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 .
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 . 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 < TL <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 . 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, NS (R), scales as the square of that distance . 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 .
With the number of stars passing within a given distance, NS (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 : 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, NL, would be fip times TL, the orbital lifetime of the object upon which the Lurker is resident, times the passing star rate, [dNS(R)/dt] from Eq. 1:
We don’t know fip, but we can calculate the ratio
Now we make estimates of NL/fip. Details of these estimates below can be found in .
4.0 Locations for Lurkers Near Earth
The time that Lurkers would be in the solar system, TL, 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 . 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 . 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 , ~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 . 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  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).
In  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: NL/fip: 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 fip, 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, fip 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 .
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 .
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, Ćuk, 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.