4]. This comes about because Earth is in a flow of stars circling the galactic center, so the cross-sectional area is what matters, which gives an R² scaling, rather than the volume, ~ R³. Figure 3 shows that several stars have approached or will approach our solar system over 10⁵ years.

Figure 2. Our most recent visitor: Scholz’s Star came within 0.82 light-years from the Sun about 70,000 years ago (NASA).

Bailer-Jones et al., using accurate 3D spatial and 3D velocity data for millions of stars from the Second Gaia Data Release has shown that a new passing star comes within one light year of our Sun every half million years, 100 within 10 light years [4].

With the number of stars passing within a given distance, N_S (R), and R the distance of the star from the Sun in light years, the rate of passing stars is:

So a new star comes within 10 ly every 5,000 years [3]: during our 10,000-year agricultural civilization, two new stars have come within 10 ly.

Figure 3. Stars come very close to Earth frequently. About 2 stars come within a light year every million years. An ET civilization that passes nearby can see there’s an ecosystem here, due to the out-of-equilibrium atmosphere. They could send interstellar probes to investigate. (stackexchange.com)

3. How Many Lurkers May Have Come Here?

To calculate the number of Lurkers that could be located at various sites nearby to Earth, such as the Moon, Earth Trojan zone or the co-orbitals, I make the following estimates. The quantities to use in calculating this concept are shown in Table 1.

There are two factors to evaluate: 1) How often do stars get within a given range of Earth? 2) How long would a Lurker reside in a given location near Earth?

Of course, a key factor we do not know is what fraction of the stars have spacefaring civilizations.

Table 1 Passing Stars Parameters

The number of Lurkers that could arrive and now be found, N_L, would be f_ip times T_L, the orbital lifetime of the object upon which the Lurker is resident, times the passing star rate, [dN_S(R)/dt] from Eq. 1:

We don’t know f_ip, but we can calculate the ratio

Now we make estimates of N_L/f_ip. Details of these estimates below can be found in [1].

4.0 Locations for Lurkers Near Earth

The time that Lurkers would be in the solar system, T_L, will be limited by the lifetime of the orbits they are in. That is determined by the stability of the orbit of the near-Earth object it lands on. This provides an upper bound to how long they could be around. The Moon, Earth Trojans and co-orbitals of Earth lifetimes are:

4.1 The Moon

Searching on the Moon has recently been advocated [5, 6]. Our Moon is thought to have formed about 4.5 billion years ago, long before life appeared. Then the Earth ecosystem would not attract attention. Later, life became evident in our atmosphere.

We have had the Lunar Reconnaissance Orbiter in low orbit around the Moon since 2009. It has photographed about 2 million sites at sub-meter resolutions. We can see where Neil Armstrong walked! The vast majority of these photos have not been inspected by the human eye. Davies and Wagner have proposed searching these millions of photographs for alien artifacts, which would require an AI for initial surveys [5]. Development of such an AI is a low-cost initial activity for finding alien artifacts on the Moon, as well as Earth Trojans and the Earth co-orbitals. A recent AI analysis of 2 million images from LRO revealed rockfalls over many regions of the Moon [9]. So we have proof a search for artifacts of ~1-meter scale could be done by AI.

Figure 4 The Apollo 17 site as seen by the Lunar Reconnaissance Orbiter. Note that Moonbuggy tracks can be clearly seen. A study of the >2 million such photos could detect possible artifacts on the Moon (NASA).

4.2 Earth Trojans

Figure 5 shows the many Jupiter Trojans, located at stable Lagrange Points near that planet. There may be many such objects in the Earth Trojan region [11], ~60 degrees ahead of and following Earth. Their lifetime is likely to be on the order of billions of years, and some objects there may be primordial, meaning that they are as old as the Solar System, because of their very stable Lagrange Point orbits [11-14].

Figure 6 shows a portion of the orbit of the only Earth Trojan found so far, 2010 TK7. It oscillates about the Sun–Earth L₄ Lagrange Point, ~60 degrees ahead of Earth [15]. Its closest approach to Earth is about 70 times the Earth-Moon distance. It is not a primordial Earth Trojan and is estimated to have an orbital lifetime of 250,000 years, when it will go into a horseshoe orbit about the sun. It is clear why there are no other Trojans of the Earth yet found: they are hard to observe from Earth.

There are large stable regions at Lagrange Points, so Trojans may exist for long time scales. It is possible that primordial Earth Trojans exist in the very stable regions around the Lagrange Points. Orbital calculations show that the most stable orbits reside at inclinations <10° to the ecliptic; there they may survive the age of the solar system, so again we use the oxygen time, ~2.5 Gyr. So Trojans’ orbital lifetimes can vary from 2 10⁵ years to 2.5 10⁹ years.

Figure 5. The many Jupiter Trojans, which lead and follow the planet at ~ 60°. (Wikipedia Commons)

Figure 6. Portion of the orbit of the one Earth Trojan found so far, 2010 TK7. (NASA)

4.3 Earth Co-orbitals

See [16] for a discussion of the co-orbitals of Earth. A large number of tadpole, horseshoe and quasi-satellites that approach near to Earth appear to be long-term stable. Figure 7 shows to orbit of the nearest one, 2016 HO3. Morais and Morbidelli, using models of main asteroid belt sources providing the co-orbitals and their subsequent motions, estimate lifetimes to run between 1 thousand and 1 million years. They conclude that the mean lifetime for them to maintain resonance with Earth is 0.33 million years (17).

Figure 7. Orbits around the Sun of Earth and the nearby quasi-satellite 2016 HO3. It comes within 5 million km of Earth (NASA).

5. Conclusions

In [1] the above remarks are quantified. Here I summarize the calculations in the Table, for probes traveling from 10 ly and 100 ly. (Note that, since co-orbitals have a finite lifetime on their orbits near Earth, Table 2 refers to this is the number of probes that may have landed on what was at the time a co-orbital but will now have wandered off somewhere.)

Table 2: N_L/f_ip: The number of Lurkers, from stars that pass by our Solar System that could have arrived and now could be found, for several nearby astronomical bodies, divided by f_ip, the fraction of stars that have civilizations that develop interstellar probe technology and launch them.

• Clearly, the Moon and the Earth Trojans have a greater probability of success than the co-orbitals.
• Of course, f_ip is the factor we don’t know: how many civilizations develop interstellar probe technology and launch them.
• The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.
• Close inspection of bodies in these regions, which may hold primordial remnants of our early solar system, yields concrete astronomical research. It will yield new astronomy and astrophysics, quite apart from finding Lurkers.
• A suggestion for SETI observers: Look at the specific stars that have passed our way in the last 10 million years and ask how many of them are ‘sunlike’ and/or are known to have habitable planets. Observe those stars closely for possible emissions to Earth [16].

For discussion of approaches to study these objects, starting with passive observations, and going on to missions to them, see Reference 14, section 4, “SETI Searches of Co-orbitals”. The actions and observations are:

1. Launch robotic probes and manned missions to conduct inspections, take samples.

2. Conduct passive SETI observations.

3. Use active planetary radar to investigate the properties of these objects

4. Conduct active simultaneous planetary radar ‘painting’ and SETI listening of these objects.

5. Launch robotic probes and manned missions to conduct inspections, take samples.

This argues for a Search for Extraterrestrial Artifacts (SETA) strategy of exploring near Earth for alien artifacts [2].

References

1. J. Benford, “How Many Alien Probes Could Have Come From Stars Passing By Earth?”, JBIS 74 76-80, 2021.

2. J. Benford, “A Drake Equation for Alien Artifacts“, Astrobiology 21, 2021.

3. E. Mamajek et al, “The Closest Known Flyby Of A Star To The Solar System” ApJ Lett., 8003 L17, 2015.

4. C. A. L. Bailer-Jones et al, “New Stellar Encounters Discovered in the Second Gaia Data Release”, Astronomy & Astrophysics 616 A37, 2018.

5. P.C.W. Davies, R.V. Wagner, “Searching for Alien Artifacts on the Moon”, Acta Astronautica, doi:10.1016/j.actaastro.2011.10.022, 2011.

6. A. Lesnikowski, L. Bickel and D. Angerhausen, “Unsupervised Distribution Learning for Lunar Surface Anomaly Detection”, arXiv:2001.04634. 2020.

7. X. L. Kaltenegger, Z. Lin and J. Madden, ““High-resolution Transmission Spectra of Earth Through Geological Time”, Astroph. Lett., 2041, 2020.

8. Y. V. S. Meadows et al., “Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment“, Astrobiology 18, 620, 2018.

9. V. Bickel V. et al., 2020 Impacts drive lunar rockfalls over billions of years, Nature Communications, 11:2862 | https://doi.org/10.1038/s41467-020-16653-3

10. R. Malhotra, “Case for a Deep Search for Earth’s Trojan Asteroids”, Nature Astronomy 3, 193, 2019.

11. M, ?uk, D. Hamilton and M. Holman, “Long-term stability of horseshoe orbits”, Monthly Notices Royal Astronomical Society, 426, 3051, 2012.

12. F. Marzari, H. Scholl, “Long term stability of Earth Trojans”, Celestial Mechanics and Dynamical Astronomy, 117, 91, 2013.

13. Zhou, Lei; Xu, Yang-Bo; Zhou, Li-Yong; Dvorak, Rudolf; Li, Jian, “Orbital Stability of Earth Trojans”, Astronomy & Astrophysics, 622, 14, 2019.

14. R. Dvorak, C. Lhotka, L. Zhou, “The orbit of 2010 TK7. Possible regions of stability for other Earth Trojan asteroids”, Astronomy & Astrophysics, 541, 2012.

15. P. Wiegert, K. A. Innanen and S. Mikkola, “An Asteroidal Companion to the Earth”, Nature, 387, 685, 1997.

16. J. Benford, “Looking for Lurkers: Objects Co-orbital with Earth as SETI Observables”, AsJ, 158:150, 2019.

17. M. Morais and A. Morbidelli, ‘The Population-of Near-Earth Asteroids in Co-orbital Motion with the Earth”, Icarus 160, 1, 2002.

Just how useful is oxygen as a biosignature? It’s a question we’ve examined before, always with the cautionary note that there are non-biological mechanisms for producing oxygen which could make any detected biosignature ambiguous. But let’s go deeper into this, thanks to a new paper on ‘oxygen false positives’ out of the University of California at Santa Cruz. The paper, produced by lead author Joshua Krissansen-Totton and team, offers scenarios that can place an oxygen detection in the broader context that would distinguish any such find as biological.

Let’s begin with the fact that in addition to its obvious interest because of Earth’s history, photosynthesis involving oxygen requires the likely ubiquitous carbon dioxide and water we would expect on habitable zone planets. Helpfully, oxygen should be readily detectable on exoplanets because of its absorption features, which are prominent not only in visible light but in the near infrared and thermal infrared, if we include ozone. Space-based missions as well as ground-based Extremely Large Telescopes should be able to find oxygen signatures.

I found the authors’ discussion of M-dwarfs fascinating. We have to weigh our strategies with these small stars in mind because they are the ones for which atmospheric spectroscopy will first become available for habitable zone rocky planets. Already we’re in deep water, because the oxygen we might find on such worlds could have complex origins. From the paper:

…several features of M?dwarfs make them susceptible to non?biological oxygen accumulation. In particular, the extended pre?main sequence of late M?dwarfs could yield habitable zone terrestrial planets with hundreds or thousands of bar O₂ from XUV?driven hydrogen loss (Luger & Barnes, 2015). At least some of this oxygen will likely dissolve in a surface magma ocean and be sequestered in the mantle, but retaining oxygen?rich atmospheres is still possible, especially for highly irradiated terrestrial planets…

Not only does this throw a spanner into the works for biosignature detection, but it could act as an active deterrent to the emergence of life by making prebiotic chemistry impossible. All this is under active study, as the paper’s numerous citations make clear, with the authors adding that “photochemical runaways yielding O₂?CO rich atmospheres remain a strong possibility for late M dwarfs.” These older red dwarfs tend to be the ones of higher astrobiological interest given that younger stars in this category are given to higher amounts of flare activity.

All of this points to the problems of oxygen as a biosignature and the need to examine how non-biological oxygen can accumulate on the planets we’re interested in, and this extends to planets around F-, G- and K-class stars as well, although the problem here seems highly dependent on the initial inventory of volatile elements, as the authors make clear. It’s also clear we have a great deal to learn about oxygen production via non-biological methods like hydrogen escape and water photodissociation, all reviewed in this crisp and clearly written paper. The interplay between atmosphere and geochemistry is the study’s central point:

The robustness of oxygen biosignatures rests on the assumption that for temperate planets with effective cold traps, small abiotic oxygen source fluxes from H escape will be overwhelmed by geological sinks. To test this assumption, it is necessary to model the redox [oxidation-reduction] evolution of terrestrial planet from formation onwards. This is because planetary redox evolution depends on both the initial state of the atmosphere and mantle after the magma ocean has solidified, and on the subsequent internal evolution and atmospheric state. Interior evolution dictates crustal production rates and outgassing fluxes, which determine the efficiency of geologic sinks of oxygen.

The authors use a model of planetary development that includes a wide range of initial volatile elements in varying abundance, taking rocky worlds all the way from their original formation up through eras of geochemical cycling lasting billions of years. The goal is to produce scenarios in which a lifeless planet around various stellar types could evolve with atmospheric oxygen. Context is all, meaning we have to know what other molecules beyond oxygen are available, and the range of outcomes is wide indeed. For a given scenario, distinguishing between false positives and genuine biosignatures is the key, and the paper explores the various options.

Image: By varying the initial inventory of volatile elements in a model of the geochemical evolution of rocky planets, researchers obtained a wide range of outcomes, including several scenarios in which a lifeless rocky planet around a sun-like star could evolve to have oxygen in its atmosphere. Credit: J. Krissansen-Totton).

The photodissociation referred to above occurs as ultraviolet light from the star breaks water molecules into hydrogen and oxygen in the upper atmosphere, with the lighter hydrogen escaping into space and the oxygen remaining as a potentially deceptive biosignature. But the paper also examines how oxygen can be removed from an atmosphere, through outgassing of carbon dioxide and hydrogen, which will react with oxygen. The weathering of rock also affects oxygen levels, all factors that need to be included in this model of geochemical evolution.

The model is given weight when we see that it can reproduce the evolution of the atmosphere both on the Earth and on Venus. Using it, then, we can explore possibilities. We can imagine a planet with more water than Earth, one whose deep oceans preclude weathering of rock that would remove oxygen. Conversely, on still molten young worlds with only a small inventory of water, the magma surface can solidify quickly, with water remaining in the atmosphere. Oxygen remains behind as hydrogen in the upper atmosphere escapes. Says Krissansen-Totton:

“The typical sequence is that the magma surface solidifies simultaneously with water condensing out into oceans on the surface. On Earth, once water condensed on the surface, escape rates were low. But if you retain a steam atmosphere after the molten surface has solidified, there’s a window of about a million years when oxygen can build up because there are high water concentrations in the upper atmosphere and no molten surface to consume the oxygen produced by hydrogen escape.”

Another scenario involves high amounts of carbon dioxide in relation to water, resulting in a runaway greenhouse. Here again, as the paper notes, we’ve got an oxygen problem:

The lack of liquid surface water precludes CO₂?drawdown via silicate weathering… Reactions between supercritical water and silicates will be severely kinetically limited by sluggish solid state diffusion, and are therefore assumed to be negligible (Zolotov et al., 1997). Consequently, a dense CO₂ atmosphere and supercritical surface temperature persist indefinitely… despite the planet residing in the habitable zone. Moreover, there is sufficient steam in the atmosphere to ensure diffusion?limited hydrogen escape provides an appreciable source flux of oxygen…

Its focus on the geochemical and thermal evolution of a planet in the habitable zone, emphasizing interactions between crust and atmosphere, make this a noteworthy addition to the ongoing attempt to understand biosignatures. We may well get a biosignature detection involving oxygen relatively quickly once we have the tools in place to delve into rocky worlds in the habitable zone. The effort to sort out its meaning will take considerable time.

I’l stand by a previous prediction: Initial euphoria will quickly wear off as we consider how deeply ambiguous any biosignature detection is going to be. I think we’ll be seeing plenty of interesting hints, but it will be many years before we can say with certainty that we have found life around another star.

The paper is Krissansen-Totton et al., “Oxygen False Positives on Habitable Zone Planets Around Sun?Like Stars,” Vol. 2, Issue 2 (June 2021). Full text.