A supercomputing cluster operated by a team at Northwestern University is giving us fresh simulations of the birth of planetary systems, with results that may dismay terrestrial planet hunters. For if this work is correct, the ‘rare Earth’ hypothesis is back, this time bolstered by computer models that are the first to simulate the formation of planetary systems all the way from earliest dust disk to full-fledged solar system.
More than a hundred simulations using exoplanet data collected over the last fifteen years went into the modeling of dust, gases and the effects of gravity. Planetary systems do seem to have a few things in common, among them a violent birth. The Northwestern team found that the dynamics of the early gas disk push nascent planets inexorably toward their central star. There they may be consumed in the star or subjected to collisions with other objects as each accumulates mass. Dynamical resonances can occur that produce increasing orbital eccentricity, with planets occasionally flung into deep space. The young system emerges out of this flux and bears the inevitable stamp of these interactions.
Frederic A. Rasio, senior author of the paper on this work, notes that the violent history of early planetary growth makes producing a relatively sedate solar system like ours problematic. A massive gas/dust disk tends to give rise to ‘hot Jupiters’ and highly eccentric orbits. A low-mass disk produces ice-giant planets no larger than Neptune. Is it possible that our mix of small, rocky worlds, ice giants and gas giants in circular outer orbits really is an exception in the galaxy? Says Rasio:
“We now better understand the process of planet formation and can explain the properties of the strange exoplanets we’ve observed. We also know that the solar system is special and understand at some level what makes it special. The solar system had to be born under just the right conditions to become this quiet place we see. The vast majority of other planetary systems didn’t have these special properties at birth and became something very different.”
A rare Earth is, of course, another way to answer the Fermi ‘where are they’ question. They’re not here because they don’t exist, or at least, not in appreciable numbers, and the reason for that is that planets capable of producing intelligent life hardly ever form. It’s a depressing thought for those of us excited by the prospect of future contact, but one that should be placed in a certain perspective. For while it is true that other planetary systems we’ve found tend to look much different than ours, it’s also true that we don’t yet know as much as we need to know about these systems. We do not, for example, know how many other planets we have yet to find in them, or how many of these may be potentially habitable.
Future space-based missions should help us sort that out. Until then, my view is that these powerful simulations do exactly what good science is supposed to do — they work with the best available data and draw conclusions that will be subject to further observation and refinement. Just as the sheer number of ‘hot Jupiters’ came as a surprise to most astronomers, so may the presence of terrestrial worlds in hot Jupiter systems, or their possible existence around close binaries like Centauri A and B, force us to look anew at our formation theories. We’ll see how this latest take on the ‘rare Earth’ hypothesis develops as our data increase and we have closer looks at systems we are only beginning to characterize.
The paper is Thommes et al., “Gas Disks to Gas Giants: Simulating the Birth of Planetary Systems,” Science Vol. 321, No. 5890 (8 August 2008), pp. 814-817 (available online).
Maybe it’s wishful thinking, but I think, that if they have put the observation bias of Hot Jupiters into the model, it can be expected that such systems are the result of the simulations.
There are also still a lot of stars that do not have known exoplanets, so these systems are probably different. It could mean that they do not have any planets at all, but that does not seem very logical to me. Where has the left-over of star formation gone to?
The solar system would probably among the systems without known exoplanets if it would be viewed by the best currently available equipment from some parsecs. The inner planets are not heavy enough and the outer planets have a too long orbital period.
Anyway, I still have some hope ;)
… but it’s better to have some model than nothing, of course.
Hans
I assume that our galaxy has 400 billion stars and 50,000 light years for the radius, I estimated the “average” nearest advanced civilization is around 700-800 light years away. I think it’s easier to check the existence of advanced civilizations in a star system than finding an earth-like planet. I just wonder whether we can detect any fusion activity (neutrino flux) around a star system from 500-1000 light years away.
I tend to incline towards the “Fairly Rare Earth” hypothesis, because even if we find approximately Earth-sized planets orbiting in the liquid water zone of a star, we may find that most of them are completely unearthlike. Consider Earth itself. In its history, it”s been a high CO2 atm. planet, a methane planet, and an ice-ball before it became by its current O2 rich atmosphere. It was also very nearly an H2S rich planet.
We need to carefully define what we mean by “Earth-like.” If we mean a planet that could have liquid water on the surface, they may not be too infrequent — and planets with mircolife on may be not uncommon — but if when we say Earth-like, we mean something with a breathable atmosphere, they could be very rare.
Just consider how altering the relative ratios of elements in primordial nebula could alter the chemical history of a planet like Earth.
Dave.
Computer simulation is useful, but must always be compared with experimental observation. This simulation does seem to offer an explanation for large numbers of “hot Jupiters” that have been observed.
On the other hand, “hot Jupiters” are the low hanging fruit in exoplanet detection.
As Hans points out, we still do not have the capability to detect solar systems like our own.
I tend to favor “Rare Earth” explanations. However, only better observational techniques will answer the question.
I have a thought that you guys will probably consider unpleasant. If “Rare Earth” is right and there are very few habitable planets out there, doesn’t this suggest that we should build O’neill habitats rather than do interstellar travel?
Isn’t interstellar travel in a “Rare Earth” galaxy like being dressed up but with no place to go?
kurt9, if ‘rare Earth’ is right then the interstellar gambit seems to be purely a scientific pursuit, one worth doing for that reason alone, perhaps, but much harder to get public support for. My own take is a variation on ‘rare Earth,’ which says that life is ubiquitous but intelligent life rare. We’re just going to have to wait to see what we discover once we can get true terrestrial-planet finding equipment in place. I suspect we’re going to get surprises all over the place.
While papers of this type are valuable, I get the sense that in this area of astronomy — solar system formation — the theorists are still pretty much groping in the dark. The good news is that raw data from observations will continue to trickle in, and that trickle will eventually become such a flood that we will end up with a massive sample of thousands if not tens of thousands of systems to analyze — probably all before we even establish a permanent settlement on another planet.
We have plenty of time before we must choose where our ultimate destination will be. We are likely to have imaged a hundred thousand extrasolar worlds before we even have the capability of visiting the nearest one.
“My own take is a variation on ‘rare Earth,’ which says that life is ubiquitous but intelligent life rare.”
I like this as well. It means that there are lots of places for us to go, but we don’t have to share ’em with anyone else (I know this sounds selfish).
You will find that just about every evolution biologist would agree with you.
I just thought I’d mention something I read lately pertaining to the cosmic version of this problem, fine tuning, which recently fell afoul of a new model. The idea is that if fundamental constants were different, life would not be able to develop because nuclear, chemical and gravitational interactions would prevent stars, planets and/or chemistry from occurring. The new simulation argued that what is relevant is whether there were bodies which stably radiated energy and could support stable orbits, and found while if we mutate once constant at a time we’d be in trouble, a wide swathe of the constant-space supports at least this minimum criterion, even if it does so in an exotic way – black holes with Hawking radiation took the place of stars, in one simulation, and I don’t see why, other things being equal, life couldn’t occur in this environment. As we can’t even work out from our knowledge of the strong force all the properties of nuclei, I think it’s reasonable to guess that dynamic systems equivalent to our chemistry might well be possible, whether or not we’re presently able to show their existence with our mathematical tools.
This, I think, is relevant to this blog’s topic, being a microcosm of the fine tuning problem. We only observe systems with planets big enough to observe, with orbits on the timescales we are able to watch the stars on. That’s one problem. The other problem is that while it may be hard to find Earth-like planets which are gravitationally dominant in their orbit, I don’t see anything about whether, for example, a hot giant might collect a habitable moon. Not only that, with the kinds of crazy extremophiles we have on earth, if there was absolutely no choice but to live in a certain environment, with the right start I can easily imagine life taking hold and developing. What might be an interesting question is what proportion of bodies would contain enough accessible metals and energy to sustain technological life – the smartest creature in the world with the most adaptive biology couldn’t build a radio telescope with no electricity and no iron.
As a note on the chaotic dynamics of the planetary system, I think it’s distinctly suspicious how our planets orbit with very low eccentricity, yet these models predict highly eccentric orbits. I’m not sure what to make of this – in our solar system, orbital resonances, tidal effects and all that have put the planets into very circular ones.
Hi All
The more simulations like this come out the more I wonder if something basic is being missed. An alternative to the Core Accretion paradigm is gravitational instability – which has a few problems in a a purely disk-based setting. But it becomes very efficient if the disk around a protostar is tidally distorted, for example by another star in the common birth nebula of the stars. Number density of stars can be very high during the early phase, and tidal distortions have been increasingly studied as a driver of cosmogonical processes. Woolfson & Dormand’s Capture Theory is just one scenario in what seems a diverse range of processes that result in capture and formation of planets and brown dwarfs.
Only more data can distinguish between all the possibilities – exciting times!
“A rare Earth is, of course, another way to answer the Fermi ‘where are they’ question. They’re not here because they don’t exist, or at least, not in appreciable numbers, and the reason for that is that planets capable of producing intelligent life hardly ever form.”
I’ve never seen the logic behind this argument. I mean, “rare” is not the same thing as “nonexistent.” Even if there’s only a one in ten million chance of something happening, if you give it 400 billion chances, it’s likely to happen tens of thousands of times. That’s considerable. In a large galaxy like ours, even “rare Earths” could be fairly numerous.
And what about colonization? Even if there were only, say, a hundred planets in our galaxy which gave birth to intelligent native life forms, those species could’ve spread out and either terraformed planets or built artificial habitats across the galaxy. That would render them essentially immortal as species — or genera, rather, as over millions of years the different branches of these colonizing civilizations would evolve into distinct species.
And that’s not even counting the possibility that the same planet could produce more than one sapient species. Maybe one civilization migrates to space and eventually either dies out on its homeworld or abandons it, and then, millions of years later, another species rises to intelligence. We already share the Earth with other big-brained, arguably sapient species — the other great apes, the dolphins, the elephants. I’d say that if sapience has arisen once on a planet, that increases the odds of its evolving multiple times there (assuming the first civilized species doesn’t kill off all the other candidates).
So even with only a smattering of birthworlds, the galaxy could still contain a vast number of species and civilizations. They’d just have a relatively small number of common origins.
I’m another one who thinks that while life may well be fairly common, intelligent life is probably very rare, so much so that we may well be alone in the Milky Way Galaxy (although not in the entire universe). I wouldn’t be surprised if the nearest civilization was not 700-800 light years away, but in the Andromeda galaxy–2.5 million light years away. Maybe even more distant than that.
The reason I feel this way comes from looking at the life forms that have evolved on Earth. As far as we know, we’re the only technological species that has ever existed on this planet (I use the word “technical” rather than “intelligent” because “intelligent” is much more subjective. There are a number of other species that can be considered intelligent). Innumerable forms of life have evolved on Earth. If the development of technical civilizations really is likely, why has it only happened (so far) to only one single species on our planet? There’s no reason humans couldn’t have been merely one of many life forms to achieve technology. Moreover, there doesn’t seem to have been anything inevitable about humanity’s existence. As has been noted elsewhere, if the meteor which killed off the dinosaurs had arrived slightly later and missed the Earth, the human species probably would never have developed at all.
Obviously, these arguments aren’t conclusive. There is only one data point–Earth–to use to guess how often life evolves into intelligence. And someone who believes intelligence is common could use Earth as a “typical” case and conclude that an average of one technical civilization evolves on each life bearing planet. Still, the (to date) uniqueness of humanity among the enormous number (and diversity) of life forms on Earth suggests to me that intelligence is probably rare.
Regarding the Fermi Paradox, while it is thought provoking, I’m less impressed by it than many others. The reason is that makes assumptions that I find questionable. It’s not at all clear to me that humans, let alone any other civilization, will ever colonize the galaxy even if and when doing so is technically easy. Human civilizations have very rarely (if ever) made mass migrations unless there was a pressing need to do so. Why should colonizing the galaxy be any different? We’re already starting to seek solutions to problems that could compel humans to move elsewhere (overpopulation, exhausting resources, etc.). If these problems are managed, I suspect that while individual humans will explore other solar systems (and maybe other galaxies), there will be no mass colonization of the galaxy. Interstellar and intergalatic exploration will be what mountain climbing is today–a few people who are interested will do it, while the rest will not.
It should be remembered that as technology advances in one area (space exploration) it is likely to advance in others as well. A civilization with technology millions of years more advanced than ours may, for example, have sophisticated enough equipment to watch the surface of another planet thousands or millions of light years away! Astronomers could know all about distant planets without ever sending spacecraft there.
If one believes that technological civilizations should be very rare (as I do, for the reasons explained above), it’s not obvious there’s a “paradox” at all.
CB: “Even if there’s only a one in ten million chance of something happening, if you give it 400 billion chances, it’s likely to happen tens of thousands of times.”
From a purely statistical stance, the expected value can be easily calculated if we assume the 400 billion samples are mutually independent. This seems like a reasonably step to me. With this, the expected value equals 40,000 for the prior probability (p=1E-7) and sample set (4E11).
However we also have to keep in mind the variance in this calculation which, using a Poisson distribution, is equal to the expected value. This huge degree of uncertainty is to be expected given the scarcity of data.
We should always remember that statistical analysis is great for determining likelihoods and choosing the direction of further investigations but it can never generate evidence. Only observations can do that.
Lee,
I also think that life is common, but intelligence is rare. I think the nearest guys are not in Andromeda, but even further away in the Virgo cluster. I looked at a directory of the known galaxies nearest us. It turns out that our galaxy is the biggest, densest, most star containing galaxy this side of Virgo.
Even though Andromeda is larger in size than our galaxy, it apparently is less dense and actually contains fewer stars than our galaxy.
Since we know that stars vary in metalicity, does anyone know if galaxies vary in metalicity and, if so, how ours compares to the rest?
Four hundred billion stars in the Milkyway is the upper limit. If our galaxy only has 100 billion stars, then the “average” distance will increase up to 1600 light years. Anyway, we’ll have the next generation space telescope in the next 5-10 years, I really hope we will find something interesting.
Galaxies do vary in metallicity, in answer to kurt9’s question, and maybe someone better versed in this than I am can jump in with more. But irregular galaxies like the Magellanic Clouds are a case in point, with the Large Magellanic Cloud showing a metallicity of about forty percent that of the Milky way, and the Small Magellanic Cloud only about ten percent.
This research makes a testable prediction: the majority of multiplanet systems (and especially those containing hot Jupiters) will not be coplanar. To test this prediction, planet hunting must move beyond the radial-velocity method, which cannot yield this kind of information, to others, e.g. astrometry or direct detection. Note that binary stars can also induce non-coplanarity via the Kozai mechanism, so multiplanet systems around single stars should be targetted.
Theoretical models are as good as the assumptions and data that went into their logic — think of those pesky infinities that routinely crop up in GUTs. It’s not surprising that this simulation tends towards the “rare earth” conclusion, given that we still lack good techniques to identify planets smaller than gas giants. As for the Fermi paradox, lack of technology and/or lack of inclination are likelier explanations.
Metallicity and Jovian planet formation seem correlated. According to oligarchic accretion modelling of the terrestrial planets the amount of water a planet receives depends strongly on the late veneer – last few major accretion events. Where that veneer comes from and how water-rich it is depends on how much the material in the outer system is stirred up. Without a Jupiter class planet the inner system forms very dry – this might mean that low metallicity stellar systems will have smallish planets and no water-worlds like Earth.
Or rather no semi-dry water-worlds. As we’re discovering Hot Neptunes seem quite able to form in low metallicity, low mass systems. But even an Earth mass object that is even 10% water is too wet to ever develop land-based life, and maybe not even an oceanic ecosystem as erosion seems to have played a role in developing an oxygenic biosphere on Earth. Seems we need Jupiters to get decent water levels on Habitable-Zone (HabZone) planets, but not too many or else there’s too many Jupiters plunging into their stars and perhaps too much stirring of the dust-disk, else the Earths that form will be too wet.
This all assumes that ETIs need to be O2 breathers. We’ve no inkling yet that they could do otherwise, but my re-reading of Hal Clement’s “Heavy Planet” (Mission of Gravity & Starlight together) forces me to keep an open mind. There’s a lot of weird biochemistry that might yet be possible and it may not look like “life” to an overly narrow viewpoint. Imagine carbon based creatures using hydrocarbon solvents and breathing CO on a world richer in carbon than oxygen – such worlds very likely exist, even if the lifeforms don’t. Or planets using foramide as a solvent, or ammonia, ammonium sulfate, or even sulfuric acid. All these are theoretically possible.
I think our ignorance is too great at this stage to really judge. But the lack of macroengineering means either we’re amongst the first intelligence to emerge in the Galaxy, or else we truly are alone. Either option compels us to find out why.
“But the lack of macroengineering means either we’re amongst the first intelligence to emerge in the Galaxy, or else we truly are alone. ”
Or else that really advanced races don’t feel the need for prove how advanced they are by altering the shape of nature. (We used to think that building huge dams and taming forests were good ideas, but now we know they do more harm than good and it’s better to let nature stay natural. Macroengineering may be a product of the arrogance of youth, an urge a civilization outgrows by the time it becomes powerful enough to reshape galaxies.) Or else that macroengineering does occur in ways that we haven’t figured out how to recognize yet. Absence of (recognized) evidence is not evidence of absence.
kurt9,
if “Rare Earth” is right and there are very few habitable planets out there…doesn’t this suggest that we should go out there either by sending our eyes or ourselves? How could we decide otherwise?
I am with Paul in suspecting “we’re going to get surprises all over the place.” Most of them at arrival, I am sure.
The model is falsifyable. Once Kepler flies next year and gathers 3 years of data we will know the prevalance of small rocky planets in HZs. That being said, color me deeply skeptical of all such models. Remember the pretty pictures from Carl Sagan’s computer models in the late 80s showing solar systems so similar to ours? No hot Jupiters there. Sure we have more data now and I’m glad folks are attempting to generate models that are testable, but I would not make any plans based on any of such models.
One other point. Yet again folks here argue for the prevalance of Earths or technical civilizations by citing the large # of stars in the Milky Way. I call this the “large number fallacy” popularized by Sagan’s “billions and billions” rhetoric. Given say 10exp12 stars in the galaxy means nothing IF the probability of long stable Earthlike planets is 10exp-13, or the probability of life arising is some tiny #, or most probable according to the best scientific experts extant, evolutionary biologists, the probability of thinking technological beings evolving on the whatever number of ‘Earths’ is very very low.
Yes, I think Kepler will answer some of these thorny questions for us.
Some of the exosolar systems Sagan generated circa 1980
did have giant Jovian type planets near the star with smaller
worlds farther out. One even had a companion sun between
the planets and their main star.
The Sagan chart can be found in the Cosmos book, which
was also based on The Rand Corporation’s Habitable Planets
for Man, which was made into a popular book that is online
here:
http://stinet.dtic.mil/cgi-bin/GetTRDoc?AD=ADA473499&Location=U2&doc=GetTRDoc.pdf
I agree that we need much more evidence before the
verdict is in either way. The same goes for alien life.
With a sample set of 10^12 and a prior probability of 10^-13 the expected value = 0.1 for phil’s proposition. Once again, assuming mutually independent samples we can calculate probabilities. I find cumulative probability density functions most illustrative for this case, so that’s what I’ll use.
Let ‘E’ represent an Earth analog (with life, intelligence, or whatever attribute as you please).
p(E>=1) ~= 0.0952
p(E>=2) ~= 0.0047
With the given assumptions our own Earth has a poor probability of coming into existence, though it is better than demanding there be 2 or more (presumably us plus 1 or more Earth analogs). The variance = 0.1, the same as the expected value, and there is a 99% likelihood of E<=0.91. Interpret this as you wish.
As to extant models and estimates, to be frank but not deliberately insulting to the many qualified scientists out there, I think they are all pretty much worth squat. Our inventions of estimates are interesting thought experiments and nothing more. We simply don’t know. We need more data.
As far as modeling of physical systems go, my own experience in modeling even modestly complex electromagnetic systems using very good to excellent finite element tools is that they are treacherously prone to missing the mark, sometimes wildly, when compared against real-life implementations. Yet these EM systems are (far) better understood than early stellar system dynamics. I take all such studies and their conclusions with a large grain of salt.
Consensus is…we need more data. Wicked cool. It’s really interesting to be at this ignorant, yet developing stage of our exploration of interstellar space. Similar to our ignorance of the solar system pre 1960s space probes. The next decades will be a wild ride.
Wise words Ron.
But here’s another modelling find…
Proceedings of the International Astronomical Union (2007), 3:305-308 Cambridge University Press
Copyright © International Astronomical Union 2008
doi:10.1017/S1743921308016748
Formation of terrestrial planets from planetesimals around M dwarfs
Masahiro Ogiharaa1 and Shigeru Idaa
Abstract
We have investigated accretion of terrestrial planets from planetesimals around M dwarfs through N-body simulations including the effect of tidal interaction with disk gas. Because of low luminosity of M dwarfs, habitable zones around them are located near the disk inner edge. Planetary embryos undergo type-I migration and pile up near the disk inner edge. We found that after repeated close scatterings and occasional collisions, three or four planets eventually remain in stable orbits in their mean motion resonances. Furthermore, large amount of water-rich planetesimals rapidly migrate to the terrestrial planet regions from outside of the snow line, so that formed planets in these regions have much more water contents than those around solar-type stars.
…note that, M-dwarf planets are probably very wet.
Regarding the Rare Earth article. Rare means few but not one and the implications for SETI is that the volume of search space will be extended. Nature does not do anything once, as far as I know.
Bob Krekorian – Former NASA SETI Signal Detection Analyst
The NASA Astrobiology Conference in April, 2008 had as one of its topics, Future SETI: Technologies, Techniques and Strategies. Its premise is that after five decades of negative results from radio and optical SETI searches, there should be new approaches to the problem like detecting the biosignature of an extrasolar planet. This premise regarding SETI is not supported by reality.
The search for an extraterrestrial civilization is one of the most intellectually stimulating and potentially rewarding pursuits open to humanity. As we approach five decades since the 1959 groundbreaking paper by Giuseppe Cocconi and Philip Morrison, Searching for Interstellar Communications, much discussion has taken place on how to detect interstellar signals. In actual fact, very little systematic exploration has been performed. The NASA SETI project and the use of the NASA targeted SETI signal processing equipment by a private organization over a ten year period was especially disappointing in what it accomplished.
The Case for Extraterrestrial Beacons
Many ideas have been put forward speculating on the existence of extraterrestrial civilizations, their number in the galaxy and their longevity. For those civilizations that become technological and do not self destruct, it is reasonable to assume that some number reach long lifetimes and are still scientifically curious. An interstellar beacon, which has as its sole purpose, communication with other contemporary technological civilizations in the galaxy is quite plausible under these circumstances.
What would be the motivation to construct such a beacon? Perhaps there is an altruistic code in the galaxy to preserve the history of all civilizations, past and present? Perhaps there would be interest in contacting new technological civilizations like us, knowing that there is a time window (hundreds of years) after the the discovery of radio when some societies disintegrate because of sociological and environmental factors. In all likelihood, we would not be the first civilization that they have made contact with, thus finding one could be the gateway to many contacts. What could they expect to learn from finding one more? They may know a lot and have great understanding of science but the Earth’s civilization with its unique biology and history will be a new one for them to put into the larger context of life in the universe. Maybe they will ask for pictures and sounds from our culture? Maybe they will ask for detailed data on our solar system? This
seems far more practicable and feasible than sending out an armada of spaceships to explore other star systems. In a way, we would be their interstellar space probes.
There is another possibility in the quest to find an extraterrestrial technological civilization. Might we detect their internal communication signals (leakage), like our TV or radar? This seems like an almost hopeless proposition and is beyond our current technological capabilities. Let twenty second century SETI researchers work on detecting leakage, should our efforts in this century prove fruitless.
When one considers all the concatenated probabilities
connected with the formation of planet Earth, its composition, its stable environment over geological timescales that allowed complex life to flourish, the inescapable conclusion is that millions of sun-like stars will have to be examined to find one that is transmitting an artificial signal. That means the search volume of space could extend from one to two thousand light years. The NASA Kepler Mission which is scheduled for launch in 2009, will for the first time, give us hard empirical data on the number of Earth-like planets in habitable zones, their orbital stability in multiple star systems and the types of stars that have them. This will be quite important in bounding the SETI search space.
Recently, the NASA Astrobiology Institute sponsored studies of M stars as potential sites where complex lifeforms could exist. Even though M stars comprise about two thirds of all stars in the galaxy, which makes them attractive from a numbers standpoint, they are however poor candidates as SETI targets for the following reason.
M type stars have an effective temperature of 1/2 and a radius of 1/3 that of our Sun. Using a basic model, one can calculate the habitable zone, the annulus around a star in which stellar flux is sufficient to allow liquid water to exist. This puts the M star planet at about 0.1 AU and it will be tidally locked, like our planet Mercury at 0.4 AU.
There are a multitude of reasons why biology, let alone a technological civilization would be ill-suited to exist on such a planet. Imagine a tidally locked Earth orbiting a M star where half the Earth, say from 0 to 180 degrees west longitude is in perpetual darkness and permanently frozen down to 200 degrees below zero centigrade. I trust none of us would be living in California.
If a targeted search is the strategy of choice and detection sensitivity is essential, a targeted search of millions of stars would take centuries to complete. And even if one of these stars is indeed broadcasting, detection could be missed because our detection sensitivity was just not good enough or interstellar scintillation degraded the signal during the observation time frame or the observation is not coincident with the duty cycle of the beacon or we are not sensitive to the particular type of signal structure being transmitted.
Based on the above considerations, it is logical (logic flows from causality) to expect that our galactic colleagues will make the detection problem for the contact as simple and straightforward as possible. Universality of the laws of physics and the logic of mathematics will govern their strategy to maximize the probability of detection.
NASA and SETI
My career with the NASA SETI project included 15 years working with Dr. Kent Cullers (now living in South Africa), the leading expert in the world on SETI signal detection. I never did stop working on the detection problem. During the 1990’s, I organized a team to construct a radio telescope dedicated to SETI research. The team included Professor Frank Drake.
When I was with the NASA SETI project, one of the scientists told us that there really has not been a new idea with SETI in the last twenty years. Maybe this will all change? I have come up with new thinking in how the interstellar communication link would be achieved. Something has been overlooked. If my ideas are scientifically sound, it is quite possible to make a detection within a decade using existing telescopes and signal processing capabilities.
The expectation is that the contact/acquisition signal will be an address (pointer), like in the C programming language. It will direct contactees to where the actual communication channel is located. The exact frequency channel of the beacon transmitter may be known. Our astronomical capabilities might be insufficient to receive the text of the extraterrestrial transmission.
The charter of NASA includes a statement, the expansion of human knowledge of phenomena in the atmosphere and space. Some of us who were with NASA for the Oct 12,1992 (500th anniversary of Columbus discovering the Americas) SETI inauguration remember the worldwide interest and excitement created by what the agency was doing. Can we not rekindle this exploration spirit with a new generation of Americans?
NASA already has in place many of the resources needed to begin the search. The NASA SETI project was based on the 1977 NASA SP-419 report. See the conclusions from the report, Yahoo [nasa sp-419] which are still valid today. The 1993 congressional mandate to end United States funding for SETI is no longer in effect. Proposals for SETI grants are now being accepted by NASA and the NSF, but NASA is the proper federal agency to carry out a comprehensive search. The agency should form a small exploratory group, uninhibited by past orthodoxy and take a fresh look at the search for extraterrestrial intelligence.
I think we will probably find a quasi-Earth less than 2000 light years away. When I estimated the “average distance” to the nearest civilization, I was thinking about either type 0 or type I civilization. Type III civilizations are extremely rare, they can be “another” 300 million light years in the region of the Great Attractor or further, up to 3 billion light years.
@phil: Agreed. And while I find myself regularly dissing inventions of probabilities (and possibilities) for extraterrestrial life, I also can’t help myself from at times speculating wildly. The slate is quite clean and so allows the imagination, and scientific creativity, to run amok.
@adam: I did come down pretty hard on modeling, but I was not attempting to bury the field. We do need to be cautious and not get too excited by models that seemingly prove or disprove our favorite hypotheses. It’s all good work. I have gone through a number of papers on modeling early stellar systems and planet formation/stability, and I have noticed that they are still fairly coarse grained. This requires some assumptions, many of which are likely to be found faulty in future, of what happens at fine levels. One example being how microscopic dust particles agglomerate into larger entities that are computationally tractable, and how stellar ignition and other processes affect the disk of these particles. I am particularly intrigued by some of the new infrared studies of forming systems; we stand to learn a lot from that, which will improve those models.
Bob says: “Rare means few but not one … Nature does not do anything once, as far as I know.”
I heartily agree. A universe that can produce Earth by natural laws is equally likely to produce an Earth analog. We just don’t know the likelihood very well. To claim exactly one requires special pleading such as the anthropic principle.
All bets are off however for non-Earth like life. That really is unknown.
The paper in question is now up on arXiv here.
Thanks, andy.
I skipped through the paper fairly rapidly, but then it’s very compact at 12 pages including references and diagrams. There appear to be some papers referenced that I’ve also looked at before, though to be sure I’d have to do additional digging that I don’t have the time to do.
While I have some uncertainty of the details of what exactly they did (the supporting material, including code, is not in this pre-print), it doesn’t seem inconsistent with comments I made earlier in this thread. They start with a swarm of 10^-3 Me planetesimals in a remnant gas disk and compute forward from there. This and other constraints were selected to make the problem computationally practical. At that mass what you have is an awful lot of large asteroids. They therefore assume earlier dynamics somehow got to that point, somehow. That’s a lot of critical early history to account for unless there is some high confidence preceding work to justify it, but which I didn’t see. They assume any remaining gas is driven off by the star gradually during the 0.5 Gyr simulation that has lit up sometime in the interval.
The results seem interesting (from my position of semi-ignorance) and they do acknowledge the limitations of their model. One in particular that is relevant to the rarity of analog earths is the mode of migration (type II) could make them significantly more common than they conclude from their analysis. They reference these papers that speak to the matter (which I don’t plan to locate):
32. A. Morbidelli, A. Crida, Icarus 191, 158 (2007).
33. S. Ida, D. N. C. Lin, ArXiv e-prints 802 (2008).
To Bob Krekorian – the 1977 NASA document is online here:
http://history.nasa.gov/SP-419/sp419.htm
And as for there being only one planet with life in the Cosmos,
an ancient Greek natural philosopher named Metrodorus of
Chios had this to say on the subject a while back:
“To consider the Earth as the only populated world in infinite
space is as absurd as to assert that in an entire field of millet,
only one grain will grow.”
(Just back from my summer vacation in Spain, took me some time to go through the new threads and discussions, Paul is so prolific).
Great discussion, great to be back!
My first reaction is similar to the very first one above, that of Hans Bausewein, but indeed may contain some wishful thinking: there could be a good deal of observational bias involved if one takes the present batch of exoplanets, which may represent a non-representative sample after all.
Kurt9: interesting how you answered your own question: even if there are few or new truly earthlike planets to be settled, going interstellar could still be worthwhile for terraforming potentially habitable planets, rather than the O’Neill colonies. And terraforming lifeless planets indeed has the added advantage of avoiding competition, conflict and disruption.
Lee: colonization, or rather settling on other planets and planetary systems, does not necessarily involve large numbers or pressing problems (I agree that most problems can better be solved on one’s own home planet), but the result will still be the same, even if in small numbers and for purely scientific reasons: making ones presence known on another planet. However, I do agree that telescopic searches will probably always be much more feasible than space travel.
I agree most of all with Ron S: the proof of the pudding will in this case be in the observing, nothing beats scientific observation and discovery.
Kurt9: as far as I know, Andromeda has many more stars that our Milky Way (about double), but the total mass of Andromeda seems to be less (though that may well be within margins of error). From what I have read, my impression is that Andromeda contains a higher percentage of main sequence and sunlike stars and metallicity compares rather well, contrary to the many dwarf galaxies in our Local Group which generally seem to be old and metal-poor. In the Local Group the MW, Andromeda and perhaps M33 are probably the best galaxies for living planets (and planets in general) by far, beyond that Virgo?
@ljk, 18 aug.: love your (Greek) quote!
As Adam also points out, metallicity of (sunlike) stars seems to be a key characteristic for earthlike planet formation, (too) high metallicity leading to high-mass dust disk leading to hot Jupiters, (too) low metallicity and resulting low-mass dust disk to subgiants (and/or dry terrestrial planets?). So this seems to confirm an optimal metallicity range. The essential question then becomes: is such an optimal, sunlike metallicity range typical of sunlike stars?
What still makes me optimistic about the chances of solar system-like planetary systems and (more or less) earthlike planets near sunlike stars, is not so much what has so far been discovered, but rather what has not, in other words the absence of certain results: the fact that so far, despite being the subject of various radial velocity studies, by far most sunlike stars in our galactic neighborhood have not shown any giant planets in close orbit (Laughlin et al.) and the indication by current data (Univ. of San Francisco, Marcy et al.) that, the smaller the planet mass, the more common.
Sometimes we get some essential information from what has not been discovered: of a list of most sunlike stars within 50 ly, that I once compiled (sent it to Paul, I think I will construct a Wikipedia page of sunlike stars within 50 ly), I am pleased to notice, that of the 45 most sunlike stars, only 4 have been found to have hot Jupiters and 2 others have giant planets at distances still ‘too close for comfort’ for terrestrial planets. And, in fact, my ‘personal’ top-12 (I could not make up my mind on the top-10) still all stand undamaged, as far as I know: 82 Eridani, Delta Pavonis, Beta Canum Venaticorum, 61 Virginis, Zeta Tucanae, Gliese 442 A, Alpha Mensae, Zeta(2) Reticuli, Zeta(1) Reticuli, 58 Eridani, 18 Scorpii, Nu(2) Lupi.
With regard to intelligent life, we may indeed, as a good publication title suggests, just be coming of age in the Milky Way.
Not all stars are equal. Stars in the core couldn’t contain habitable planets due to them being to close together, stars in the spiral arms will be exposed to to many Gamma Ray Bursts, leaving a Habitable zone in the galaxy that Sol is in. Then the star has to be the right size with the right elements. Then life has to develop. What would Earth be like without life? Like Venus?
We are alone in the glaxy, but possibly not the local group. Andromeda may have another intelligent species in it.
Tobias, the Sun and our Solar System are already in one of the spiral arms, the Orion Arm. As to other civilizations, there is insufficient evidence to know whether or not we are alone in the galaxy, but it’s certainly possible.
The core of our MW and the inner parts of the spriral arms are probably too reactive for life.
The extent of the Galactic Habitable Zone is not yet well understood, but is may have a lot of overlap with the galactic disc.
Direct Imaging and Spectroscopy of a Planetary Mass Candidate Companion to a Young Solar Analog
Authors: David Lafrenière, Ray Jayawardhana, Marten H. van Kerkwijk (University of Toronto)
(Submitted on 8 Sep 2008)
Abstract: We present near-infrared imaging and spectroscopy of a planetary mass candidate companion to 1RXS J160929.1-210524, a roughly solar-mass member of the ~5 Myr-old Upper Scorpius association. The object, separated by 2.22″ or 330 AU at ~150 pc, has infrared colors and spectra suggesting a spectral type of L4(-2/+1) and a temperature of 1800(-100/+200) K.
The H- and K-band spectra provide clear evidence of low surface gravity, and thus youth. Based on the widely used DUSTY models, we infer a mass of 8(-1/+4) Mjup.
If gravitationally bound, this would be the lowest mass companion imaged around a normal star thus far, and its existence at such a large separation would pose a serious challenge to theories of star and planet formation.
Comments: Submitted to ApJL
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0809.1424v1 [astro-ph]
Submission history
From: David Lafreni\`ere [view email]
[v1] Mon, 8 Sep 2008 20:00:05 GMT (218kb)
http://arxiv.org/abs/0809.1424
Sentient Developments says the Rare Earth concept is a load
of organic waste:
http://www.sentientdevelopments.com/2009/03/rare-earth-delusion.html
Maybe the solar system had a hot gas giant, but it migrated all the way into the sun. Meanwhile, a more distant gas giant (Jupiter itself) formed to cut off the accretion for the protoplanets that had been forming between the inner hot gas giant (now consumed by the star) and Jupiter.
The Flip Side of Exoplanet Orbits
by Jason Major on May 12, 2011
New research reveals the possible cause of retrograde “hot Jupiters”.
It was once thought that our planet was part of a “typical” solar system. Inner rocky worlds, outlying gas giants, some asteroids and comets sprinkled in for good measure. All rotating around a central star in more or less the same direction. Typical.
But after seeing what’s actually out there, it turns out ours may not be so typical after all…
Astronomers researching exoplanetary systems – many discovered with NASA’s Kepler Observatory – have found quite a few containing “hot Jupiters” that orbit their parent star very closely. (A hot Jupiter is the term used for a gas giant – like Jupiter – that resides in an orbit very close to its star, is usually tidally locked, and thus gets very, very hot.) These worlds are like nothing seen in our own solar system…and it’s now known that some actually have retrograde orbits – that is, orbiting their star in the opposite direction.
“That’s really weird, and it’s even weirder because the planet is so close to the star. How can one be spinning one way and the other orbiting exactly the other way? It’s crazy. It so obviously violates our most basic picture of planet and star formation.”
– Frederic A. Rasio, theoretical astrophysicist, Northwestern University
Full article here:
http://www.universetoday.com/85585/the-flip-side-of-exoplanet-orbits/