Taking advantage of the fact that most major bodies in our Solar System orbit in roughly the same plane around the Sun, Cornell’s Lisa Kaltenegger, working with Joshua Pepper (Lehigh University), has gone on to ponder the implications of the ecliptic plane, traced out as the plane of Earth’s orbit around the Sun, for exoplanet studies. We’re in the realm of transit detection here, because what the authors want to know is not what we see on the ecliptic so much as what extraterrestrial observers would see if they were on the same plane. Says Kaltenegger:
“Let’s reverse the viewpoint to that of other stars and ask from which vantage point other observers could find Earth as a transiting planet. If observers were out there searching, they would be able to see signs of a biosphere in the atmosphere of our Pale Blue Dot, And we can even see some of the brightest of these stars in our night sky without binoculars or telescopes.”
Kaltenegger is director of Cornell’s Carl Sagan Institute and she takes a Saganesque perspective: What nearby stars exist that would see Earth as a transiting exoplanet? It’s a question with SETI potential because another civilization detecting biosignatures on Earth might conceivably target us for deliberate broadcasts. The paper defines an Earth Transit Zone (ETZ), that region from which the Earth could be seen transiting the Sun. It is a thin strip around the ecliptic as projected onto the sky, with a width of 0.528°.
This is a region of the search space that has seen recent attention as the quality of our catalogs improves. A 2016 paper by René Heller and Ralph Pudritz had identified a slightly narrower ‘restricted’ ETZ defining a region where exo-astronomers would see Earth transit for more than 10 hours, finding 82 stars within 1000 parsecs (3260 light years) in this zone. Heller and Pudritz used Hipparcos data and extrapolated their result to estimate that about 500 stars should exist in that region, while a later paper by Robert Wells found roughly twice that number (citations below).
Mindful of the limitations of the datasets used in these studies, Kaltenegger and Pepper take advantage of the more precise measurements of stellar distance now available through the Gaia mission’s Data Release 2 (DR2), while excluding evolved stars and those with poorly measured distances. The authors note that one particular advantage of the Gaia data is the measurement of parallax distances to fainter, late-type dwarf stars. They operate with a focus on main sequence stars as the best targets to search for biosignatures. Also in play here is the TESS Input Catalog version 8, used by TESS scientists to select target stars, which provides stellar distances, effective temperatures and approximate luminosity.
Out of this Kaltenegger and Pepper identify 1004 main sequence stars within 100 parsecs (326 light years) that would see Earth as a transiting planet. Two of these are known from the K2 mission to be exoplanet hosts. If Earth-size planets in the habitable zone occurred at a rate of 10 percent, this would lead to roughly 100 Earth-class planets in the habitable zone capable of observing a transiting Earth, although the authors are quick to note that estimates of the occurrence rate of such planets range widely and depend upon how habitable zone limits are chosen. We are also early in the game — how complete are our catalogs, and what are we not detecting?
“Only a very small fraction of exoplanets will just happen to be randomly aligned with our line of sight so we can see them transit.” Pepper adds. “But all of the thousand stars we identified in our paper in the solar neighborhood could see our Earth transit the sun, calling their attention.”
Animation: Cornell astronomer Lisa Kaltenegger and Lehigh University’s Joshua Pepper have identified 1,004 main-sequence stars – similar to our Sun – that might contain Earth-like planets in their own habitable zones within about 300 light-years of here, which should be able to detect Earth’s chemical traces of life. Credit: John Munson/Cornell University.
The closest star in the ETZ is found to be 28 light years from the Sun. The bulk of the sample consists of red dwarf stars. 12 percent are K stars; 6 percent G-class stars like the Sun; 4 percent are F stars and 1 percent A-class stars. Breakthrough Listen is already searching the ETZ, and the paper notes that TESS is to begin searching for transiting planets in the ecliptic next year, so the list generated here provides a helpful set of targets.
This interesting note on stellar motion ends the paper, reminding us how mutable is our view of the heavens:
For the closest stars, their high proper motion can move them into and out of the vantage point of seeing our Earth block the light from our Sun in hundreds of years: For example, Teergarden’s star – which hosts two known non-transiting Earth-mass planets – will enter the ETZ in 2044 and be able to observe a transiting Earth for more than 450 yr (Zechmeister et al. 2019) before leaving the ETZ vantage point.
Thus, the stars which could have seen Earth when life started to evolve are a different set to the ones which can spot signs of life on our planet now compared to those which will see it transit in the far future: Therefore, our list presents a dynamic set of our closest neighbours, which currently occupy a geometric position, where Earth’s transit could call their attention.
The paper is Kaltenegger & Pepper, “Which stars can see Earth as a transiting exoplanet?” Monthly Notices of the Royal Astronomical Society: Letters, Vol. 499, Issue 1 (November, 2020), pp. L111-L115 (full text). The Heller & Pudritz paper is “The Search for Extraterrestrial Intelligence in Earth’s Solar Transit Zone,” Astrobiology Vol. 16, No. 4 (15 April 2016). Abstract. The Wells et al. paper is “Transit visibility zones of the Solar system planets,” Monthly Notices of the Royal Astronomical Society Vol. 473, Issue 1 (January 2018), pp. 345-354 (abstract).
“What would they think?” They would think “wow those aliens are putting a lot of CO2 into their atmosphere. Don’t they know that will have deadly consequences?”
Nice work by Kaltenegger and Pepper. Keeping unknowns at reasonable levels.
To Expand possibilities beyond the main study, it would be interesting to assume that an ET is maybe 100 yrs technologically advanced, In this case our transit and bio signature would be an easy catch for them, even further away. This more advanced civilization would possibly double or triple candidate stars they can analyze with a HZ that has terrestrial planets. So ETI could have been aware of us by analysis of our atmosphere for quite a while now. Even if our broadcast signals have not reached them, you can bet they have dedicated equipment trained on our solar system, and any others (unlikely IMO) that hint at some advanced life.
Yes Robert, I’ve been saying this for a while. The more advanced civilization spots the lesser one first. Especially if they are thousands or millions of years ahead of us technologically, they probably have a handy dandy catalog of all interesting planets in the galaxy and may have been aware of us for thousands or millions of years.
I don’t see the value of this approach for 2 main reasons:
1. Any ETI is most likely to be technologically far beyond us. This means that our technological limits that make transits the best method to detect life are not going to be relevant to such a civilization. Thus the restricted view along our plane is unnecessary.
2. If any civilization is within 100 parsecs, this implies that the galaxy must be chock full of civilizations. This seems extremely optimistic. The distance limit appears to be determined by our technology (cf point 1).
IMO, this just artificially restricts potential targets for ETI, which may not even exist at all. Better to focus limited resources to look for life rather than another SETI strategy, even if that makes it tactically easier to allocate SETI resources, both radio and optical.
Watching yesterday’s SETI presentation about radio telescopes, I gather that small sensor telescope arrays allow for much more of the sky to be searched than with large dishes. The receivers are cheap and rely on the plummeting cost of computers to handle the bandwidth. It was argued this was the future for radio astronomy which would help the SETI program too. So why artificially restrict the search parameters just as we can increase the search space?
Alex,
I really can’t agree with you. Anything to improve the odds of us detecting a probing civilization, in this case by looking toward where our “shadow puppet show” can be seen, as in transits across potentially several hundred parsecs and 10^5 star systems (my WAG), or, as the joke goes, looking at where the lighthouse of our civilization points (meaning the plane of our strongest radar emissions which are sent along the ecliptic toward planets and asteroids, albeit for only the last ~60 years) reduces the search parameter size. A visualization I did some years ago may be of interest https://vimeo.com/208013337
If you could explain why my 2 points are flawed or wrong that would help. By all means disagree, but provide an explanation so I don’t repeat the errors.
I will try to be less flip. Try everything: go wide, go deep.
1. Any ETI is most likely to be technologically far beyond us. This means that our technological limits that make transits the best method to detect life are not going to be relevant to such a civilization. Thus the restricted view along our plane is unnecessary.
The earth-sun transits observed by an advanced civilization would have detected O2 for ~2.5 billion years. Per classic SETI arguments, advanced civs would be long lived. Would they care as per AlexTru’s spot on (and funny) post? I don’t know, but if they did care, across hundreds or thousands of parsecs, the odds are at least modestly better that they might “knock on our door” once in a while. Thus, while narrowing the search space, it deepens it.
2. If any civilization is within 100 parsecs, this implies that the galaxy must be chock full of civilizations. This seems extremely optimistic. The distance limit appears to be determined by our technology (cf point 1).
I would not argue with that at all. We’re up from searching a dropper, to a cup, to a hot tub’s worth of water out of Earth’s oceans (CD 9/9/20) so the SKA is a super worthy instrument to bring online. But if the Webb telescope detected Oxygen on a distant plant, it’s have our full attention.
Put the billions of bucks for a yet larger Hadron Collider (with little chance of a new horizon being reached according to Sabine Hossenfelder) into the search for life and solar exploration and there’d be solid results, possibly nul. Either way: humbling.
Yet to AlexTru’s point (Alex Tolley’s Pen Name?) all of science seems a near fairly tale of late. It may be my zeitgeistdepressed mood but man, I wish those UFOs the Navy filmed would just land somewhere or hold still!
I would characterize what you are saying as “looking for a black cat that is [probably] not there in a dark room”. Better to look outside around the town to find a black cat. [Or set traps if possible.]
The search is still dropper-sized, but now more restricted to fewer samples near the seashore. In marine terms, few fish live in the surf, and most live in deeper water. If the nearer stars were like a coral reef, I might agree with you, but there is no reason to further stretch the analogy beyond breaking point.
That at least supports my opinion this is about hoping for communication (or a visit to the WH lawn). But if one wants to go for nearer stars, why restrict it to stars that can view Earth as a transit, rather than all suitable stars, just because transits are what we have to characterize atmospheres? But I reiterate my first point about cats and suggest we search all neighborhood dark rooms, not just those in the same street. ;)
[And no, AlexTru is a totally different person].
Well, I have a little bit different take on this. When I first started looking at their sample I noticed that they included stars only down to M 5.0. After digging thru the data there should be at least 300 more M dwarfs between M5.5 to M 9.0 that are not included, these stars have temperatures below 2800 degrees Kelvin. Now researching this I came across an article that I had seen earlier reports about but had new data:
Getting Deeper into the Main Sequence with Gaia.
https://aasnova.org/2020/08/21/getting-deeper-into-hr-diagrams-with-gaia/
What they had discovered was the actual cutoff in Hertzsprung–Russell (HR) diagrams of the main sequence that shows an actual gap between partial convective M dwarfs to being fully convective at masses of around 0.35 solar masses.
But something that they brought up and has puzzled me for some time is that later M6.0 to M10 become rarer when their lifetime become much longer. These should be the most common stars in our galaxy and should be making up at least 50% of the M dwarf population. So have the civilization’s in our galaxy shanghaied these minute stars for their own purposes?
Now before getting to far off track let me explain why this make sense;
The galaxy is an ecosystem just like earth and screwing around with the giant short lived stars could have long term problems with stellar cycles.
Now small low power stars with long life times have very little influence on the the stellar cycles in our galaxy and would not be missed over long time periods. The largest effect would be their gravitational effects for a extremely large number of them.
The obvious result would be a large number of invisible low mass stars with some being much closer then Proxima Centauri. As Clark said, it would be indistinguishable from magic. There are ways to spot them but the standard ways will be useless, so start thinking like a invisible aliens or invisible tecnosignatures…
This is an intriguing observation and your suggestion as to the cause is worth pursuing. If these stars have Dyson swarms or are being reduced in mass to cool them further, this would suggest that there should be more IR-only stars. If the total mass remains in the system, then gravitational lensing should be able to detect those stars where the apparent spectral output is shifted to the longer wavelengths. If the stars have been collapsed to black holes and the bulk of the stellar mass is being fed to the BH for energy, that would represent an extreme case of this solution.
Do you have any suggestions for testing your hypothesis that might be doable with the data we have? Alternatively, is there an alternative explanation[s] for your observation of the missing stars?
I think there may be a much simpler explanation;
Plasma metamaterials as cloaking and nonlinear media.
https://iopscience.iop.org/article/10.1088/0741-3335/59/1/014042
Metamaterial cloaking.
https://infogalactic.com/info/Metamaterial_cloaking
I still researching but on the stellar level this looks like a good possibility even to the extent of a natural phenomenon to some degree.
My slow DSL internet has been up and down because of a tropical storm for the last two days so will update when working.
Let me add some detail to my original comment.
The paper is implicitly asking “Where might we best look for ETI?” The assumption is that ETIs will send some sort of signal in our direction if they can be sure there is life on Earth, and presumably a technological civilization to respond.
The 100-parsec distance limit could be justified if an industrial civilization on Earth is detectable up to 600 odd years ago (2x the time to detect and send a message detectable by us) and would be detectable. IOW, European civilization could be detectable as industrial or likely to be shortly industrial with interstellar communication technology around 1400 CE. More likely is that the 100-parsec limit is based on our technology limitations and the TESS data.
The number of candidate stars is reduced to those most likely to have ETI, F, G, K, and M types.
The resulting set is around 1000 stars.
For this list to be search targets that result in success, that is the detection of a signal beamed at us by an ETI that has detected and characterized our planet based on transit analysis, there must be at least 1 civilization that is beaming at us with a detectable em signal.
Note that the assumption is that ETI uses transit technology to detect us. That they in turn assume that we could detect their signal and perhaps return a signal acknowledging their signal. Or they might be altruistically sending us an Encyclopedia Galactica to uplift our knowledge, or perhaps to damage us (c.f. “A for Andromeda” – Fred Hoyle).
Given the depth of time, how likely is it that there is an ETI with approximately our current level of technology in that target set? Within a few hundred years we may be sending out swarms of sublight probes to look for life and possibly civilizations, obsolescing transit technology other than an early method to target our probes towards inhabited worlds. [I have just reread Silverberg’s “Tower of Glass” where an industrialist is determined to send an FTL signal to a source that has sent a radio message to Earth. He fails and ultimately leaves Earth in a sublight starship to find teh source.]
If there was even 1 ET civilization within 100 parsecs, however old, that would be either a remarkable coincidence or imply that the galaxy is full of ETIs. This then runs up against the Fermi Question, if only because the SETI has not turned up any results so far.
Now it is entirely possible that ETI uses some form of FTL communication that we have yet to discover, and that when we do the sky will light up with messages. However, when Europeans went exploring unknown lands, especially rainforests, looking for other people, they did not use technological communication methods to signal their presence and hope for a reply using the same medium but used the lowest common denominator communication method – physical presence. The main objection to assuming ETI would use radio or optical communication rather than FTL communication (if it exists) is if the cost of the former was considerably higher than the latter, or that the discovery of FTL communication was a prerequisite to contact less-advanced species. (cf warp drive signature in “Star Trek: First Contact”).
At this time, we do not know how common life is elsewhere, let alone ETI (The Drake equation results in 1 (us) to many thousands of ET civilizations, depending on input assumptions). We have an anthropomorphic view that brains and intelligence tend to increase over evolutionary time, and that technological species will almost necessarily arise and prove to have a long-term (i.e. on an evolutionary time frame) advantage. That may be a conceit on our part. Our pre-H. sapiens ancestors had those intelligence advantages, yet they failed to endure in any numbers. If one uses the excuse that modern humans out-competed our near cousins, how is it that far earlier primates and hominids did not dominate the planet with their intelligence without competition from modern humans? No other lineage has acquired that spark of cultural evolution that in the space of tens of millennia (an eyeblink in evolutionary time) brought us civilization, and initially in Europe, global industrial civilization starting mere centuries ago. What if the Anthropocene proves transient, like a large meteor impact?
In summary, I find the assumptions underlying this paper deeply flawed. I don’t understand Kaltenneger’s motives in writing this paper. Her previous paper with Pepper (2019 in the references) was far more mainstream.
The authors merely stated “Here, we ask, from which stellar vantage points would a distant observer be able to search for life on Earth in the same way?” They do not need to justify it any further. I see no underlying assumptions. It is not somehow out of the mainstream.
I read this as arguing that stars in this target list are the best for SETI search as ETI in these systems can use the transit method to characterize our planet, and therefore know that life, and possibly intelligent life, is on Earth.
While not as cynical as AlexTru below, I think there is some truth in his comment.
“Cornell astronomer Lisa Kaltenegger and Lehigh University’s Joshua Pepper have identified 1,004 main-sequence stars – similar to our Sun – that might contain Earth-like planets in their own habitable zones within about 300 light-years of here, ” That is a lot a stars similar to our Sun and there might one hundred Earth sized exoplanets in the life according to this article. The age of the stars could help refine the search.
Transit spectroscopy has a range limit of several thousand light years as Kepler has already proven. It would be nice to start searching those nearby planets for year long transits right away.
There might be a lot of Earth sized exoplanets in the habitable zone which should be looked at any way, but if life is more rare, then we would have to look further, but starting the search nearby is a good idea. I agree with Alley Tolley that a very advanced civilization would not be limited to transit spectroscopy, and might have a extremely large space telescope. There is no doubt that an ET civilization on one of these nearby Earth sized exoplanets could see us even if they are not too much more advanced than us. A signal might still take hundreds of years to reach us. If they could see our spectra, the biosignature gases and also carbon monoxide, CO and air pollution, then they might think we were like them and have a similar technology and level of advancement.
SETI suppose to find some advanced civilization, but always limiting ETI ability by anthropomorphic argumentation and our current technological level and methods :-)
May be this article more dictated by financial motives, than anything else.
Transit astronomy is on the science edge now, so it seams that in this article SETI adopted idea to connect transit astronomy achievements to a new SETI methods, and sell it as “new approach” in distinguish to old unsuccessful long time searches. This is done with hope to find some new financial investments.
Other words : “in the past we (SETI) searched in all direction and found nothing, so from now we will listen mostly in ecliptic plane and now, using “new”approach”, for sure will find the ETI who adopted transit astronomy tech.
SETI people – funny people, who Like to tell fairy tales.
I don’t deny thinking something similar, but this seems rather harsh. Let’s invent another means of finding exoplanets, at least in our minds, before we put too much effort into bashing this thesis.
Hmmm… how hard would it actually be to spot exoplanets by measuring transits of suns not their own from a constellation of space telescopes, each of which watches every star in the sky? How many would it take? If Gaia can see a billion stars, that’s 1/8E7 steradian per star with a radius of 6.3E-5 radians = 0.2 arcseconds, leaving all those stars about 0.4 arcseconds apart on terrible average. (Someone tell me how wrong I am) Planet Earth at 6.4E6 m at a distance of 100 ly (9.5E15 m) is about 6.7E-10 radians = 0.4E-7 arcseconds. So we need maybe 10 million telescopes to spot it as it glides past the nearest distant star it doesn’t orbit. Maybe each telescope is a tiny carbon nanotube pointed at its single star of interest, calibrated by the reflection from some external source and redirected by a gentle beam of weak EM. But if we multiply that figure by a deep field it becomes quite intimidating, as is every manner of solar wind and light pressure for such a project. I can picture a time could come for some civilization when they could do it, or maybe there’s a smarter way, but I’m not ready to jeer at the notion of looking for mundane transits just yet.
Don’t we already know how to analyze planets w/o transits? For large space telescopes, a starshade will expose planets around a star whatever the orientation. Gravitational lensing will allow relatively good imaging of those planets. Deploy sufficient numbers of these telescopes and ETI could probably maintain periodic monitoring of planets in a large volume of space. This with technology we could deploy in the relatively near future. For technological species carelessly beaming out em waves from radars, broadcast media, etc., any sufficiently large radio telescope can detect technological species inside the light cone. By all means, add in transits but my point is that ETI, if it exists, has a number of techniques to observe Earth that do not require being in a special direction to see our transits.
Wups, I forgot to multiply by that 100! Parentheses are evil… Our alien astronomers will need more thralls and more grog to get this done.
There are approximately 250,000 stars within 100 parsecs (round numbers). Without even filtering these down to likely candidates by stellar type, with planets, etc, a long-lived ETI could launch 100 interstellar probes every [Earth] year at 0.1c and with 4 millennia every star could have a monitoring lurker that has reported back conditions on any living world and the conditions. Assume 1 star in a 1000 has a living world with complex life. That leaves just 250 stars to be monitored over deep time. As long as that ETI civilization can sustain itself as hi-tech, and a probe needs to be replaced every 100 years, they need to launch on average 2.5 probes per year to maintain surveillance on the promising worlds.
For Earth, once a probe has detected artifacts like cities, there are but 10 millennia to reach our level of technology. So by the 100th probe after the first city detection, ETI will have established that Earth has a technological civilization that the civilization can decide when/if to contact.
So even a brute force method to detect technological civilizations around other stars is not unreasonable. In practice, the 250,000 stars could be filtered by remote observation first, if to only send probes to those with rocky planets in the HZ [CHZ?] detected by transits, radial velocity, and astrometry using technology that is only a little more advanced than what we have already achieved in so very short a time, as well as more advanced methods like gravitational lensing.
Once a promising planet has been located by the first probe, it might make sense to build local infrastructure to aid the flow of replacement probes. For beamed radiation propulsion, a comparable beamer at the target star will be useful for deceleration and to maintain a fast transit between stars. Or local construction of replacement probes could be established, rather like the von Neumann replicator concept, but restricted to only replacement within the system. Or perhaps von Neumann replicators to cover all the stars in the 100 parsec volume to remove the economic cost of the launches from teh home world.
IMO,by far the biggest problem is that civilizations appear many millions of years apart. There are not going to be even remotely contemporaneous, at least not on a technological level. Either they are millions of years old or they will be fleeting and disappear before the next civilization arises. Far more likely is that when a civilization arises, it will start colonizationas soon as it is practical to ensure its long term survival. What form those colonies take is unknown, but theoretically they could colonize every star in the galaxy within a million years, a time period that is possibly shorter than any indedependant civilization can emerge.If that is a reasonable scenario, if they are not already here or very nearby, they are not anywhere.
1. Mike, it is huge fault to limit our future (any ETI) astronomical technology current homo sapience situation – it is way nowhere, accepting that approach you are implicitly denying progress.
As Alex Tolley wrote, even today we have alternative ways for exoplanets detection (starshade – one of options). And I believe that in close future when scientists will have image sensors (matrix) that have higher dynamic range and better pixels resolution – this technology will allow direct imaging of distant planetary systems, and those that oriented perpendicular to ecliptic plane will have huge advantage. Wandering, Should we expect from Kaltenegger a new article that will shift preferred SETI searches direction by 90 degrees, only because our technology changed direction?
Finally Kaltenegger argumentation try to prove that ETI activity depends on our current technology, some type of “miracle” civilization entanglement :-)
2. Our current exoplanets detection technologies do not allow to detect planets around stars similar to our Sun, so what conclusion should we get from this fact?
3. Our exoplanets sampling and distribution mostly shows our technology limitation.
The fact that we know to detect planets mostly around Red dwarfs, does not mean automatically , that planetary systems are possible only around Red dwarfs.
But if we apply planets detection rate around Red dwarfs to all type of the stars, I am sure we can accept that most (probably all) stars own planets.
If we accept that most stars own planetary system, we can conclude that for SETI searches every direction (relatively to our ecliptic plane) is equal.
– Not we, nor ETI are not choosing relative location in the Galaxy,
– physical laws are same in all directions in our Galaxy.
4. So I cannot accept Kaltenegger’s argumentation as scientifical.
I do see your point, but I’m not ready to be dogmatic about it. Future technology isn’t something we can predict or accurately account for. For example, the Fermi paradox could still imply some sort of stagnation after our current state of development, or civilizations with a lifespan as short as ours is likely to be. Alternatively, maybe aliens would still focus on transits because they have a highly efficient neutrino detector that can make a high resolution 3D scan of every room on Earth using the solar neutrinos. Though I don’t deny you’re likely right, the other possibility is still worth attention.
If the proximity of numerous stars near the galactic center makes for an unhospitable cirumstance, and low metallicity in the outer reaches of a galaxy militates against biology, then maybe the Galactic Habitable Zone could be a useful guide on where to look for the elusive – and so far mythical – creatures.
Almost like cryptozoology. CryptoETI?
I don’t know about the broadcast media. I read somewhere online that if there was a large amount of radio telescopes we might be able to pick up TV and radio signals, but I don’t think that is correct due to the inverse square law which applies to electromagnetic radio signals. The broadcast media spread out to fast and they are not strong enough. Someone has to be sending a signal with a large radio telescope with enough watts of power in order for us to pick it up even with a square kilometer array or many radio telescopes.