TOLIMAN: Looking for Earth Mass Planets at Alpha Centauri

Why the renewed focus on astrometry when it comes to Alpha Centauri (a theme we saw as well in the previous post on ALMA observations from the surface)? One problem we face with other detection methods is simply statistical: We can study planets, as via the Kepler mission, by their transits, but if we want to know about specific stars that are near us, we can’t assume a lucky alignment.

Radial velocity requires no transits, but has yet to be pushed to the level of detecting Earth-mass planets at habitable-zone distances from stars like our own. This is why imaging is now very much in the mix, as is astrometry, and getting the latter into space in a dedicated mission has occupied a team at the University of Sydney led by Peter Tuthill for a number of years — I remember hearing Tuthill describe the technology at Breakthrough Discuss in 2016.

Out of this effort we get a concept called TOLIMAN, a space telescope that draws its title from Alpha Centauri B, whose medieval name in Arabic, so I’m told, was al-Zulm?n. [Addendum: This is mistaken, as reader Joy Sutton notes in the comments. It wouldn’t be until well after the medieval period — in 1689 — that the binary nature of Centauri A and B was discovered by Jesuit missionary and astronomer Jean Richaud. The name al-Zulm?n seems to be associated with an asterism that included Alpha Centauri, though I haven’t been able to track down anything more about it. Will do some further digging.]

The name is now fixed — a list of IAU-approved star names as of January 1st, 2021 shows Alpha Centauri A as Rigel Kentaurus and Centauri B as Toliman, so what goes around comes around. Centauri C’s name, according to the IAU, remains the familiar Proxima Centauri.

Image: This is Figure 5 from an online description of this work. Caption: Habitable zones of Alpha Cen A (left) and B (right) in green, along with dynamical stability boundary (red dashed line), 0.4” and 2.5” inner and outer working angles (IWA and OWA) of a small coronagraphic mission. The inset shows the Solar System to scale. Planetary systems of Alpha Cen A & B are assumed to be in the plane of the binary (the likeliest scenario) and orbits of hypothetical Venus-like, Earth-like, and Mars-like planets are shown. Credit: Tuthill et al.

The TOLIMAN Technology

The TOLIMAN mission is designed to use astrometry by means of a diffractive pupil aperture mask that in effect leverages the distortions of the optical system to produce a ‘ruler’ that can detect the changes in position that flag an Earth-mass planet in the Alpha Centauri system. Tuthill’s colleague Céline Bœhm presented the work at the recent Breakthrough Discuss online.

In TOLIMAN, we actually have an acronym — Telescope for Orbit Locus Interferometric Monitoring of our Astronomical Neighborhood — but at least it’s one that’s applicable to the system under study. According to Boehm, there are certain advantages to working with nearby binaries. Astrometry normally uses field stars (stars in the same field as the object being studied) as references, making larger apertures an essential given the faintness of many of these reference points and their distance from the target star, and requiring a wide field of view.

But a bright binary companion 4 arcseconds away, as we find at Alpha Centauri, resolves the problem. Now a small aperture telescope can come into play because we have no need for field stars. The TOLIMAN concept puts a diffractive ‘pupil’ in front of the optics that spreads the starlight out over many pixels, a diffraction caused by features embedded in the pupil. Here stability is at play: We’re trying to eliminate minute imperfections and mechanical drifts in the optical surfaces that can create instabilities that compromise the underlying signal. The pupil mask prevents the detector from becoming saturated and reduces noise levels in the signal.

The astrometric ‘ruler’ used to measure the star’s position is thus created by the light of the stars themselves. The diffraction pattern cast by the pupil positions a reference grid onto the detector plane, which registers precise stellar locations. As a 2018 paper on the mission puts it: “Drifts in the optical system therefore cause identical displacements of both the object and the ruler being used to measure it, and so the data are immune to a large class of errors that beset other precision astrometric experiments.”

Narrow-angle astrometry, where the reference star is extremely close, makes for greater precision than wide-angle astrometry, and angular deviations on the sky are larger. Boehm makes the case that TOLIMAN’s diffractive pupil technology should be effective at finding Earth analogs around Centauri A and B, with a more advanced mission (TOLIMAN+) capable of finding rocky worlds around secondary targets 61 Cygni and 70 Ophiuchi.

Image: This is from Figure 3 of the online description of TOLIMAN referenced above. Caption: Left: pupil plane for TOLIMAN diffractive-aperture telescope. Light is only collected in the 10 elliptical patches (the remainder of the pupil is opaque in this conceptual illustration, although our flight design will employ phase steps which do not waste starlight). Middle: The simulated image observing a point-source star with this pupil. The region surrounding the star can be seen to be filled with a complex pattern of interference fringes, comprising our diffractive astrometric grid. Right: A simulated image of the Alpha Cen binary star as observed by TOLIMAN. Credit: Tuthill et al.

Astrometry and Earth-like Planets

Unlike both transit and radial velocity methods, the signal generated by a potential planet increases with the separation of planet and star, which makes astrometry an ideal probe for habitable zones. We should also throw into the mix the fact that radial velocity detections vary depending on the spectral type of the star, which is why we have had pronounced radial velocity success at Proxima Centauri and Barnard’s Star, but face a steeper climb in doing the same around G- and K-class stars. Astrometry becomes more sensitive with rising luminosity of the host star even as we wrestle with the tiny distortions that a planet like this would produce.

Both HIPPARCOS and Gaia have shown what can be accomplished by moving astrometry into space, and we saw yesterday that Gaia is likely to produce gas giants in the thousands, but the idea behind TOLIMAN is that astrometry now needs a dedicated mission to get down to rocky planets in temperate orbits. Tuthill and team have pointed to Alpha Centauri as an ideal target, one that would yield signals potentially 2 to 10 times stronger than the next best systems for study.

The researchers believe their work yields the optical innovations needed to acquire astrometric data on habitable-zone rocky planets with a telescope with an aperture of 10 cm. To explore the concept further, a CubeSat mission called TinyTol is scheduled to fly later this year, serving as a pathfinder to demonstrate astrometric detection. The full-scale TOLIMAN mission is funded and under development, with launch planned for a 3 year mission in 2023. TOLIMAN+ is envisioned as a 0.5-meter aperture telescope capable of sub-Earth class detections at Alpha Centauri and Earth-mass planets at both 61 Cygni and 70 Ophiuchi.

The most recent paper I know of on this work is Tuthill et al. “The TOLIMAN space telescope: discovering exoplanets in the solar neighbourhood,” SPIE Proceedings Vol. 11446 (13 December 2020). Abstract.


Closing in on Centauri A and B with Astrometry

When it comes to finding planets around Centauri A and B, the method that most intrigues me is astrometry. At the recent Breakthrough Discuss sessions, Rachel Akeson (Caltech/IPAC) made the case for using the technique with data from the Atacama Large Millimeter Array (ALMA). My interest is piqued by the fact that so few of the more than 4300 known exoplanets have been discovered using astrometry, although astronomers were able in 2002 to characterize the previously known Gliese 876 using the method. Before that, numerous reported detections of planets around other stars, some going back to the 18th Century, have proven to be incorrect.

But we’re entering a new era. ESA’s Gaia mission, launched in 2013, is likely to return a large horde of planets using astrometry as it creates a three-dimensional map of star movement in the Milky Way. Dr. Akeson’s case for using ALMA to make detections on the ground is robust, despite the challenges the method presents. She points out that if we viewed the Solar System from a distance of 10 parsecs, Jupiter’s impact on the movement of our star would be 500 microarcseconds (µas), which works out to 1.4 X 10-7 degrees. Keep that figure in mind.

Astrometry is about measuring the movement of a star’s position on the sky, and it has been put to good use for a long time in identifying binary star systems. As the star’s position changes over time, the gravitational pull of an orbiting planet should likewise be revealed. This is something like the well established radial velocity technique for planet detection, but it operates within the plane of the sky instead of along the line of sight (radial velocity measures the Doppler shift in the star’s light as it is pulled alternately toward and then away from Earth along the line of sight). The beauty of astrometry is that it complements radial velocity by being best suited to planets with a wide separation from their star.

Image: Astrometry is the method that detects the motion of a star by making precise measurements of its position on the sky. This technique can also be used to identify planets around a star by measuring tiny changes in the star’s position as it wobbles around the center of mass of the planetary system. Credit: ESA.

Back to Gaia for a moment. It’s a mission that has given astrometry a dazzling upgrade, offering 10 to 20 µas performance for a large sample of observed stars, making the upcoming release of its exoplanet catalog an event that should vastly enlarge our statistical understanding of planetary systems. Remember that figure from Jupiter as seen from 10 parsecs — the movement of the Sun is 500 µas. Gaia should identify tens of thousands of exoplanets out to 1600 light years from the Sun, all of this by tracking minute changes in the stars’ position. But for the nearest star system, Gaia is far less effective because of Alpha Centauri’s brightness.

Enter ALMA. 5000 meters up on the Chajnantor Plateau in Chile’s Atacama desert, the site offers 66 antennas with baseline lengths of 150 meters to 16 kilometers. At the longest baseline, the resolution is 12 milliarcseconds (one thousandth of an arcsecond, abbreviated mas). We’ve looked often in these pages at ALMA studies of protoplanetary disks (Akeson refers to the famous image of HL Tau, shown below, with the roughly one million year old disk dividing into clearly visible rings and gaps).

With ALMA, the proximity of Centauri A and B becomes an advantage. The relative positions of the two stars can be measured, as opposed to having to compare them to reference stars in other fields, meaning that the precision of the measurement is greatly increased. To be sure, this remains a tough measurement — in 2015 observations, position uncertainties were in the range of 10 milliarcseconds, a figure that needs to be reduced to hundred of microarcseconds.

Can ALMA handle this level of precision? Akeson’s team requested three pairs of observations in 2018-2019, with two achieved and the third incomplete, a dataset that will be complemented with future observations to identify deviations in the orbital motions of these stars. The need for high resolution has meant that only a few of the configurations, using the longest baselines between ALMA antennas, can be used, which puts limits on the observing time available.

Image: ALMA image of the protoplanetary disc around HL Tauri – This is the sharpest image ever taken by ALMA — sharper than is routinely achieved in visible light with the NASA/ESA Hubble Space Telescope. It shows the protoplanetary disc surrounding the young star HL Tauri. These new ALMA observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system. Credit: ALMA (ESO/NAOJ/NRAO.

The highest resolution the team has yet achieved is in the August 2019 dataset. Here the absolute position of the stars was limited to 3 milliarcseconds (atmospheric noise is a factor here), and their relative separation showed uncertainties of 300 µas. Out of all this the plan is to use ALMA’s astrometrical data along with archival data (this includes radial velocity) to tighten up constraints on the orbits of both stars. The data allowed the team to improve our calibration of the stars’ masses by 2-3 percent. From a paper (citation below) describing the ALMA work, illustrating how data from numerous sources play into this effort:

The combination of historical measurements of ? Cen A-B position angle and separation, more recent PRV [precision radial velocity] measurements from HARPS, and absolute astrometry from Hipparcos, and ALMA yields improved ? Cen A and B orbital elements, system proper motion and parallax, and component masses.

But I also noticed, from the same paper, this intriguing bit about Proxima Centauri, whose status as bound or unbound to the central binary has remained problematic. The ALMA work seems to resolve the issue:

The significance of the gravitational link between AB and Proxima has therefore increased over the last three years from 4.4? in (Kervella et al. 2017b), to 5.5? in Kervella et al. (2019) to 8.3? in the present work (< 10?15 false alarm probability). Proxima becomes a yet more valuable check on lower main sequence stellar modeling…

Leading to this:

Our accurate determination of the ? Cen AB system parallax, RV [radial velocity], and proper motion, when compared to those values for Proxima Centauri, confirms that the three stars constitute a bound system.

All this is obviously painstaking work, and in the way of astronomical observation, it allowed the scientists to now use the tightened orbital information as a baseline for their continued search for deviations caused by planets. Akeson argues that the highest resolution configuration of the ALMA antennas will allow a resolution 2.5 times higher than the August 2019 data, which would take ALMA into the 100 microarcsecond range. As the paper notes:

Our estimates of ALMA’s relative astrometric precision suggest that we will ultimately be sensitive to planets of a few 10s of Earth mass in orbits from 1-3 AU, where stable orbits are thought to exist.

We may be closing in on a potential planet like Kevin Wagner’s tentatively identified C1. Here we can see the synergy between detection methods, with direct imaging (the subject of multiple proposals) exploring the same space.

But both these methods play off radial velocity studies as well. Remember that astrometric data is best suited to planets in wider orbits than radial velocity. While the latter continue and the ALMA work pushes ahead, we have imaging possibilities through the James Webb Space Telescope to add to the mix. The observational ring around Centauri A and B, then, is tightening as astrometry yields more refined orbital parameters for both stars. We can hope for exoplanet discovery here that can be confirmed in short order through multiple approaches.

For more on the ALMA work on Alpha Centauri, see Akeson et al., “Precision Millimeter Astrometry of the ? Centauri AB System,” in process at the Astrophysical Journal (preprint).


Imaging an Alpha Centauri Planet

At some point, and probably soon, we’re going to be able to identify planets around Alpha Centauri A and B, assuming they are there and of a size sufficient for our methods. We may even be able to image one. Already we have an extremely tentative candidate around Alpha Centauri A — I hesitate even to call it a candidate, because this work is so preliminary — which could be a ‘warm Neptune’ at about 1 AU. One of the pleasures of the recent Breakthrough Discuss meeting was to hear film director James Cameron on the matter. Cameron, after all, gave us Avatar, where a habitable moon around a gas giant in this system plays the key role.

Despite his frequent protestations that he is not a scientist, Cameron was compelling. He’s obviously well-enough versed in the science to know the terminology and the issues involved in the ongoing deep dive into the Alpha Centauri system, and he’s done wonders in fixing the public’s attention not only on its possibilities but also on presenting a starship concept that, using hybrid propulsion methods, makes a bid for most realistic starship ever in film.

I imagine Kevin Wagner thinks about the Avatar scenario now and again, given that his work on Centauri A has turned up the observation he refers to as C1. Let’s put it in context (and I’ll also send you to Imaging Alpha Centauri’s Habitable Zones, which ran here in February), delighting in the fact that we have more than one habitable zone to talk about.

Wagner (University of Arizona Steward Observatory) and team run NEAR (New Earths in the Alpha Centauri Region), which thus far has been a full-on 100-hour attempt to look into the habitable zones of Centauri A and B. It’s fascinating to realize that these stars are close enough to us that with technology like Hubble, we can actually observe the habitable zones, for these are at separations we can see, at about 1 arcsecond, which is resolvable with large telescopes. Think Sagan’s ‘pale blue dot’ when you imagine the ultimate goal of actually imaging an Earth-like world, although it will take future instrumentation to get us to that level of sensitivity.

Image: This is a familiar image from Hubble showing Centauri A at left and B on the right. Kevin Wagner superimposed the circles showing the size of the habitable zones. The image was captured by the Wide-Field and Planetary Camera 2 (WFPC2), and is drawn from observations in the optical and near-infrared. Credit: Kevin Wagner/ESA/NASA.

NEAR, whose first 100-hour run is complete, used an adaptive secondary telescope mirror working in combination with light-blocking and masking technologies in the mid-infrared to suppress light from each of the binaries in sequence. The NEAR equipment is mounted on the Very Large Telescope’s Unit Telescope 4 in Chile, and the key, according to Wagner, is the deformable secondary mirror, which maximizes adaptive optics without adding warm optics downstream in a tertiary mirror that would degrade the infrared signal. About 1600 magnetic actuators zone out atmospheric distortion even as the coronograph nulls out star light.

Imaging something on the order of a pale blue dot around another star is quite a goal. We’ve only imaged a dozen or so exoplanets thus far, and all of these have been young and massive gas giants that still radiate brightly, no more than tens of millions of years old. Mature planets like those in our own Solar System are much cooler, and if we are after a planet like the Earth, we have to look in areas where the infrared signal is swamped by our own atmosphere. Adds Wagner:

“The earth is a 300 K black body. Here the primary radiation is at 10 microns, which is where we have to look at more mature exoplanets. And the problem is that the atmosphere of our own planet is what we have to look through, and it also radiates at about 10 microns. The sky, the telescope, the camera, everything is glowing at us.”

I ran the figure below in February, but I want to introduce it again, as it shows not only the C1 observation but also, on the left, the systematic artifacts that have to be removed to come up with what the astronomers hope is a clean image. Remember, we are in early days here, and when discussing C1 as a possible planet, we have to keep in mind that other explanations are possible, including distortion in a not yet recognized effect within the equipment itself.

Image: This is Figure 2 from the paper. Caption: a high-pass filtered image without PSF subtraction or artifact removal. The α Centauri B on-coronagraph images have been subtracted from the α Centauri A on-coronagraph images, resulting in a central residual and two off-axis PSFs to the SE and NW of α Centauri A and B, respectively. Systematic artifacts labeled 1-3 correspond to detector persistence from α Centauri A, α Centauri B, and an optical ghost of α Centauri A. b Zoom-in on the inner regions following artifact removal and PSF subtraction. Regions impacted by detector persistence are masked for clarity. The approximate inner edge of the habitable zone of α Centauri A13 is indicated by the dashed circle. A candidate detection is labeled as ‘C1’. Credit: Wagner et al.

The C1 candidate looks, says Wagner, like what the team’s simulated planetary sources look like, but it could also represent, in addition to a systematic error, dust in the habitable zone, bearing in mind that while the Sun has its own zodiacal light from such dust, the Alpha Centauri system is known to have 50 times more dust. We could be looking, in other words, at dust that is off-center simply because of the orbital perturbations within the binary. “We can’t attribute this to any of the known systematics,” says Wagner, “but we don’t know all the systematics in this new system.”

What’s truly newsworthy in the NEAR work is the sensitivity of the dataset, which demonstrates that a habitable zone planet somewhere between Neptune and Saturn in size is detectable around the Alpha Centauri stars. NEAR is, in other words, sensitive to planets smaller than Jupiter at about 1 AU, and thus we can expect further work to find out whether C1 can be verified as a planet. This could be done through imaging using the James Webb Space Telescope, or through another observing run with NEAR, or via astrometry (about which more in a day or so) or even time-tested radial velocity using the hugely sensitive ESPRESSO.

The current limit on radial velocity detection around Alpha Centauri is on the order of 50 Earth masses in the habitable zone, Wagner added. NEAR itself is not currently in operation but could be reinstalled at UT-4 on the VLT, and of course on top of the other options, we have the next generation of ground-based telescopes coming, extremely large instruments that could accomplish within a single hour what it took the NEAR instrumentation 100 hours to do.

NEAR has demonstrated a technology, then, that is apparently capable of imaging mature Neptune-class planets in this system. Ramp its sensitivity up four times and we get to ‘super-Earth’ detection capability. We’re not yet at Earth-like planet imaging, but within decades, the ELTs should make it possible. We can consider NEAR a pathfinder experiment that has demonstrated the limits of the possible and shown us the way forward as, step by step, Alpha Centauri yields its secrets.

For more, see Wagner et al., “Imaging low-mass planets within the habitable zone of α Centauri,” Nature Communications 12: 922 (2021). Abstract / full text.


Proxima Flare Captured at Multiple Wavelengths

I’ve been wanting to explore some of the observing campaigns for the Alpha Centauri system — their approach, design and early results — and we’ll start that early next week. But let’s home in first on an event within that system, a flare from Proxima Centauri that is fully 100 times more powerful than any flare ever detected from our own star. That Proxima was capable of major flares was already known in 2018 when, according to data from the Atacama Large Millimeter Array (ALMA), an earlier flare at millimeter wavelengths (233 GHz) was detected.

It was an interesting moment, captured in a paper on the work in Astrophysical Journal Letters (citation below). Lead author Meredith MacGregor, an assistant professor at the Center for Astrophysics and Space Astronomy (CASA) and Department of Astrophysical and Planetary Sciences (APS) at the University of Colorado Boulder, also found it provocative. “We had never seen an M dwarf flare at millimeter wavelengths before 2018, so it was not known whether there was corresponding emission at other wavelengths,” says the astronomer.

That led MacGregor to initiate a 40 hour observing campaign over the course of several months in 2019, this one involving nine instruments at various wavelengths, both on the ground and in space. The attention quickly paid off. In May of that year, five of the instruments detected a seven-second event, a flare that surged in both ultraviolet wavelengths (Hubble) and millimeter wavelengths (ALMA) and, according to MacGregor, brightened the star by a factor of 14,000 in the ultraviolet. No flare around a star other than the Sun has ever been observed across such a wide range of wavelengths. The effects near Proxima should be egregious. Adds MacGregor:

“Proxima Centauri’s planets are getting hit by something like this not once in a century, but at least once a day, if not several times a day. If there was life on the planet nearest to Proxima Centauri, it would have to look very different than anything on Earth. A human being on this planet would have a bad time.”

Image: Artist’s conception of the violent stellar flare from Proxima Centauri discovered by scientists in 2019 using nine telescopes across the electromagnetic spectrum, including the Atacama Large Millimeter/submillimeter Array (ALMA). Powerful flares eject from Proxima Centauri with regularity, impacting the star’s planets almost daily. Credit: S. Dagnello, NRAO/AUI/NSF.

This was the largest ultraviolet flare ever detected from Proxima, a red dwarf and thus representative of the most common stellar type in the galaxy. It’s widely understood that these are highly active stars, so that protective mechanisms during flare activity would have to evolve for life to survive. Evgenya Shkolnik (Arizona State University) notes the importance of working across wavelengths as we try to understand the nature of UV flaring in this environment:

“This research is a benchmark of how best to study flares from many angles. It reminds me of the old parable about the blind men and an elephant, where different people observe small parts of the elephant and conclude that one part is a snake, another a wall, another a tree. Only when they look at all the angles together, do they finally understand it is an elephant. This multiwavelength flare from Proxima Centauri is our first snapshot of the whole elephant.”

Image: Artist’s conception of a violent stellar flare erupting on neighboring star, Proxima Centauri, from the viewpoint of Proxima Centauri b. The flare is the most powerful ever recorded from the star, and is giving scientists insight into the hunt for life on planets in M dwarf star systems, many of which have unusually lively stars. Credit: S. Dagnello, NRAO/AUI/NSF.

The huge surge in ultraviolet and millimeter radiation marks a kind of activity that could strip planetary atmospheres as well as exposing surface life to dangerous levels of flux. Numerous other flares were detected during the course of the 40 hours of observation, but none so powerful as this. There is conceivably a driver for evolution here as well, with radiation enabling chemical reactions that may eventually become the precursors for life. Untangling these contradictory threads will require continued focus on flare activity and its effects.

From the paper:

Proxima Cen is a unique target given that it hosts a planet in the habitable zone but also produces anomalously powerful flares for its old age. Can a planet truly be habitable in this environment? It is clear that necessary pieces are missing from our current understanding of M dwarf flares in order to answer that question. We expect to learn much more as we synthesize the available data from this project and from future flare campaigns. This paper presents the results from just a few minutes of the available data. Many other flaring events are detected simultaneously across multiple facilities (including ALMA and HST) during the full 40-hour campaign. If the correlation between FUV [far ultraviolet] and millimeter flaring emission holds, there is potential for future all-sky millimeter surveys (e.g., the Atacama Cosmology Telescope, Naess et al. 2020) to be able to provide constraints on the high-energy radiation environment of exoplanet host stars and inform discussion of planetary habitability.

The current paper is MacGregor et al., “Discovery of an Extremely Short Duration Flare from Proxima Centauri Using Millimeter through Far-ultraviolet Observations,” Astrophysical Journal Letters Vol. 911, No. 2 (21 April 2021). Abstract / Preprint. The 2018 paper is MacGregor et al., “Detection of a Millimeter Flare from Proxima Centauri,” Astrophysical Journal Letters Vol. 855 L2 (1 March 2018). Full text.


A Drake Equation for Alien Artifacts

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


“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 TR. And I also multiply by:

fR = 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, NL, can similarly be expressed as the rate of creation of radiating civilizations, times the fraction that also develop interstellar probe technology fip, times the sojourn that Lurkers would be in the solar system, TL:

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

TL = time that Lurkers could reside in the solar system

(Note that for such civilizations, fC =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 fip/fR < 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 TL/TR > 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 VL, the volume over which Lurkers can travel, and its corresponding range RL vs. VB, the volume over which Beacons can transmit and be plausibly detected, and its corresponding range RB. 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, VL/VB:

As volume scales as the cube of the distance to them, RL/RB:

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 TR, time that ETI Beacons radiate

In the literature, estimates of TR fall between a hundred and 100 million years, a very wide range. Michael Shermer estimated TR 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 TL, 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, TL, 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 TL 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) RL and RC are equal.

If we take as an example a Beacon at 100 ly and a Lurker probe launched from 100 ly, then RL and RC 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 fP/fR = 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 < TL < 2.5 109 years.

Next, one guesses the transmit time of the Beacon: estimates of civilization radiating times TC 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, fP/fR = 1, and, as we’re talking about a single civilization, RL/RC = 1. Consequently the Success Ratio NL/NC = TL/TC, 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 NL/NC = 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 NL/NC >>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 TL ~0.33 million years. 1) For TR , choose the 300-year lifetime estimate of Shermer for the Beacon to radiate. Then TL/TR = 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 RL/RB = 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 fP/fR = 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, fR~fp, 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 TC vary from 102 – 105, then TL/TR 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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