The dreams of Alpha Centauri I used to have as a boy all focused on visual effects. After all, the distance between Centauri A and B ranges from 11.4 to 36.0 AU. What would it be like to have a second star in our Solar System, one that occasionally closed to a little more than Saturn’s distance from the Sun? What would a day be like with two stars, and even more, what would night be like with a star that close lighting up the landscape? I also wondered about how much effect a second star would have on the planets in our system, curious as I was about gravitational effects and even the possible repercussions for weather and seasonal change.
Image: The Alpha Centauri star system and other objects near it in the sky. Image copyright Akira Fujii / David Malin Images.
You can imagine, then, that Duncan Forgan’s new paper hit close to home. Forgan (University of Edinburgh) has taken discussions of habitability around Centauri B to a new level by analyzing the effect of Centauri A on habitability using latitudinal energy balance models that allow him to study how small changes in the properties of a planet can affect the overall climate there. Such models have been useful in studying things like climate variability due to orbital eccentricity and other factors, and Forgan puts them to work to chart the effect of a binary companion.
Alpha Centauri in the Last Fifteen Years
Before I get into the results of the habitability study, though, I want to go through some of the more recent work on Alpha Centauri, all summarized carefully in the Forgan paper. Indeed, I point you to this paper with great assurance that if you are interested in the Centauri stars, you’ll find a useful bibliography and summary here that will quickly get you up to speed (though the bibliography would be better if it listed paper titles along with the rest of the citations). Let’s run through some of the more salient work — in most cases I’ll skip the authors and citations in this discussion, knowing that Forgan’s work containing all of these is freely available at the arXiv site.
Centauri A and B, being high metallicity stars, are presumably prime candidates for circumstellar disks with a high solid material component, making the building blocks of planets readily available, and deepening the spectral lines for improved precision in radial velocity studies. Another useful factor for observations is that the binary is inclined by only 11 degrees with respect to our line of sight, an important fact because it means that any planets we discover through RV methods will yield a mass that is fairly accurate, assuming that the planets around these stars have formed in the same orbital plane. Without such knowledge, the mass figures from RV studies vary widely depending on assumptions about the target system’s inclination.
Studies on planet formation have shown that both Centauri A and B should be capable of forming terrestrial planets even when the perturbations caused by the binary companion are taken into account. Early studies on this question have found that the planetesimal disks seem to be stable out to about 3 AU of the parent stars, assuming a reasonable inclination of the disk relative to the binary plane, meaning something less than 60 degrees. More recent work by Thébault and colleagues has shown that the later stages of accretion may not be efficient because the binary companion can inhibit the growth of larger objects outside 0.75 AU (Cen A) and 0.5 AU (Cen B).
What does this mean? Most likely that the formation of gas giants is unlikely here (a finding that squares with previous radial velocity surveys), while if we can get past the problem of forming larger planetesimals referred to above, Earth-mass planets should be able to form in the habitable zone of Centauri B, assuming an eccentricity of no more than 0.3. A 2009 study I’m not familiar with by Michtchenko & Porto de Mello makes the case that any terrestrial planets that do form in Centauri B’s habitable zone should be dynamically stable despite perturbations from Centauri A under certain conditions of eccentricity and orbital inclination, but planets with inclinations to the orbital plane larger than about 35 percent should experience strong instability.
So where is the habitable zone around Centauri B? Kasting and team used a model that assumed Earth-mass planets with similar atmospheric composition and found a habitable zone ranging from 0.5 to 0.9 AU, although this 1993 study did not include the perturbing influence of Centauri A. But Forgan notes this with regard to the light reaching Centauri B planets:
If main sequence relations for the luminosity of each object are assumed, the insolation experienced by planets in the habitable zone of ? Cen B due to ? Cen A would be no more than a few percent of the total insolation of the ? Cen AB system at the binary’s periastron, and around one tenth of a percent at apastron. This insolation can be diminished further by eclipses of ? Cen A by ? Cen B, the duration of which is estimated to be of order a few Earth days.
Tuning the Model for Centauri B
Kasting was using a global radiative balance model (GRBM), but he and other researchers later deployed latitudinal energy balance models (LEBMs) of the kind Forgan uses in his new study, the latter being more complex and incorporating assumptions about latitude and season and other properties that would be temperature dependent. Forgan adjusts the model to include the effects of the binary (neglecting the distant M-dwarf Proxima Centauri). From the paper:
A planet in global radiative balance is not in general in local radiative balance, and by extension habitability is not a discrete concept (i.e. either habitable or uninhabitable), but a continuous one, where a certain fraction of the planet’s surface will be habitable at any given time. In the LEBM, the evolution of the planet’s temperature T (?) is described by a diffusion equation made nonlinear by the addition of the heating and cooling terms, as well as an albedo which makes a rapid transition from low to high as temperature decreases past the freezing point of water. As a result, small changes in the properties of a planet can strongly affect the resultant climate.
The latitudinal energy balance model, then, seems the best approach for asking how the perturbations caused by Centauri A might affect planets in the habitable zone of Centauri B.
So what does Forgan find? It turns out that calculating the habitable zone of Centauri B’s inner and outer boundaries can be roughly correct if we leave Centauri A out of the picture — the dimensions of the habitable zone remain more or less the same. But adding Centauri A does create oscillations in the planet’s climate that happen when Centauri A is at its closest to Centauri B. The temperature variations caused by Centauri A are no more than several K, and could alter the fraction of habitable surface on planets at the habitable zone boundaries by about 3 percent, a figure made flexible depending on the size of oceans or planetary obliquity.
The paper goes on to note the possible effect on life (science fiction writers take note):
It is reasonable to speculate that if life were to exist on planets around ? Cen B, that they may develop two circadian rhythms (cf Breus et al. 1995) corresponding to both the length of day around the primary, and the period of the secondary’s orbit (approx 70 years). Altering the available habitat by a few percent may also in?uence migration patterns and population evolution.
Small changes over time, though, can lead to big results, as Forgan goes on to remind us:
While we have demonstrated that the temperature ?uctuations for planets around ? Cen B due to ? Cen A are relatively small, the consequences of a periodic temperature forcing of a few K to long term climate evolution cannot be fully understood from this work. To fully appreciate the impact on (for example) ocean circulation and carbonate-silicate cycles requires further investigation with more advanced climate models.
Simulations and Their Limitations
The paper analyzes the results of the simulation for different classes of planets, from fully habitable worlds to uninhabitable hot planets, uninhabitable snowball planets, and two other classes — eccentric transient planets and binary transient planets — that are both partially habitable, with the habitability oscillating according to either the planet’s orbit around Centauri B (eccentric transients) or the period of Centauri A (binary transients). Forgan is careful to comment on the limitations of the LEBM model, which is not sensitive to long-term climate processes, and he notes that adding clouds and a carbonate-silicate cycle into the mix would potentially extend the outer edge of the habitable zone. Another limitation: The model is not sensitive to planets with extremely slow rotations.
Nevertheless, previous work with such modeling has shown its effectiveness, and the picture of the potential Centauri B system that emerges is one in which habitable worlds could well flourish. On the latter score, one other note:
The inner edge of the habitable zone is less well-de?ned than the outer edge – atmospheric changes could allow liquid water above 373 K, and the runaway greenhouse effect may become important at temperatures nearer 350 K (Spiegel et al. 2008 and references within). In any case, the outer edge is likely to be more interesting from an astrobiological standpoint, as current and future instrumentation will be more capable of prob[ing] spectral features of planets at larger semi-major axes (see e.g. Kaltenegger & Selsis 2010).
What’s needed now, of course, are radial velocity results from the ongoing studies of Alpha Centauri, which will begin to tell us whether or not rocky terrestrial worlds actually exist there. This is tricky work — radial velocity methods are much happier with huge gas giants in close orbits than with small rocky planets, which demand a much longer analysis. The paper is Forgan, “Oscillations in the Habitable Zone around Alpha Centauri B,” accepted for publication in Monthly Notices of the Royal Astronomical Society (preprint) and highly recommended to anyone with an interest in planets around our nearest stellar system.
Any planets around B should experience tides from the interaction of B with A. Such a planet would have lunar-like tides even though it would not have a large moon. The same dynamics would also apply to plate tectonics as well as maybe stabilizing the axial tilt of the planet. Thus, a habitable planet around B might even be Earth-like.
that what i do not get about kepler. Why waste our time finding a good planet a 1000 light years away. When the chance is almost 100 % that alpha centauri b have a habitable planet. Every model say the same that the chance is very high that alpha centauri have life somewhere.
It’s hard to read this without being reminded of Brian Aldiss’ Helliconia. There are so many cultural observations in that book that relate to a binary system. It’s fun reading for anyone vaguely interested in habitability in these systems.
Centauri Dream hits the bullseye with this post: Centauri B: A Close Look at the Habitable Zone. As a gambling man I’ll wager this bet the instant a rocky earth-like planet is found circling Centauri B: “Life will find a way.”
Author: Michael Crichton, in his book, Jurassic Park.
Might I add that with this post is launched the spine of Volume Two in human evolution: developing a good relationship with an alien race.
“Little Baby Steps,” the voice of Sagan says in the movie, “Contact”.
James D. Stilwell
Why waste our time finding a good planet a 1000 light years away.
The purpose of Kepler is to pick a region of the sky and to detect all of the planets within that region that can be detected in order to generate statistically meaningful data on the presence and type of planets. Kepler was designed to find any detectable planets within the chosen region of the sky, regardless of the actual distance. This is the only way to derive statistically meaningful data without having to move the probe to search different areas of the sky (which would be more difficult to do).
Kepler will tell us what percentage of FGK stars have planets and the size and orbits of those planets. This data can be used to extrapolate how many Earth-sized planets lie within 30 lyrs of us.
re henk’s comment above, Abelard Lindsey got to it before I could, and I certainly agree with what he has to say. Kepler is indeed about giving us a statistical perspective on the exoplanet population. It will be up to subsequent missions to target nearby stars. One such mission concept was TESS:
https://centauri-dreams.org/?p=1788
There will be others.
A habitable planet around the nearest star would be like the cosmic gods smiling upon us.
If there is such a planet in that binary system, I’m sure the interactions of the two stars on the climate and exobiology of that world will be thoroughly fascinating. On earth, there are all kinds of complex adaptations made by organisms, such as built-in magnetic sense (magnetoception) as a biological compass. An exotic habitat would have a chain of evolution that we can only speculate on.
You find the planets your detection methods are able to find, which may not necessarily be the most representative examples of the overall planet population, and they may not be the planets henk wants to find.
For starters the transit method which has been used by Kepler is not particularly good for studying individual systems – if your planet is tilted more than a few degrees out of the line of sight then you see nothing. So you do a search of a large number of stars, many of which will be located at a large distance.
And in any case, Alpha Centauri has been searched for planets using other techniques. We have already ruled out gas giants, the detection methods are only now getting to the point where hunting for potentially-habitable planets is feasible. If you hope to find habitable planets there, the current lack of known planets is probably a good thing.
@Abelard: A will have very little tidal effect on B’s planets. The sun is 1 AU from Earth and has less tidal effect than the far less massive moon. “Lunar” tides at B’s habitable planet zone from A which is always > 10 AU away won’t happen.
The Centauri stars are lovely in their own right. It would be nice if they have interesting planets, but no tragedy if they do not. Any interstellar transport system robust enough to remain functional for 500 year passages (for artilects or dormant life) is probably equally effective for 5000 year passages. Best of all, we already know of some planetary systems within 10 parsecs.
Since an Earthlike planet was discovered around a low metallicity star, Centauri`s high metallicity seems less important. Since oceans even out temperatures, transiently or partially habitable worlds would most likely be mostly desert. With something to prevent water vapor from leaving the atmosphere of hot worlds (a protective magnetic field, or high gravity) the result would be a wet sauna, not a Venus. Also, climate change can drive the evolution of intelligence, since when conditions change, it pays to approach objectivity to predict how things work, instead of animalistic/superstitious superficial understanding (assuming the organs to use the ideas practically).
“Every model say the same that the chance is very high that alpha centauri have life somewhere.”
I don’t know ANY model that says this, nor does the article discussed here claim this. There are even models that rule out any Earth-sized planets around binaries like Alpha Centauri. And much more than this would be needed for life.
Although the literature seems somewhat split as to probability of the existence of planets around the Centauri stars, I tend to err on the side of those who believe planets are probably waiting to be found in that environment. The reason has to do with the history of extrasolar planet science and what we have learned thus far. Here is what I have taken away from the past 20 years of planet hunting results:
1. Planets are not just common, occurring around a significant minority of stars. They are very common, occurring around the majority of stars. This we are able to tell even though ALL of the techniques used for finding planets (radial velocity, transit photometry, gravitational microlensing, and especially direct imaging) are far from reaching their theoretical limits!
2. Planets form in a dazzling variety of environments. They have been found around old stars, young stars, metal-rich stars and metal-poor stars, pulsars, small stars and large stars, very closely spaced binaries, moderately separated binaries, and widely separated binaries.
Not only do the current results favor the existence of planets in the Centauri system, they are going along way toward resolving a crucial astrobiological issue—determining the number of potential starting points for Life in the Universe. I suspect that over the course of the next decade we will have proof that terrestrial roughly earth-sized planets exist in the habitable zones of FGK stars in copious amounts. This will represent a monumental step forward in getting to know the Universe in which we dwell and it will clear the stage for solving the next part of the puzzle: finding Life.
We may indeed find an earth or super-earth in the habitable zone of Alpha Centauri B in the near future, but “will life find a way” in such an environment? I would like it if the Universe were brimming with Life and there exist biospheres with creatures of myriad shapes, sizes, dispositions, intellects, etc. My belief may or may not match up with reality. Alas, the next term in the Drake equation after Fp Fl may be close to O and we have to be prepared for the possibility that the Universe may not only contain no other intelligent Life but also no other Life period. I would like to coin a new term here in Centauri Dreams to describe this bleak situation, to describe a new class of extrasolar planet whose type I hope is not out there….alas, the Universe may be filled with what I shall call Monodian class earth-like planets.
This hypothetical class of planets, Monodian Earths, I define as planets meeting the following conditions:
1. Rough earth-size: 0.5 to 2.0 Re.
2. In the habitable zone of FGK stars.
3. Surface temperatures, pressures, and geology roughly similar to earth.
4. Water rich, rich in organic matter.
So, in effect, this type of planet may well have oceans and a nitrogen, CO2, and water vapor atmosphere—perhaps such a world will be found to orbit Alpha Centauri B. Human explorers of such a world would experience surface temperatures and atmospheric pressures familiar to their home world. The big difference: the human visitors would need Oxygen tanks/masks in order to breath and the landscapes, lakes, and seascapes of these, if you will, cosmic teases, would be as sterile as an autoclave. Essentially, these orbs would be just what the late 20th century French biochemist Jacques Monod would have expected to exist and perfectly in line with his famous quote regarding the prospects of finding biology in the larger Universe beyond Earth:
“…The universe was not pregnant with life nor the biosphere with man. Our number came up in the Monte Carlo game…”
Those who currently argue in favor of a Universe filled with sterile planets often cite the following:
1. There is no evidence of multiple origins of Life on our own planet. If live arose easily, then why haven’t we found other types of Life with biochemistries different from the familiar one.
2. Laboratories studying the origin of Life have not produced chemically evolving systems. Creating life from scratch in the near future, although exciting, will be done in highly unnatural circumstances and would only qualify as an instance of human intelligent design rather than the disorderly situation on prebiotic earth.
3. The existence of extreme Life does not have any bearing on the issue of whether non-living matter transformed into living matter in the first place. It only says that once Life arose it became amazingly adaptable.
Paul, where do you come down on the issue of Fl? Do you expect a Universe of Monodian class earths or a Universe teeming with Life? Do you think the Copernican Principle applies to biology, as many of our discussions here at Centauri dreams deal with trying to reconcile the Copernican Principle with Fermi’s Paradox?
Couldn’t there also be an Earth-like planet or two orbiting Alpha Centauri A?
Is this circadian rhythm you speak of similar to the rhythm in the strength of earth’s magnetosphere relative to its orbital eccentricity? Where are we in the period and what variables, if any, control this aforementioned rhythm? Furthermore are the variables, given they exist, controls as popularly purported to be or do they exist as things one can exert some control over? Maybe we can ponder these questions during the designs of future space travel modes and geoengineered system controls.
@spaceman: Very well said. We will know fairly soon, because the detection of oxygen in the atmospheres of exoplanets should not remain beyond reach of astronomers for long. Let me also add that while Monodian planets are certainly less interesting, they can be seeded and in due time become the oases of life that we wish they were now.
Do you think the Copernican Principle applies to biology, as many of our discussions here at Centauri dreams deal with trying to reconcile the Copernican Principle with Fermi’s Paradox?
Bacterial life is common. Complex (Eukaryote) life is likely rare.
To spaceman, you mention at the beginning of your post that you’re convinced that planets are very common. Why? Because this is what the observational evidence indicates.
Just a few decades ago some astronomers still asserted the premise that planetary formation was a rare freakish occurence caused by for example a near miss between two passing stars drawing material from the stellar surface to later form planets. And why not propose this? There was no evidence of extrasolar planets, none at all.
Now we have abundant evidence of exo-planets and more evidence arriving all the time. And yet no astronomer can describe precisely the formation process of planets from start to completion in anything like a convincing manner. Too many gaps in our knowledge. In some ways planetary formation still seems to be unlikely occurences. And yet out there, planets by the billions, pretty much everywhere, as you pointed out at the start of your post.
So now we have our present situation where there is no evidence for alien life. None at all. Not even in our own solar system. Abiogenesis is not understood at all either. Biologists can’t describe how life originated with any degree of confidence. Except maybe to state how unlikely it appears to be. And why shouldn’t they? There is no evidence for alien life.
But absence of evidence isn’t necessarily evidence of absence as shown by our recently acquired evidence for the long speculated existence of exoplanets.
To paraphrase the late Allan Sandage I’d like to see what the universe says about this. I truly hope that the money will be found in my lifetime to launch some kind of space telescope with the ability to detect bio-markers ( if they are there) on a few nearby exoplanets. I’m getting old and impatient.This speculating gets tiresome and I crave to see some evidence that will help us better understand the answer to this monumental and historic question.
@spaceperson. Agree that terrestrial planets in HZs will likely be found to be common, not rare. This space cadet admits that science has no evidence refuting the Monodian Earth hypothethis. It could well be so based on what we know today.
However as a life long space cadet I ‘believe’ that the universe teems with life. How complex that life is, is another matter altogether.
This is the reason why I think complex (Eukaryote) life is rare:
http://journalofcosmology.com/Abiogenesis107.html
The Hydrogen hypothesis of endosymbiosis is the most plausible explanation for the origin of Eukaryote life. Currently known evidence suggests that it was a singular rare event that occurred only once in the history of the Earth.
If Nick Lane’s reasoning is correct (and I see no reason to believe otherwise) it is quite likely that the Earth is the only planet with complex life in our galaxy. Bacterial life, on the other hand, is probably quite common throughout the galaxy and universe.
I also think our future in space is in O’neill style habitats. We will need to learn how to make and live in these in order to do interstellar travel. Once we get used to this kind of living, many people will not return to planetary surface life, even if habitable planets are found and we can get to them.
Also, an Earth-sized planet in the HZ does not necessarily mean habitable. It could have a 5 bar, CO2 rich atmosphere. We would have to bio-engineer ourselves to live in that atmosphere without having to wear breathing apparatus or complete pressure suits. The rotation period could be, say, 13 hours and 17 minutes. Or it could be 87 hours. Both of these would play hell with one’s sleeping cycle without bio-engineering.
If Nick Lane is wrong and complex life is not that uncommon, any alien complex life will be inedible and perhaps poisonous to humans. Again, we would have to bio-engineer ourselves to live on such a planet.
I’m reading a 20-year old SF anthology called “Murasaki”, where humans (about 2 centuries from now) discover a double planet system orbiting an M star about 20 lyrs away. Both of these planets are considered “Earth-like” but suck really hard to live on. My thought while reading the novel is, instead of trying to settle these planets (which have sentients on them, by the way), would it not be easier and more comfortable simply to build lots of O’niell habitats and then make short visit to the planets for study?
Abelard Lindsey writes “Bacterial life is common. Complex (Eukaryote) life is likely rare.”. This inspires me to a way to restate the Fermi paradox in a non anthropocentric way.
Put the average number of primitive life carrying suns in a Milky Way sized galaxy at p(1) and put the probability that primitive life on an average planet can evolve into a technological civilisation at p(2). Now, as all here know, once such a society exists it is extremely difficult to prevent its rapid spread to every star in the galaxy that is even modestly capable of supporting its sort of life. From data we already have, we have to multiply p(2) by at least a billion (if all higher life turns out to be addicted to just planet dwelling), and, more likely, by a few hundred billion when comparing the number of spontaneously generated primitive life infected planets, to technologically infected planets. So p(2) has to be a less than one in a billion to a hundred billion in order for primitive life to be more common that complex life.
That we find that we are the only technological civilisation on our planet is just the human version of The Fermi Paradox. Given a belief that evolutionary progress does not have a probability of much less than one in a billion, we can restate this statement to apply to all biosystems, and not just our own.
Bacteria grade life is rare. Complex life (associated with technological civilisations) is common!!
@Mike: I think your analogy between planets and life is a bit of a stretch. In particular, I would expect some planetologists (?) to protest the proposition that they do not know more about the formation of planets than we know about the formation of life. They have some fairly good models that explain a lot, and many of them predate the actual discovery of exoplanets. In contrast, we have to rely mostly on handwaving to explain the formation of life.
Another common fallacy hinted at in your comment: Because the naysayers on planets were wrong, the naysayers on life must be wrong, too.
Anyway, I totally agree on letting the universe have its word, and I am optimistic that this will happen in the next decade or two, as we master exoplanet spectroscopy and start to answer the question of whether there are other oxygen atmospheres out there.
Endosymbiosis is extremely common. A close analog to the mitochondrion is the chloroplast. I fail to see what should make the evolution of Eukaryotes (or something like them) rare.
That’s still habitable.
Still habitable.
Still habitable.
Habitable and “comfortable for humans” are two completely different things.
Endosymbiosis AFTER the evolution of phagocytosis is easy. There have been multiple examples (the chloroplasts alone represent several different instances of endosymbiosis).
Endosymbiosis WITHOUT phagocytosis, on the other hand, might be hard. The problem being that phagocytosis is a very energy intensive adaption, and might well have REQUIRED mitochondria to already be present before it could evolve. If that is the case, then the very first endosymbiotic event, the one that created the mitochondria, could not have arisen as a result of phagocytosis, and might have been a rare, rate-limiting event.
However, this:
does NOT lead, logically or probabilistically, to this:
The endosymbiosis that created eukaryotic grade life on earth may well have been indeed an event rare and unlikely enough to have occurred only once in the history of life on earth.
It DOES NOT FOLLOW from this that earth is “quite likely” (or ANY OTHER probabilistic statement) to be the only planet with complex life in our galaxy.
The observation that something happened just once on earth only means that the cumulative probability of that event happening on an earth-like planet at least once in 4.5 billion years is 100% or less.
In other words, it could STILL BE 100%. As in the probability is such that EVERY SINGLE EARTH-LIKE PLANET AS OLD OR OLDER THAN THE EARTH will have had it occur at least once.
It could also be that the probability is such that EARTH IS THE ONLY PLANET IN THE ENTIRE HISTORY OF THE UNIVERSE where this event has happened.
The observation of an event happening only once in earth’s history does not limit the probability to anything more exact than this range. In other words, it is utterly useless and inaccurate to say anything at all about the probability of some aspect of life in the universe using only an observation of earth as supporting evidence. It simply cannot be done.
Both the statements “life (or ANY individual aspect of life, intelligence, technology, civilization, etc, you care to name) must be very common in the universe because on earth we see X” and “life (or any aspect thereof) must be very rare and almost exclusive to earth because on earth we see Y” are unsupportable. Both statements are simply an expression of wishful thinking. If you really WANT there to be life elsewhere in the universe, you might gravitate to the first. If you really WANT life on earth to be unique, you might prefer the second.
The evidence obtained from one planet CANNOT distinguish between the two. And even if we somehow learned EVERYTHING about life on earth, we STILL would be no closer to determining this probability. You cannot determine a probability with a denominator of one.
The ONLY way of actually figuring out a valid probability is to find another habitable planet and check to see if eukaryotic grade life (or whatever aspect of life you are interested in) exists there or not, and compare it with the example seen on earth.
We also cannot say that just because on earth we observe that a eukaryotic grade of life is necessary for multicellularity, intelligence, and technology to evolve, that will necessarily be the case elsewhere.
The eukaryotic state may have been just one of several available solutions to the energy problem for increasing complexity. It may even have been entirely contingent on the specific nature of how abiogenesis occurred on earth (ie the use of chemiosmosis across membranes for energy generation) which may or may not turn out to be a universal requirement.
It may be that, GIVEN chemiosmotic respiration across membranes as the primary source of energy, prokaryotic life on earth could only evolve higher complexity after evolving an eukaryotic grade of organization. And it may be that an eukaryotic grade of organization is ONLY possible with a rare and unlikely endosymbiosis event, such that, once chemiosmotic prokaryotic LUCA conquered the planet, complex life on earth could ONLY arise via endosymbiosis.
But are we so sure that chemiosmotic respiration is the only way that life can gain independent energy production? That a chemiosmotically respiring prokaryotic-grade cell is the only type of lifeform that has a chance of becoming the LCA of all subsequent life on a planet?
In short, what features of life on earth are universal (or necessary) to the phenomenon of life, and what features are actually incidental to earth’s unique contingent history? No amount of study of earth alone will give us the answer to this question, and without this it is impossible to answer any questions of probability whatsoever on the universal or galactic scale. The only way forward is to find another habitable world and compare it to earth.
I am not optimistic about exoplanet spectroscopy answering questions regarding biomarkers in planetary atmospheres even by 2030. Perhaps the nearest life infested planet is yet too many LY away for 1st generation instruments like the proposed but nonexistent TPF to analyze. But most importantly, such projects are NOT funded. Worse yet the US Decadal Astronomy Advisory group (forget the actual name) did not recommend TPF or anything like it for funding this entire decade. No, zero Kepler or other type planetary detection primary missions to be funded. Equally depressing is the reduction in funding for NASA planetary missions announced just this week. Progress cannot be assumed to be guaranteed.
@spaceman
Quite a good post. The Monodian hypothesis has as much scientific validity as any other at this point. The odds of abiogenesis on a Monodian class planet are truly unknown. Any value from 1 to <10^-22 is possible. Personally, I favour very high odds based largely on the Copernican principle, but that could be wrong.
The Fermi paradox only adresses the failure to detect alien spacecraft visitations to Earth and the failure to detect civilizations beaming profligate quantities of EM energy into to void for altruistic reasons. There could even be a planet circling Alpha Centauri B with sentients with a Victorian level of technology and we would have no means of detecting even the planet, much less the technology. So evidence for the non-existence of life, even quite complex life with rather advanced technology, is also completely lacking.
We do know that life, and complex life, happened at least once. We do not yet know that fusion reactors can ever be made to work. And if ITER is ever built, and a follow-on commercial scale demonstrator is ever built, will that plant in its lifetime make enough energy to finance the cost of its production, including the mining of all the rare earths for the magnets and so on? What the Fermi paradox might be telling us is that sustainable fusion power, with a positive energy return on energy invested is actually impossible. Thus no civilizations anywhere with surplus terawatts to pour into space transmissions or space travel.
@Abelard Lindsey – 5 bar without a pressure suit? Been there, done that, it was quite pleasant, we call that scuba diving to 40 metres with a wetsuit. In fact people have lived and worked offshore in conditions over 50 bar without a pressure suit! That is called saturation diving. I also have a different opinion about the odds of life making the bacterial to eukaryotic transition. I think it is pretty likely after you have an oxidizing atmosphere. After the Earth had the Great Oxygenation Event, and recovered from the resulting Snowball Earth glaciation, eukaryotic life booted up pretty quickly.
@Eniac, The hydrogen hypothesis of symbiosis does not posit that endosymbiosis is rare. Instead it puts the energetics of a cell whose high metabolism and size make it capable of energetic acts of phagocytosis first needs an abnormally large source of energy, that for complex life on Earth was only obtained by urkaryotes association with alpha purple bacteria (mitochondria). It sort of works but assumes so much that it is doubtful that it is the only path to higher life.
@Rob:
But, what then makes it rare? The fact that endocytosis somehow requires too much energy for prokaryotes? A chicken and egg thing? I think it is a blatant baseless assumption that phagocytosis requires more energy than a prokaryote could muster. It is much more reasonable to say phagocytosis is a form of predation, and predation makes sense only when there is an oxygen atmosphere to make the ingestion of biomass energetically favorable. Thus, the Great Oxygenation is to be seen as the trigger for these developments, not waiting for a chance event.
Amphiox:
I agree with pretty much all you say. Actually, I would go further: Just because something appears to have evolved only once, does not mean it has. I don’t find the way Lane dispatches extinction convincing. Any sufficiently disruptive advance will be found in only one version after sufficient time, i.e. become universal. We see this with proteins, DNA, photosynthesis, many metabolic processes, even more innocuous things like bilateralism among higher animals. I am not convinced that the universality of Eukaryotes is any different from these, or less likely to happen.
In my opinion, the disruptive advance that drove Eukaryotes to conquer the planet was one or both of two things: 1) Their genomic organisation, providing a way of packaging DNA so that arbitrarily large amounts could be transmitted, or 2) Mitosis and the advent of true sexual reproduction, which changes the very nature of evolution. The two may or may not be connected. All the remaining characteristics of Eukaryotes, in my opinion, are either riders (i.e. happened to be characteristics of the Eukaryotic progenitors) or consequences (i.e. were enabled or encouraged by the larger genome or the more efficient natural selection).
Oops, that should be Meiosis, not Mitosis under 2)
Joy said, “There could even be a planet circling Alpha Centauri B with sentients with a Victorian level of technology and we would have no means of detecting even the planet, much less the technology. ”
I now have the incentive for an interstellar mission to B. Imagine the reality TV ratings for Alpha B’s version of Downton Abbey! Finally the at large public would support space exploration.
How long has Alpha Centauri A got left before it enters the red giant phase, and what will that mean for any life on B’s planets?
A quick wikki search reveals that eukaryotes came about as long ago as 2 possibly 2.5 billion years ago. About half the time that life has been present. There are so many variables. Adaptation to land may have been critical, as could sexual reproduction or something as unrelated as the battle between the viruses and uni/multi-cellular life forms…
As I understand it, Alpha Centauri A and B both share similar issues from the standpoint of planet formation. So I’d guess that both stars have roughly similar likelihood of hosting a habitable-zone terrestrial planet. There is more of a focus on Alpha Centauri B because it has significant advantages in terms of our ability to detect such planets. Remember, you only detect the planets you can detect.
@Mike – Alpha Centauri A may have planets in its HZ; they’re just harder to detect via the radial velocity method because A’s spectra is noisier than B’s, making detection harder.
@kzb – The Alpha Centauri system is roughly as old as ours (I’ve seen values as high as 6 BY, but most cluster around 4.5-4.8 BY) so Alpha Centauri A probably has a billion or so years before it becomes a red giant.
A good site to follow is Greg Laughlin’s systemic (http://oklo.org/).
Eniac says of eurkaryote origins “It is much more reasonable to say phagocytosis is a form of predation, and predation makes sense only when there is an oxygen atmosphere to make the ingestion of biomass energetically favorable.”, and that is an interesting contrast with the hydrogen hypothesis of end symbiosis.
The hydrogen hypothesis of endosymbiosis puts the mitochondrial ancestor as an anaerobe, some descendants of who became aerobic after phagocytises, and others degrading (and metabolically reverting?) to hydrogenosomes. Under its auspices the eukaryote origin did not wait (here we must strengthen that to could not wait) for the oxygenation event. In its defence we can point to the photosynthetic generation of oxygen accelerating after eukaryotes plants joined the game.
Eniac says of eurkaryote origins “It is much more reasonable to say phagocytosis is a form of predation, and predation makes sense only when there is an oxygen atmosphere to make the ingestion of biomass energetically favorable.”, and that is an interesting contrast with the hydrogen hypothesis of end symbiosis.
I don’t think you can say that.
Phagocytosis, is just as likely to have come about as a form of extracellular sensing mechanism or recycling of membrane proteins or removing and isolating extracellular toxins.
FrankH
Here it claims that alpha centauri system is between 5.6 to 5.9 billion years old
http://arxiv.org/abs/astro-ph/0611733
here is a other site that give information about alpha centauri
http://www.solstation.com/stars/alp-cent3.htm
on wikipedia the article about alpha centauri say that the age is 4.85 billion years old.
but when you read the article further it say under the Alinea Binary system
Stellar evolution theory implies both stars are slightly older than the Sun at 5 to 6 billion years, as derived by both mass and their spectral characteristics.
so it contradicts itself
Both Eniac and tesh imply that the advent of sexual reproduction might have been the most important step to the development of higher life. I feel it timely to point out that the benefits of sexual reproduction have been oversold. For example, although it is good at forming new great gene combinations, it is even better at destroying them. Actually, I can’t think of a single advantage of sexual reproduction in a population of infinite size.
General (general = applies to all life , not just terrestrial versions) disadvantages include
1) it is more complex so typical infertility rates will be higher
2) elaborate apparatus needed to control gene dose while still shuffling genome
3) energy must be expended on finding mates with accompanying dangers
4) powerfully inhibits single cell to multicelled transition
5) drive towards dioecy must be resisted or fertility further drops by half
General advantages
1) Muller’s ratchet is significantly weakened in populations of low size.
@Rob, I think you are way underselling sexual reproduction. The key difference, I think, is that a given diploid organism has an entire population in its ancestry. A haploid organism only a single line. Since ancestry is the “reservoir” from which genes are combined, the effect on evolution is more likely to be profound rather than marginal.
The many disadvantages you mention just underscore the point, since practically all higher organisms take them in stride. They MUST be balanced by substantial advantages, and while there is no real consensus what those are, they nevertheless exist.
I’ve done 5 bar without a pressure suit as well. The last time was in Costa Rica. However, you would still have to use breathing apparatus to walk on a planetary surface with such an atmosphere. Yeah, you can do saturation diving at much greater pressure. But I don’t know if you can live indefinitely at such pressures.
I stand by my comments about the emergence of the Eukaryote being a singular rare event. One of the reasons why it was such a rare event is because the composite cell had genes that were not necessary for its survival until the Oxygen environment was created, which took millions of years. Bacteria are under strong selective pressure to shed superfluous genes in order to optimize proliferation rate. This is why bacteria tend to have minimal genomes. The initial hybrid bacteria (which lead to the Eukaryote cell) was not optimized in this manner. Thus, its survival was a matter of extreme happenstance.
BTW, diving at 5 bar pressure (40 meters down) can only be done for a limited time (1-2 hours) before you start to have Nitrogen narcosis. Also, the partial pressure of Oxygen is way too high for optimal health. This is why Helium-Hydrogen gas mixtures are used by people who do this diving commercially or who have lived in the undersea habitats like SeaLab and the like.
I’ve seen various estimates of the age of the system, ranging from 200m to 1bn years older than the sun. Since “A” is 1.5 times as luminous as our sun, I would have thought its red giant stage was either starting now or at least imminent, depending on its precise age.
@Abelard Lindsey: the tidal effect of Alph Cen A on a planet near B at closest approach would only be about 0.8% of that of our sun on the earth.
The Habitable Zone Gallery
Authors: Stephen R. Kane, Dawn M. Gelino
(Submitted on 10 Feb 2012)
Abstract: The Habitable Zone Gallery (www.hzgallery.org) is a new service to the exoplanet community which provides Habitable Zone (HZ) information for each of the exoplanetary systems with known planetary orbital parameters.
The service includes a sortable table with information on the percentage of orbital phase spent within the HZ, planetary effective temperatures, and other basic planetary properties.
In addition to the table, we also plot the period and eccentricity of the planets with respect to their time spent in the HZ. The service includes a gallery of known systems which plot the orbits and the location of the HZ with respect to those orbits.
Also provided are animations which aid in orbit visualization and provide the changing effective temperature for those planets in eccentric orbits.
Here we describe the science motivation, the under-lying calculations, and the structure of the web site.
Comments: 6 pages, 3 figures, accepted for publication in PASP
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1202.2377v1 [astro-ph.EP]
Submission history
From: Stephen Kane [view email]
[v1] Fri, 10 Feb 2012 22:27:59 GMT (23kb)
http://arxiv.org/abs/1202.2377
Paul, excellent post and very interesting referred article.
amphiox (February 19, 2012 at 22:30 and February 19, 2012 at 22:41): very well said! Too many blunt statements both ‘pro’ and ‘con’ appear based on unknowns and a sample of 1.
All we can say now is that life is likely to depend on, or at least likes more or less earthlike planets, water, carbon and a certain (not too extreme) temperature range, and since those conditions are very common in our galaxy and the universe, there are no fundamental showstoppers so far.
spaceman (February 18, 2012 at 16:44): the type of planet you describe and call Monodian, is in fact a primordial earthlike planet. Although I also hope with you that life will appear to be common, even if it is not, but such Monodian planets are very common, it is still good news: lots of (potentially) habitable planets for us earthlings to be settled, terraformed and seeded with (genetically adapted) life. In fact, the moral issue of how to deal with inhabited planets would largely disappear.
Or, as our famous former Dutch soccer player Johan Cruyff used to say: “every disadvantage has its advantage”.
Abelard Lindsey: as I have argued several times before in various posts, I do not think that the future will be O’Neill like space colonies, at least not as primary habitats for a species that is able to travel interstellar. Apart from the fact that we humans and probably also other planet-originating species simply like planets, the way people who can afford it like the green and quiet suburbs (it is literally in our genes), space colonies are very risk-prone and very, very expensive per area unit and time unit. Once you manage to reach them, planets are dirt cheap.