Author Topic: General SETI Thread  (Read 33337 times)

Offline jebbo

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Re: General SETI Thread
« Reply #140 on: 01/17/2018 10:32 AM »
For the Kepler system, as jebbo just mentioned, its not yet fully clear what this means. But on top of that, there might be an anthropic bias involved (of which we are not yet aware), so that compact systems might be less habitable. There is a similar situation with the Sun not being an M-dwarf although the latter being very common: you could explain this by "luck", but perhaps M-dwarfs are just not as habitable as G-dwarfs.

Given their activity when young, I lean towards the view that planets around M-dwarfs are less likely to be habitable due to atmosphere / volatiles being stripped.  Some might be habitable, but this depends on formation models and the initial volatile fraction ...

--- Tony

Offline Bynaus

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Re: General SETI Thread
« Reply #141 on: 01/17/2018 10:55 AM »
For the Kepler system, as jebbo just mentioned, its not yet fully clear what this means. But on top of that, there might be an anthropic bias involved (of which we are not yet aware), so that compact systems might be less habitable. There is a similar situation with the Sun not being an M-dwarf although the latter being very common: you could explain this by "luck", but perhaps M-dwarfs are just not as habitable as G-dwarfs.

Given their activity when young, I lean towards the view that planets around M-dwarfs are less likely to be habitable due to atmosphere / volatiles being stripped.  Some might be habitable, but this depends on formation models and the initial volatile fraction ...

--- Tony

I lean towards agreeing with you. :) Stripping of atmospheres, combined with tidal lock. While I know that some models suggest that tidal lock and backside freezout is not a big problem if the atmosphere is thick enough, it still erases quite a bit of the habitable parameter space (i.e., if the atmosphere is not thick enough). Combine this with atmospheric erosion and the lack of a strong magnetic field due to slow, locked rotation (further helping with that atmospheric erosion) and we end up in a situation where most rocky planets around M-dwarfs would be poor in volatiles. There might be caveats (e.g., the low density of some of the Trappist-1 planets, although within large errors), and perhaps habitability doesn't really need an atmosphere (-> Europa, Enceladus), but at least Earth-like planets will have a hard time to survive long enough to come up with complex life around an M-dwarf.

Offline jebbo

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Re: General SETI Thread
« Reply #142 on: 01/17/2018 11:08 AM »
Yes, and I'm interested to see whether those low densities hold up when we get RV data (hopefully soonish from SPIRou).  In particular, for "b", which I had pegged as an Io-analogue ...

I love these compact systems with near-MMR resonance chains!

Hopefully TESS will find a few more of them (there's a new TESS M-dwarf yield paper on arxiv today: https://arxiv.org/abs/1801.04949)

Edit: this does relate to the discussion on "mediocrity" and is not just tangential: if the chances of M-dwarf planets being habitable are substantially less than those for FGK stars, the fact that we orbit a G star becomes less surprising, even though most planets may be around M-dwarfs.

--- Tony
« Last Edit: 01/17/2018 11:16 AM by jebbo »

Offline Alpha_Centauri

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Re: General SETI Thread
« Reply #143 on: 01/17/2018 11:13 AM »
You weren't claiming that the Earth or the Solar System were rare, you were claiming that life such as ours is extreme compared to life elsewhere in the universe.  Those are two different arguments.

Simple probability dictates that it is unlikely by chance to happen to find yourself a member of an extreme group, just as it is unlikely to pick out the one red ball in a bag of 9 other black balls, so the default (though not necessarily correct) assumption is that we are unlikely to be that unusual in the context of other life.  This of course is rather negated if we are the first example of sentient life so the bag is low on balls.

This is a separate question to the rarity of the conditions in which we formed.  Think of it like this, you have 5 oases in a 1000 sq km desert.  The chances that the first oasis you come across is hosts radically different life to that in the other four is low, but this has no relation to the rarity of the conditions (an oasis in the desert) that allows any of these ecosystems to develop.

Sorry, I thought the whole point of the Mediocrity Principle is that neither the Earth nor the fact that it has life on it is particularly special.

If every star system in our galaxy has 8 planets with only one of them being habitable, then is the Earth special for being a one-in-eight occurrence or nothing much to write home about because there would be ~100 Billion earths in our galaxy?  You are getting hung-up on the term mediocrity, we are really talking about probabilities.  Probabilities are set-dependant. "Special" is relative to the set you are considering.
« Last Edit: 01/17/2018 11:38 AM by Alpha_Centauri »

Offline Alpha_Centauri

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Re: General SETI Thread
« Reply #144 on: 01/17/2018 11:23 AM »
As for the M-dwarf habitability thing, personally I would first like to see what the inferred volatile content of the first few M-dwarf planets to have their atmosphere probed (and properties well constrained) is like.  We have a lot of educated guesses theory but very little actual observational evidence to back it up as yet.
« Last Edit: 01/17/2018 11:27 AM by Alpha_Centauri »

Offline jebbo

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Re: General SETI Thread
« Reply #145 on: 01/17/2018 11:28 AM »
Yup.  Which is why I brought up TESS and that paper, which has the opening line:

Quote
NASA’s TESS Mission (Ricker et al. 2014) will furnish the vast majority of small, rocky planets for atmospheric study

 :)

--- Tony

Offline M.E.T.

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Re: General SETI Thread
« Reply #146 on: 01/17/2018 11:36 AM »
You weren't claiming that the Earth or the Solar System were rare, you were claiming that life such as ours is extreme compared to life elsewhere in the universe.  Those are two different arguments.

Simple probability dictates that it is unlikely by chance to happen to find yourself a member of an extreme group, just as it is unlikely to pick out the one red ball in a bag of 9 other black balls, so the default (though not necessarily correct) assumption is that we are unlikely to be that unusual in the context of other life.  This of course is rather negated if we are the first example of sentient life so the bag is low on balls.

This is a separate question to the rarity of the conditions in which we formed.  Think of it like this, you have 5 oases in a 1000 sq km desert.  The chances that the first oasis you come across is hosts radically different life to that in the other four is low, but this has no relation to the rarity of the conditions (an oasis in the desert) that allows any of these ecosystems to develop.

Sorry, I thought the whole point of the Mediocrity Principle is that neither the Earth nor the fact that it has life on it is particularly special.

If every star system in our galaxy has 8 planets with only one of them being habitable, then is the Earth special for being a one-in-eight occurrence or nothing much to write home about because there would be ~100 Billion earths in our galaxy?  You are getting hung-up on the term mediocrity, we are really talking about probabilities.  Probabilities are set-dependant. "Special" is relative to the set you are considering.

I think that is what I'm trying to get at. Which is that the definition of the "set" is based on a lot of assumptions, and if you add enough conditions that must be met by the set, the population of the set might well drop very low.

For example. You used an example population of 100 billion stars in the galaxy. Then yes, if that is the extent of the criteria that allows for inclusion in the set, there should indeed be many planets like ours.

However, if we narrow it down to sun-like stars, that drops the size of the population to say 20 billion (random number for sake of argument). If only 10% of those have rocky planets in the habitable zone, that drops it down to 2 billion. If only 10% of those are in the right region of the galaxy, that drops it down to 200 million.

If only 10% of those have plate tectonics, then you are down to 20 million. Add the need for a Moon similar to ours, and it may cut that number by a factor of 1000, bringing us down to 20,000.

These are just random criteria with random probabilities added  for sake of demonstration. But the point is, if you define the "set" with sufficient criteria (many of whom we may not be aware of yet), it is quite conceivable that we may indeed end up being a typical member of a set containing a grand total of 1 star system or fewer.

I think that is basically the Rare Earth Hypothesis just rephrased in fairly clumsy manner by me.
« Last Edit: 01/17/2018 11:42 AM by M.E.T. »

Offline Alpha_Centauri

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Re: General SETI Thread
« Reply #147 on: 01/17/2018 11:50 AM »
Yes that is all well and true but, getting back to the original point, we are unlikely to be an extreme member of the set of sentient life. How rare that set is is dependant on a lot of variables.  If it is so rare you could count the members on your hand it makes little sense to be talking about extremes in the first place.
« Last Edit: 01/17/2018 11:54 AM by Alpha_Centauri »

Offline M.E.T.

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Re: General SETI Thread
« Reply #148 on: 01/17/2018 12:03 PM »
Yes that is all well and true but, getting back to the original point, we are unlikely to be an extreme member of the set of sentient life. How rare that set is is dependant on a lot of variables.  If it is so rare you could count the members on your hand it makes little sense to be talking about extremes in the first place.

OK, maybe I'm arguing about the wrong thing here, then. In my experience the Mediocrity Principle is usually invoked to argue against the idea that life (let's focus on sentient life in this case) is rare in the Universe. I have never seen it used to argue that other sentient life should be fairly similar to us. But I guess it can be applied in that manner too. But sure, that's not the argument I was engaging in.

My focus was on whether there is other sentient life at all.
« Last Edit: 01/17/2018 12:05 PM by M.E.T. »

Offline Star One

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General SETI Thread
« Reply #149 on: 01/17/2018 04:34 PM »
For the Kepler system, as jebbo just mentioned, its not yet fully clear what this means. But on top of that, there might be an anthropic bias involved (of which we are not yet aware), so that compact systems might be less habitable. There is a similar situation with the Sun not being an M-dwarf although the latter being very common: you could explain this by "luck", but perhaps M-dwarfs are just not as habitable as G-dwarfs.

Given their activity when young, I lean towards the view that planets around M-dwarfs are less likely to be habitable due to atmosphere / volatiles being stripped.  Some might be habitable, but this depends on formation models and the initial volatile fraction ...

--- Tony

Well being as nature constantly surprises us, plus these models are still being refined almost it seems on a yearly basis I wouldn’t bet your house on the inhospitably of M dwarf class systems. Especially as we know nothing of the atmospheres of the planets in such systems.
« Last Edit: 01/17/2018 04:35 PM by Star One »

Offline CuddlyRocket

Re: General SETI Thread
« Reply #150 on: 01/17/2018 07:14 PM »
Next, our Sun, based on some cursory Googling, appears to be of a type (G-type main sequence), that only represents 5% or so of all observable stars in the Universe.

'In the Universe' over-estimates the strength of our knowledge of star population distributions. Basically, we can only do surveys within range of our telescopes - which is longer for brighter stars - which we can call our 'solar neighbourhood'. And then we have to add the caveat that our neighbourhood may not be typical - but we can pray in aid the principle of mediocrity there!

So, in the solar neighbourhood, 7.5% of stars are Class G main sequence. Class K are 12% and Class M, 76%. (For completeness, Class F 3%, Class A 0.63%, Class B 0.13% and Class O 0.00003%.) Though Gaia will be doing a more complete survey, so it will be interesting to see if these figures shift significantly.

Offline Star One

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Re: General SETI Thread
« Reply #151 on: 01/20/2018 08:12 PM »
New article from Jason Wright pointing out how SETI is marginalised by other astronomers

http://sites.psu.edu/astrowright/2018/01/20/seti-is-not-about-getting-attention/

Offline ikke666

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Re: General SETI Thread
« Reply #152 on: 01/21/2018 04:47 PM »
a little question  :D is it possible to put seti @home on a raspberri pi? if yes, where can i find the documentation to do so step by step?   ::)

Offline Paul451

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Re: General SETI Thread
« Reply #153 on: 01/29/2018 09:45 PM »
if you add enough conditions that must be met by the set, the population of the set might well drop very low.
For example. You used an example population of 100 billion stars in the galaxy. [..]
if we narrow it down to sun-like stars, that drops the size of the population to say 20 billion [..]
If only 10% of those have rocky planets in the habitable zone, that drops it down to 2 billion.
If only 10% of those are in the right region of the galaxy, that drops it down to 200 million.
If only 10% of those have plate tectonics, then you are down to 20 million. [etc]

Congratulations, you've just reinvented the Drake Equation.



Can someone help me to understand some of the parameters of the Mediocrity Principle.

It's just the idea that across measurable traits, 95% of the time we should be within the middle 95% of measured values. 50% of traits we should be in the middle 50%, 25% of traits we should be in the top 25%, another 25% we should be in the bottom 25%. And so on and so on.

It's... well, a pretty mundane principle.

The thing you seem to miss is that the same Principle says that we shouldn't expect to be "average". By chance alone, you've the same probability of being in the top 10% as in the middle 10%. Measure enough traits and we're going to fall in the high or low end, or dead centre, of some of them simply by chance. The Mediocrity Principle just serves as a warning not to read too much into that.

For example, in our own solar system, we are the only one of eight planets that has intelligent life, most likely the only one out of eight that has any complex life, and as far as we know, the only one with even simple life.

1 in 8. 12.5%. Mundane result.

(I'd also say that 100% of planets within the Sun's habitable zone have life, but that's circular reasoning, obviously. Anthropic principle.)

Next, our Sun, based on some cursory Googling, appears to be of a type (G-type main sequence), that only represents 5% or so of all observable stars in the Universe.

Within the middle 95%. Not "average", but still mundane. (5% also leaves you with 10 billion stars to play with in just this galaxy.)

But that's an arbitrary, human made category. Why only G and not G+K? Or F+G+K? Or why all of G, and not just G2V? This is where having a sample size of 1 hurts us. We know that the range of conditions supporting life, intelligence and technological civilisation includes Earth, but how narrowly is it prescribed around Earth? We don't know.

My point being, under what parameters do we feel that our planet should be considered "Mediocre"? Or is it based more on a philosophical principle, rather than any kind of physical evidence?

In a Bayesian probability calculation, it serves as your prior probability. That's all.

For example: Kepler (and other systems for detecting planets) have a bias towards finding larger and closer planets. So when you look at the data, how do you tell what's significant? You assume that all planets are equally likely, then predict (based on Kepler's bias) what that distribution of planets would look like to Kepler, then you look at what Kepler actually found and note the differences.

Result? Sub-Neptune/Super-Earths (which our solar system lacks) appear to be more common than Earths, and much more common that Jupiters/Saturns. Neptunes are slightly more common than Jupiter/Saturns. Earth-likes (Earth/Venus) are not particularly rare. (Sub-Earths (Mars/Mercury) are still too hard to find to draw a meaningful conclusions.) It looks like there's a weird pattern to the size of sub-Jupiters, they tend to congregate around certain values. Most common are around 2-3 Earth-radii, about 20% of stars. A sharp drop off above 3 Earth-radii (Neptune/Uranus are around 4.) But also a weird drop around 1.8. Then an increase to the next most common value, 1.3 or so Earth-radii, about 10-15% of stars. Then another dip around 1.2 E(r). Then 1 Earth-radius, about 6% of stars. So Earths aren't the most likely, but they are common as muck. One of the three most common sizes of planets. Pretty mundane, you might say.

Other results? Every multi-planet system seems to be fairly unique. Some are dominated by large planets, most by small planets, some have a mix. Some have large outers, small inners, some alternate large/small. So you could say that the Solar System is unique. Or that it's mundane. Depending on how you look at it. Absence of a pattern is the pattern. Definitely not what we were expecting.

And planets in multi-planet systems are typically distributed across what we think is the habitable zone. It's rare to find a system orbiting entirely within, or entirely without the habitable zone.



No papers, but some images that popped up easily...







Offline Paul451

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Re: General SETI Thread
« Reply #154 on: 01/29/2018 09:55 PM »
Oops, missed this one.

In my experience the Mediocrity Principle is usually invoked to argue against the idea that life (let's focus on sentient life in this case) is rare in the Universe. I have never seen it used to argue that other sentient life should be fairly similar to us.

It applies to everything. It's just the baseline assumption behind any traits you are measuring. Any traits. It's not a SETI thing.

I have never seen it used to argue that other sentient life should be fairly similar to us.

They should neither be very like us, nor very unlike us. On a bell curve of key traits, we shouldn't be exactly in the middle, but we certainly shouldn't be out off the end of the curve. OTOH, someone has to be. So measure enough traits and we'll be in the heart of the bell-curve on most of them, on the fringes of a few.

That's all the Mediocrity Principle is saying.

But it means that when someone shouts "Why should They be like Us!" The answer is, "Why shouldn't they?" Because neither has preference.
« Last Edit: 01/29/2018 09:56 PM by Paul451 »

Offline ExoExplorer

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Re: General SETI Thread
« Reply #155 on: 02/04/2018 07:35 PM »
Given their activity when young, I lean towards the view that planets around M-dwarfs are less likely to be habitable due to atmosphere / volatiles being stripped.  Some might be habitable, but this depends on formation models and the initial volatile fraction ...

--- Tony
In fact, recent water loss modelings on planets around M-dwarfs found the lost amount during PMS is rather insignificant. Energy-limited formula predicts no more than a few Earth oceans would be lost. The planet mantle also has the potential to store tens of Earth oceans of water and degasses it back to surface at later time.
Earth was assembled from giant impacts during the first 100 Myr. Each impact was strong enough to melt the mantle and form magma ocean lasting for several Myr, so Earth also spent a lot of time in runaway greenhouse state during the first 100 Myr. The young sun emitted strong XUV and stellar wind that were comparable to or even stronger than MS M-dwarfs, but no evidence supports that Earth has lost a large amount of volatile.

I lean towards agreeing with you. :) Stripping of atmospheres, combined with tidal lock. While I know that some models suggest that tidal lock and backside freezout is not a big problem if the atmosphere is thick enough, it still erases quite a bit of the habitable parameter space (i.e., if the atmosphere is not thick enough). Combine this with atmospheric erosion and the lack of a strong magnetic field due to slow, locked rotation (further helping with that atmospheric erosion) and we end up in a situation where most rocky planets around M-dwarfs would be poor in volatiles. There might be caveats (e.g., the low density of some of the Trappist-1 planets, although within large errors), and perhaps habitability doesn't really need an atmosphere (-> Europa, Enceladus), but at least Earth-like planets will have a hard time to survive long enough to come up with complex life around an M-dwarf.
N2 is resistant to atmospheric collapse (condensing on the dark side). Earth-like atmosphere or 1 bar of N2 is already enough to avoid condensation in the most part of habitable-zone. Indeed, lack of planetary magnetic field poses a threat to habitable planets around M-dwarfs, but the erosion would take at much longer timescale, so it can be replenished through tectonic and volcanic outgassing. The indication of CO2 atmosphere on TRAPPIST-1b observed by Spitzer, if confirmed, would be the evidence of volcanic outgassing secondary atmosphere.
There are actually a few advantages. Tidal locking can extend the habitable-zone by weakening Coriolis force and increasing dayside albedo, so the planets at much closer distance can still possess moderate surface temperature. Tidal heating can provide extra energy and extend the lifetime of geological cycles. M-dwarf planets are much less susceptible to snowball, glaciation and limit cycle (very unstable climate), because M-dwarfs emit much light at near-IR wavelengths.
The low-density nature of TRAPPIST-1 planets, if confirmed, implies they have volatile mass fractions up to a few to tens percent, equivalent to hundreds of Earth oceans (Earth has only ~0.1%). It is debatable that such volatile-rich planets can be habitable. In fact, a planet with Earth-like volatile is indistinguishable from a pure rock planet based on today's technology and radius-mass chart.
« Last Edit: 02/06/2018 12:17 AM by ExoExplorer »

Offline Dao Angkan

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Re: General SETI Thread
« Reply #156 on: 02/04/2018 10:52 PM »
Why would the Coriolis force be significant? Surely in tidally locked planets then the coriolis force would be insignificant.

Offline ExoExplorer

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Re: General SETI Thread
« Reply #157 on: 02/05/2018 12:03 AM »
Atmospheric circulation depends much on the Coriolis force. Strong force would make the clouds banded and distribute them evenly. In contrast, weak force would cause strong rising on the dayside and descending on the nightside. In the later case, optical convective clouds form and cover much area of dayside. They cool the surface down by reflecting over half of radiation back to space.

Offline CuddlyRocket

Re: General SETI Thread
« Reply #158 on: 02/05/2018 07:38 PM »
Why would the Coriolis force be significant? Surely in tidally locked planets then the coriolis force would be insignificant.

Tidally locked planets still spin - once per year, in fact! The significance of the Coriolis force for a tidally locked planet therefore depends on the length of its year. For the seven planets of the TRAPPIST-1 system, the year ranges between 1.5 and 18.8 Earth days. For the innermost planet, the Coriolis force would seem to be roughly as significant as it is on Earth (assuming it has an atmosphere that is!).

Offline jebbo

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Re: General SETI Thread
« Reply #159 on: 02/05/2018 07:56 PM »
Earth was assembled from giant impacts during the first 100 Myr. Each impact was strong enough to melt the mantle and form magma ocean lasting for several Myr, so Earth also spent a lot of time in runaway greenhouse state during the first 100 Myr. The young sun emitted strong XUV and stellar wind that were comparable to or even stronger than PMS M-dwarfs, but no evidence supports that Earth has lost a large amount of volatile.

Figure 1 of https://arxiv.org/abs/1411.7412 suggests otherwise, by large factors.

--- Tony
« Last Edit: 02/05/2018 08:15 PM by jebbo »

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