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Our Solar System Is Even Stranger Than We Thought (scientificamerican.com)
331 points by extraterra 4 months ago | hide | past | web | favorite | 139 comments

> To test whether the peas-in-a-pod pattern was real, I concocted (on my laptop) imaginary planetary systems in which the sizes of planets orbiting a given star were random. Could some sort of bias in Kepler’s method of finding planets—which favors the detection of large planets close to their stars—contrive to make the planets in each of my imaginary systems appear to fit the pattern?

> The answer was no: in more 1000 trials with randomly assigned planet sizes put through a virtual Kepler’s detection scheme, a pattern of similarly-sized planets in the same systems never emerged. This computational experiment did not reproduce what we observe in the Kepler planetary systems. Thus, the regular sizes of planets is a real astrophysical pattern.

This is actually a really cool use of computer simulations that I haven't noticed before (but I'm sure has been done in the past): simulating expected output for a known bias.

I wonder if it would be feasible to do the reverse too for a kind of parameter fitting to fix our model:

1. Create a reasonable approximate model for both Kepler's expected detection bias and for planet size distribution.

2. Constrain parameter space for these models as much as possible, to minimize the search space.

3. Using these models, generate a set different planetary sets for different parameters.

4. Find the outputs with the closest match in statistical "shape" of the actually measured data.

5. generate new input parameters based on these output that lead to set outputs to refine shape until certain level of matching has been found

If done naively this would probably take a lot of computation and result in large a solution space: the more parameters, the higher the conditionality of this space. So the models should be approximations with as few parameters to produce reasonable results, and with constraints on said parameter values based on known physics. This would narrow down both the parameter space to search the possibly valid outputs of this space.

Based on these parameters differ from the output we had expected before, we might then get a direction of where to look for the physics that does explain the distribution.

This kind of stuff is regularly done at LHC - they simulate the detectors and the collisions, and then compare real data against that, to both confirm that the detectors are working properly, and for searching of new physics.

Also in the LIGO project, it's a branch of astrophysics called numeric relativity. The gravity wave signatures predicted by the simulations was what helped confirm the LIGO detections of black hole and neutron star mergers.


Not only LHC, this has been the standard procedure in high energy physics since at least the early 1970ies.

If you want more, search about "inverse problem"

Thanks for the tip! From the wiki page it does look like the thing I'm describing, and I guess I've encountered it many times but never interpreted it as such.

[0] https://en.wikipedia.org/wiki/Inverse_problem

Really cool, this is the first problem that popped into my head upon reading the headline. Currently the discovery of planets is biased so drawing conclusions seems flawed.

So, the Kepler method of detecting planets only works if the orbital plane (maybe not the right term) of the star we are looking at is such that the planets are between us and the star, right? So if we are looking down at the "top" of the star then no planets will obscure the star, right? Also the planets need to be passing in front of the star at precisely the right time that the planet and star and our telescope are all lined up more or less. Someone looking at our solar system would need to wait quite awhile to see Pluto pass by. If all of that is correct, how are we able to see any exoplanets? My layman's guess would be that it would be incredibly unlikely that everything would line up so that we could see anything. What am I missing in this picture?

There are several methods of detecting exoplanets[1], the method you're referring to is the easiest and is called "transiting". Since it's the easiest, most exoplanets have been discovered with this technique. As you've guessed, it is highly unlikely for the stars and planets to align this way, and the general hypothesis as to why we've discovered so many is that there's a lot of planets out there[2].

[1] https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanet...

[2] https://www.nasa.gov/home/hqnews/2013/feb/HQ_13-057_Kepler_T...

There's an awful lot of stars, it would be very unlikely that we look at all of them from the top down where we could not detect a planet passing in front of the star.

Kepler glares at single piece of our galaxy and monitors about 150,000 stars simultaneously.

Kepler is nice for finding large planets that orbit their stars in days.

If you wanted to find planets like Pluto using the transit method, you'd have to look at its star for about 750 years (generally one wants to observe at least 3 transits) - albeit Pluto being even smaller than our own Moon, I'd guess it could not be detected.

I don't stay up on the latest developments in this field day to day, but my understanding is that most of the extrasolar systems that we can observe are a little on the wonky side to begin with, otherwise we wouldn't be able to detect that there are planets in them at all. Super-Jupiters orbiting close enough to make their stars wobble, binary star systems, all kinds of stuff. I'm not sure I'd hazard a guess at any kind of definite conclusion at the makeup of the universe and our overall specialness in it, with tools of observation as crude and limited as we've still got; there's a long way to go just to survey what is out there.

I believe the article attempts to account for your concern.

The author may well have found a real pattern, but it may only apply to a subset of planetary systems. Kepler is biased towards detecting large planets with short orbital periods. Perhaps tightly packed systems have similar sized planets because this is a more stable arrangement. However, more widely spaced systems - which we have far less data on - may not show this pattern.

We're learning something here but we don't really have enough information to know how remarkable our solar system is.

Humanity has only been searching for exoplanets for a very short time. That's why there are relatively few candidates farther than 1 AU out. It's somewhat like arguing that all lattes are comprised of milk only because we've only taken the first sip.

I'm not sure why you were downvoted, that's a beautiful analogy.

I wonder which planets Kepler would see if someone used it to look at the Solar system from a far distance.

It's regularly detecting exoplanets down to slightly less than Earth-sized now, so assuming the orbits happened to be aligned the right way to make transits detectable at all, it should be able to detect all the planets except Mercury and Mars. Our hypothetical alien astronomers would put us down as a six-planet system, possibly worth a paper or two and a page in Alien Wikipedia :)

> It's regularly detecting exoplanets down to slightly less than Earth-sized now...

But at what distance from the host star can it detect planets of this size?

That's pretty comforting. As the solar system we live in seems more unusual it seems more credible that planets with advanced life might be rare. In which case the emptiness of the galaxy seems more explicable.

Putting the Great Filter not merely behind us, but way behind us, would be a pretty cool scientific result.

Although it's science fiction, the second book from the "Three Body Problem" series by Cixin Liu offers an intriguing idea he labels as the "Dark Forest". The gist is that the primary motivation for all civilizations is survival and other civilizations are a threat. This Quora answer gives more detail, but I highly recommend reading the books; there are many compelling science-based ideas within it.


That's one way to go about it but it might be a bit simplistic and there's no reason why cooperation wouldn't work out for some civilizations as well. To completely hide one's signals is both hard to achieve and not necessarily beneficial. For example in ancient times there were blooming trades between empires which would not have been possible if every civilization just thought about hiding themselves. I was not a fan of "Three Body Problem" from its beginning.

Why would anyone want to spoil the books for themselves?

I spoiled this book for myself, and it was still fun. There was a lot of dramatic tension as I waited for all the characters to realize the horrifying things that lay in wait.

There may be more than one Great Filter. We could have passed one of them, but we might not survive the next one.

The point of the Great Filter argument is that something seems to be preventing widespread intelligent life. If the Filter is behind us, then while that is not proof that there are no impassible obstacles in front of is, we have no particular reason to believe that there are impassible obstacles in front of us. That is in stark contrast to what has been the scientific dogma for the past several decades, which is that there is nothing special about us whatsoever, which does give us reason to believe there are impassible obstacles in front of us. To be able to put the filter way behind us is still a real change.

It depends on how fully the initial Great Filter explains the lack of observed exolife. It may not be a binary thing.

The entire point of a "Great Filter" is that there is one that wipes out the majority of species that make it that far. If there are multiple then they're just likely cataclysmic events.

No, the Great Filter is more general than self-destruction. Some examples I've seen of possible Great Filters we've already passed include:

* The formation of any life at all

* The jump from single-celled life to multi-cellular organisms

* The ability to create/use/pass on knowledge of tools

There could be one factor, or several. What matters is if the species can or cannot survive at the end to spread out or at least to make their presence detectable.

This aggregate of 'filters' is what's important.

The article is interesting but I don't think we can draw too many conclusions yet due to Kepler's observational biases. For example, the similar size pattern seen may only apply to closely packed systems rather than more widely spaced ones, and we don't have a great deal of information on the latter type.

Only if you assume solar systems with advanced life need to model our solar system, which I'm not sure is a foregone conclusion yet.

Re: the solar system we live in seems more unusual

We and we alone have the one and only Trump ;-)

So to test for unknown bias and sensor anomalies, they put raw modelled data through the statistical machinery looking for homogenized outputs.

This strikes me as wrong. If you want to know if your filter is bad, you plug in ground truth and add known noise models. If you want to know if your noise models are wrong, you cant do the same thing, you need to point your sensor at a known object (e.g. calibrate).

They seemed to have done a cursory analysis of the former type, which is not the same as saying "our pea-pod hyoothesis is correct".

I don't believe this was an attempt to find bias or noise from the physical sensors, but rather was looking at the method itself.

The method has some known biases that they were ensuring wouldn't explain the given pattern in the data. Depending on the depth of the simulation, it may have also been able to account for some unknown biases in the method.

Now even under your interpretation, I'm curious about which known object you'd suggest they use.

As a side note, the drawing attached to this article is of Edward Tufte's quality, so much information condensed into a simple-looking drawing and it's so easy to get the idea from a single glance.

I'm actually pretty confused by that chart because the bigger planets of SOL are off the side.

The planets detected by Kepler will tend to be unusually close to their star due to its detection method. I believe that’s why Sol looks like an outlier in that regard — it’s really that the other systems are the outliers.

Could be. Detecting smaller planets further out is less and less likely.

My first thought is this should call into question the method(s) we are using to measure and observe planets in other solar systems.

Think on this, the one solar system we can empirically confirm with multiple observation techniques, i.e. we've sent satellites to other planets in, is way different than solar systems we are observing by measuring light differences from light years away.

Yes but the solar system is a biased sample: life evolved in it. Maybe it was helped by Jupiter shielding us from asteroids that could wipe out life on the planet? If it's statistically more likely that intelligent life evolves in a system with large planets then we would expect to see a difference between our solar system and others that we observe.

All asteroid crashes aren't equal. Earth got most of its water from an asteroid crash.

Basically a huge of range of things have to work on large timescales for intelligent life to evolve on a planet.

Which is why intelligent life apart from us could just be absent or very very rare and far away.

The article discusses an "oligarch" model of planet formation, where there are roughly early sized and spaced oligarchs early on in planet development.

Because our solar system didn't end up with the "peas in a pod" pattern, planet formation they also posited a period of violent collisions that stirred things up. Wild, uneducated, and necessarily oversimplifying speculation: perhaps all these early collisions brought water which helped encourage development of life, and led to the formation of Jupiter, which then protected that life's development.

My point is that a large planet in the system reduces the probability of large asteroid impacts later in a star system's history. I didn't mean that it immediately removes all asteroids from a star system after forming.

True, but you need water rich asteroids to crash in the immediate phases after planet formation, and then you need life to develop. After which you need asteroids to not crash into the planet. Of course in the meanwhile, you need a lot of other things like formation of atmosphere, plate tectonics, volcanic eruptions, lakes sitting on geysers etc etc.

Basically things have to (not) happen in a series of steps for life to form, even if other things like a rocky planet being situated in the goldilock zone come true. This is apart from a lot of other factors, like not having a big sun etc etc.

All this suggests even if we consider 'Terrestrial mediocrity' to be true. Life could just very very rare across the universe, if not totally absent.

The "Jupiter as a shield" hypothesis has been questioned recently: https://phys.org/news/2016-02-jupiter-role-planetary-shield-...

That's fair enough, the point I tried to make is: Statistically, we would expect our solar system to be average among solar systems that can support intelligent life, not among all solar systems (following the Anthropic Principle). Therefore, the difference we see between our solar system and the average star system might imply something about what kind of conditions make intelligent life more probable in a star system.

If we see that in all star systems we know that evolved intelligent life (even if there's only 1 we know of) there is a large Jupiter-like planet, and that's very atypical among star systems, that suggests that Jupiter might have increased the probability that intelligent life evolved here.

How do you know the statistic probability of life evolving in a solar system? We don't have an observation technique to definitively observe this.

Again, we've only ever observed a single solar system up close.

Yet, life here on earth has happened more than one way. Seems like its pretty hardy if one planet gets more than one start on life? Isn't that a statistic of sorts?

There are currently no evidence that life on earth happened more than once.

How about the multiple principles of life observable in what we see? Not just plants vs animals or fungi, Sulphur-based life chemistry, iron vs copper blood in higher orders and so on?

Currently, all life we know of fits into a single tree of life. All known lifeforms for instance agree on using RNA for something, hence the RNA world hypothesis. I would also be interested in a study which points to the opposite.

Thats not true is it? life on earth started more than once, source pls!!!

Keep in mind that this isn't true for every system they've detected, it's just more likely than not given current examples - 909 planets in 355 systems according to the abstract: http://iopscience.iop.org/article/10.3847/1538-3881/aa9ff6/m...

There's plenty of differences from our own solar system out there that also don't fit that pattern. Gas Dwarfs are a planetary type that we don't have in our own solar system, for example: https://en.wikipedia.org/wiki/Gas_dwarf

I always thought something so far away would appear to us as Pixels.

Its kind of mind blowing, we have techniques both theoretical and practical to only see pixels and yet come up with their exact dimensions.

We don't usually detect them directly. We observe a star dimming (planetary transit) or wobble and infer the planet's location, speed, and mass from that. We are only just developing techniques to observe very large planets close to their sun directly by efficiently cancelling out the sun's light.

That is precisely what is so interesting.

Even a star wouldn't be anything more than a fat pixel, and the planet would be more like a thin pixel. Plus the star dimming, given the orbit would also be a long drawn phenomenon, like in case of earth(1 year).

I understand we ideally decide these things based on color of the light we see. And light waves stretch as the distance increases, and it can all be figured out by doppler effect.

But its still kind of mind blowing how much we can infer from just these things.

> The answer was no: in more 1000 trials with randomly assigned planet sizes put through a virtual Kepler’s detection scheme, a pattern of similarly-sized planets in the same systems never emerged. This computational experiment did not reproduce what we observe in the Kepler planetary systems. Thus, the regular sizes of planets is a real astrophysical pattern.

Any explanation to this? I'd assume small planets are just harder to detect. Large planets could be gas giants that react differently to our observation technique. Also they tend to form further away from the star, leading to a longer orbital period and fewer observations. Could these contribute to an observation bias and does the simulation include these factors?

> does the simulation include these factors?

Yes, that's what the "put through a virtual Kepler’s detection scheme" part refers to. So she:

1. Generated a bunch of random planet sizes.

2. Simulated what the detector would have reported for each.

3. Compared those virtual detections against the real detection data.

If the two results had matched, it would imply real solar systems have random planet sizes and the bias they see is from the detector. Since the two results are different, it implies the actual solar systems are not randomly sized with the apparent uniformity coming from detector bias.

(There could, of course, be a third factor where planets are randomly sized but the measured bias comes from something else.)

2. Simulated what the detector would have reported for each.

I think this would be really hard. For example, you need to simulate how the detector responds to different planet compositions, different signal to noise ratios at different distances from the star. A sphere in vacuum model may be a gross simplification.

Well, we already deduce exoplanets' masses, orbits, diameters, and so on from the sensor data that basically amounts to a tiny change in the light curve of the star. The astronomers are probably pretty well aware of the limitations of their models and potential confounders.

How would the composition of a planet affect how much light it blocked of its star when passing in front? And signal to noise ratios are presumably quite easy to simulate. So not really sure what's so hard about it.

Composition determines density. A rocky planet would block less light than a gas giant planet of the same mass, because it would be denser and therefore smaller.

But does density have to do with anything Kepler measures?

Since it only measures how much light decreases, I assumed it's only measuring planet size, and that we're totally ignorant about planet mass and density?

Another possibility is intergalactic bar billiards[1]

[1] https://sites.google.com/site/h2g2theguide/Index/i/540914

I'm not sure the author is taking into account observation time and the orbit diameter of planets.

In other words, it could just be that planets in similar orbits have similar sizes, which would be roughly consistent with our own Solar System.

Jupiter alone takes almost 12 Earth years to get around the Sun. It seems to me that an observer on a remote planet on the same plane as our own Solar System, observing us, would likely only see the inner 4 planets (if anything at all) during a 4 year observation window similar to Kepler.

Yeah I don't like the finality of the statement the `answer was no`. Couldn't the simulation (virtual Kepler’s detection scheme) simply be incorrect?

Anything is possible. You can never prove anything. In science/real life true means very likely and false means very unlikely. So yes the simulation could be wrong, but I guess with everything we know and the error bars the answer was clearly no.

Following up with "Thus, the regular sizes of planets is a real astrophysical pattern" seems particularly premature.

The answer is "no" because the author needs to come to a conclusion for the paper that they are writing. It doesn't matter if they are correct, it matters that they produce a paper, so that's why they came up with this conclusion.

I wish researchers weren't biased to produce papers, it doesn't help us with honestly understanding our universe.

Actually, it appears that while gas giants form at a distance from their star due to requirements around accretion and temperature, they generally tend to end up closer to their parent star than around Sol, although this again could be selection bias.

The Nice model provides a potential explanation for our setup, which essentially requires Jupiter to have formed far out, moved inwards under drag, bombardment, and transfer of angular momentum to planetesimals, and then moved outwards to resonate with Saturn after it formed.

There’s also the Grand Tack model which sees Jupiter careening from 3.5 to 1.5 to 5.2 AU.

As an added bonus, Grand Tack also explains tiny mercury and smallish Mars.

There’s also the question of age - how old are these systems relative to ours? Are we seeing them earlier, or later in their evolution? We don’t have good enough models to yet know.

We need more data, and we also need to understand Uranus and Neptune, which are currently a bit of a question mark.



Judge planets by their size do you?

Fascinating stuff. ..and all deduced from the slightest dimming of starlight hundreds of light years away. Badass.

The thought of we may be the only species in the Universe scares me. And, we don't have a great insurance policy.

The good news is, the Universe is cruelly indifferent. So when we do go, there won't be anyone to care.

Sorry, I guess that wasn't as comforting in print as it was in my head.

Not being the only species is equally frightening.

It is far, far more frightening to consider we are all alone, given our modern understanding of scope, statistics, and molecular biology. Maybe we are all alone! But that's way more terrifying because not only are we alone, but it's just damned irregular from what we might expect.

Regardless of the truth, for practical purposes we are alone. The speed of light is far to slow on the scale of even the galaxy (must less universe) to do anything useful about any intelligent life that might be there.

I can't believe that we're alone, even if intelligent life is extremely rare, and life in general somewhat rare. It's a big universe.

Or is it just as terrifying to be First?

Do you mean "the only planet with life"? Humans aren't the only species in the universe.


All of us living now will be long dead before our species die out, so what difference does it really make?

Because if we're the only one's actually capable of appreciating the beauty of the universe, if we are in actuality the only mindful expression of the universe, we are more incomparably valuable than any other element existing as part of that universe, and an irretrievable loss once gone.

Why is perceiving beauty so valuable? It does exactly nothing except for the one perceiving.

That's why it's valuable. To us.

When this species becomes capable of observing our feelings objectively and not simply saying "I feel it, it's mine so it must be truth" people like you are going to feel something very different.

Truth != value. Of course my feelings are valuable to me; I don't need them to be true (and I don't even think being true or false _applies_ to judgement values).

"I once heard a wise man say that if anyone were to know the whole answer, he would cease to exist."


An irretrievable loss to whom?

You're asking for the meaning of life there. For some it's pure unadulterated hedonism, for others it's religion, for others it's science or technology, for others it's their children, and so forth and so on. But for many the continuation of our species, whether or not we're a part of it, is certainly quite a crucial part of it.

As an aside from this, you also don't know your statement to be true. Our planet has, multiple times, killed just about every single living species on it. We'd probably stand a decent chance of partial survival due to technology and intelligence, but on the other hand we might not. Think of all the [relatively] near miss large asteroids that we detect only long after it would have been much too late to do anything. And that's something we can actually control. An unfortunate gamma ray burst or other form of harmful electromagnetic radiation could be GG earth with little to no warning or even chance to react.

The chance of an extinction level event happening at any given point is very low of course, but they have happened and will continue to happen. It's just like winning a lottery, one that you'd very much prefer not to win.

Because it keeps us from slipping into the endless reducible hole of nihilism.

Because not all of us are so selfishly narcissistic and are rather able to consider the effect upon the lives, extant and not yet born, of our current decisions. Your apparent lack of empathy, and similarly our cultural worship of selfishness and self-deception, are foul clues as to our eventual doom. If choices today are critical in determining the viability of human life over the next several decades, but those choices conflict with the selfish wants of children in the present, and those present-day children choose to disregard future generations, then what hope do we have?

The Great Filter for our species is capitalism.

Woah! Settle down! It was just a question dude; no need to go all fire and brimstone.

If an asteroid wipes out life on Earth in a thousand years does it help to worry and mourn over future generations or something?

Random question.. If they are looking for transits, doesn't that presuppose the data will find close in planets? I believe that's not the only method, but wouldn't that push more close planets than distant ones? Would it find transits of things like Neptune or Uranus that take hundreds of years?

apologies if this is a stupid question, but I always wondered can there be multiple objects/planets in an orbit?

Not a stupid question at all. The answer is: yes. https://en.wikipedia.org/wiki/Trojan_(celestial_body)

Theories for the formation of our moon suggest a Mars-sized Trojan collided with earth before it was earth: https://en.wikipedia.org/wiki/Theia_(planet)

Langrangian points L4 and L5 are stable places where objects can orbit ahead of and behind a larger one, like a planet: https://en.wikipedia.org/wiki/Lagrangian_point

Asteroids occupying these spaces in our solar system are called Trojans: https://en.wikipedia.org/wiki/Trojan_(celestial_body)

They recently announced the first detection of an exo-moon, but said the moon was the same size as the Earth (orbiting a much larger gas planet) so the definition of planet and moon is blurry. Unfortunately the terms 'dual-planets', 'double-planets', or 'binary-planets' are not (yet) recognized by the International Astronomical Union (IAU) according to wikipedia.

Depends of your definition of planet, according to IAU a planet needs to clear his neighborhood, in this case, only one planet can be the major object in his orbit.

But co-orbital objects exist and one of the most interesting ones is the pair of Saturn satellites, Janus and Epimetheus, their orbit describe a horseshoe and there are pretty cool videos on YouTube with their orbits.

The short answer is "yes", but the longer answer is "we haven't observed this happen in practice for any significant planetoids".

There was an observation five or ten years back that originally looked like it might be two planets in the same orbit, but it turned out that one planet was in an orbit with a period exactly twice that of the interior planet.

Can anybody explain why the chart stops at 3 AU? All our bigger planets are off the chart.

Seems likely that the planets with larger orbits have not yet been detected since they will take longer to reach a point in orbit when they are transiting their respective stars. It seems premature for the paper to reach it's conclusion when Kepler has been in operation for less than ten years, but hey... publish or perish.

This result says that random sizes and orbits are not consistent with observations. But there is a huge gap between "random" and "uniform", even ignoring all the possible different distributions.

We know that our solar system is not random -- e.g., we have the "platonic solids" spacing, for our inner planets. So, our solar system is also not described by the disallowed model.

It's great that some subset of random distributions are ruled out for systems that happen to have planets packed close to their star, but that says nothing at all about how we compare to the the overwhelming majority of other systems systematically excluded from the sample.

As with most results that hit the popular press, any actual significance is garbled to the point of meaninglessness.

But a good graphic can be worth a lot if you ignore the headline.

Stranger? We already knew our solar system was strange. At least compared to the other solar systems we observed ( admittedly a tiny fraction - a couple hundred out of 200 billion solar systems ).

We already knew gas giants ( a major cause of our peculiarity ) are rare. And we knew that most solar systems had uniform sized planets ( within a particular range ) orbiting very close to their suns.

The article doesn't offer anything new that would make us think the solar system is stranger. It just reaffirms everything we knew and why we thought it was strange to begin with.

I am fascinated about how less we know overall about the earth, and the solar system. I would not be surprised if we found life in the solar system. May not be advanced, but multi cellular life. I mean life as it was on earth some 10,000 years ago, with early humans and all.

Edit: Sorry I shared my fictitious fascination. It would probably make a good movie, humans finding pre-historic humans on Titan or Europa.

I think it's helpful to look at a timeline of life on Earth when thinking about that issue.


Life's gone through a lot of changes over the time it's around. 10,000 years ago there were already humans and the Neolithic revolution was well underway and agriculture was just starting.

Multicellular life was one of the big revolutions but eukaryotes and before that photosynthesis seem to have taken a very long time to evolve. Interestingly we don't have a lower bound for how long it took life to arise once the Earth's surface started having liquid water, so maybe very simple life is pretty common.

I would be very surprised to see life anything like life on earth 10,000 years ago in our solar system. No planet in our solar system can support that level of bio diversity. They are either too hot, small, cold. Additionally none of them have the right mix of atmospheric gases. Plants massively changed the makeup of Earth's atmosphere, some hundreds of millions of years ago, before the first animals on land.

I think there is a chance of very simple life on some moons of the gas giants, if they turn out to have liquid water. Most likely this would be single cell organisms (which can grow quite large, a few inches even) or basic multicellular life.

I would be shocked if we found life like trilobites, or even simple plants. It would be the biggest discovery of our lifetime.

The watery moons are really interesting. Consider this - if we dumped a few tons of (earth originated) diverse bio-matter inside and on these moons, I would be shocked if not some of the earthen critters couldn't if not thrive, at least gain a foothold in these moons. I mean, considering we find microbes in the weirdest places here on Earth.

So - given that they could live in these moons:

if there is life there already, is it related to Earth life? If not, why not?

If there is no life there, but we could seed these moons, how come they are not seeded already? (From meteorite ejecta etc.)

Oh almost certainly! Earth grown animals have been found alive on the outside of space stations [0]. Dumping a bunch of earth stuff in these places would likely lead to a new ecosystem. So long as there was an energy source, like thermal vents.

This is actually a major problem for looking for life elsewhere in the solar system. How do we know it isn't just an Earth grown hitchhiker? Instruments sent to other planets have to be thoroughly cleaned.

In theory they could have been seeded as you describe. The problem is, to quote Douglas Arthur, that "space is really, really big"[1]. It's also not great for life, even the hardiest Earth animals can only survive for decades, and it could easily be centuries or millenia until they were deposited anywhere.

Additionally the gas giants, due to their mass, act as a big attractor to stray mass. And any biological life adrift is likely to be sucked into one of these, rather than land on an orbiting moon.


[1]https://en.wikipedia.org/wiki/The_Hitchhiker%27s_Guide_to_th... fiction, doesn't back up any point. But hilarious and correct about the size of space.

Collect all of Earth's extremophiles--those tolerant to heat, cold, radiation, drought, high pH, low pH, organic pollutants, etc.--and pack them all into an artificial spore casing. Pack those into a spacecraft.

Aim the spacecraft at a detected planet or moon, and set it to start ejecting spores whenever it nears a massive object. Make the spores break open and release their biological payload when an accelerometer detects an impact spike, starting a timer, which is reset by changes in acceleration.

Most spores will break open in an environment too hostile for the extremophiles, but some will land in conditions that at least partially overlap those found on Earth, and those could reproduce. A cold desert planet could become a lichen-encrusted planet. A wet volcanic planet could become a vent-tubeworm planet.

If we ever have the wherewithal to reach those planets eons later, the extremophiles that evolve there could be sequenced and broadcast around explored space for use in other spore-ships.

Panspermia from meteorite ejecta would be a slow and painfully random process. An engineered--and, more importantly, aimed--spore craft would increase the odds tremendously.

Maybe, what sort of chemistry happens in there water columns?

Your timeframe of 10,000 years ago as "multicellular life" and "early humans and all" is WAY off. There have been human-like animals for 2 million years, there have been multicellular life forms for probably a few billion.

Early humans were pretty damn advanced, at least in terms of biology.

I would be surprised if even our concept of cells translates that easily.

Unless of course both forms of life share the same origin, but that just raises more questions.

Cells are a pretty fundamental structure. Sci-fi can have fun with undifferentiated masses of protoplasm like The Blob, but there's a lot of reasons to expect that cell walls are going to be pretty fundamental on one level or another. Without them, instead of isolating the task of giving permission to enter and exit the life form to one structure, everything in the life form has to be capable of doing so. It is exceedingly unlikely that this is a good trade.

I would also say that before you start drawing up crazy ideas about alternate structures that you should check your own body first; we already have a lot of variants on the theme! Blood is a liquid that has a lot of cells in it. There's a lot of other such circulatory fluids that have cells within them, but are not "cellular" in the sense that they contain nothing but cells. White blood cells do interesting things with cell walls, as does the immune system in general. Cells have structure well beyond just little cubes already, like nerve cells, etc.

Personally I find that when one tries to come up with some funny sci-fi idea for an alternate life form, either A: it's physically implausible or B: something, somewhere on Earth is already doing something very like it, and I don't see middle ground very often.

This is the thing I also always wonder about. Consider that about 60% of a banana's DNA is shared with us. And for cats it's 90%. That's just really stupefying to think about. But the more important point here is that I think it is literally impossible for us to even imagine what life that had no genetic similarity to us would even begin to be like. We just have no basis for comparison whatsoever since we are all so closely related.

Though yeah, genetic similarity implying exogenesis would also be an equally interesting result. Would that similarity be organic/natural in nature, or might we just be some species' lab experiment? Perhaps they seeded us with their same genetic origins to discover more about their own evolution and development. This is an experiment I'm certain we'll eventually carry out ourselves. So many interesting possibilities.

So there's two high-confidence bets we can make about alien life:

(1) They will probably share huge swaths of the basic chemistry and machinery: proteins, sugars, starches, fats, and even more surprising things like cell walls and digestive acid and (something like) DNA. The building blocks of life are basic chemistry, and are some combination of "the simplest possible molecule that can do a thing" and "things that arise naturally from inorganic processes".

(2) There will almost certainly be 0% overlap in terrestrial and alien DNA, even assuming aliens use DNA and use the same four amino acids to encode their DNA and even assuming they have identical proteins and cellular structures. This is because the encoding from DNA to proteins is more-or-less arbitrary, and determined by both the exact cellular machinery used to turn DNA into proteins and by our genetic history. Even in our own cellular machinery, different DNA sequences can code for the same protein, so you can encode any individual protein in a bajillion different ways. The only real reason for to different organisms to share any common DNA sequences is because they both derive from a common ancestor. If our DNA and alien DNA overlap in any meaningful way, even with the tiniest of percent, it's basically conclusive proof of exogenesis of some sort.

So basically, it is very likely we'll be able to eat aliens (probably no more or less likely than how likely it is you can eat any particular plant or animal on Earth, to be honest), and almost impossible that we could ever hybridize alien and terrestrial plants and animals.

I think it goes further than that, that alien and native lifeforms will probably be toxic to one another. All life on this planet evolved in tandem, and have some of the same basic compounds for life, so there are few toxic interactions between life forms, excluding those evolved for defense (and even then there are exceptions, plenty of plants contain. And even then there are plenty of exceptions. But lifeforms from a completely different evolutionary line may use completely different proteins for basic functions. Even using the same proteins but of the opposite chirality could result in toxic reactions.

I think that even if we found alien life that evolved and developed on a planet much like ours, and used the same amino acids, metabolic processes, and cellular structures, the odds are high that contact with them would result in toxic interactions.

You've got to weigh "we didn't evolve to specifically eat it" against "it didn't specifically evolve to prevent us from eating it". I'm not sure we'll get an answer to which is a bigger factor unless we find some xenobiology and take a bite.

A lot of digestion is pretty elementary, and built upon our stomachs' acid bath -- proteins go in and are unwound and broken down. Most foods that cause problems later down the line contain a protein that both (a) is resistant to the stomach acid, and doesn't break down before hitting the intestines and (b) causes a problem when it hits the intestines.

Barring that - or incorporating elemental poisons like arsenic at levels we can't cope with - I'd personally bet that if you charred a random alien critter over a fire and wolfed it down, it's not crazy that your stomach acids would break down anything that survived the fire and would otherwise harm you, and you'd be able to extract a reasonable mix of sugars, fats and proteins from what you'd ate.

It's not impossible that an entire planet would have some proteins common to all of their lifeforms that both (a) didn't break down easily when cooked or digested and (b) were incredibly lethal to terrestrial life, but there's no ab initio reason to think they would or wouldn't, one way or the other.

The stomach isn't a perfect barrier, there's always something that gets through. If it didn't we wouldn't be able to take drugs orally. Our bodies have an entire subsystem devoted to processing compounds that made it through the main digestive system into the blood stream. The liver is dedicated to metabolizing these substances, in a process literally called "xenobiotic metabolism".

Sure, the odds of a particular compound found in some alien lifeform having a toxic interaction with our bodies is small, but I consider the odds of one of the hundreds of thousands of different types of compounds in that lifeform having a toxic interaction is on the higher side.

Oh, and to be clear, by toxic interaction I don't necessarily mean a poison, per se. It could also have a carcinogenic effect, or if the compound is common enough in the creature and undigestable through the stomach and intestines it could overload the liver, etc.

Yeah, not a perfect barrier, but if it's good enough that I'm not poisoned, do I care? I eat lots of delicious food that doesn't digest optimally.

As for carcinogenic effects, well, depends on what the timelines and cancer-rates you're talking about look like. We eat lots of things on Earth that are mildly carcinogenic over a lifetime, and the fact they are doesn't really stop us.

I wonder if some of the early core functionality such as replication, splitting, energy generation, etc. only has a few simple and effective possibilities? Are there any really simple RNA/protein patters that almost just have to occur from a statistical/mechanical view?

> They will probably share huge swaths of the basic chemistry and machinery: proteins, > So basically, it is very likely we'll be able to eat aliens

Their proteins may be folded in the opposite direction though. That would make things difficult.

Do you mean from a digestive standpoint or are you thinking prion diseases?

But how quickly did they get started in that case?

I mean, the ago of the Earth is quite a nice chunk out of the timeline since the Universe got started.

200,000 years ago we did not exist as a species.

2500 years ago stabbing each other with iron weapons was literally cutting edge technology.

In the past 200 years we have invented: production automobile, airplanes, space flight, launched probes into interstellar space, computers, the internet, ... We have discovered relativity, quantum mechanics, galaxies, ...

Assuming we are able to survive, imagine where we'll be in 200k years. Actually you can't. It's just impossible to even begin to try to imagine that. On that scale we'll likely have even begun to experience evolutionary speciation as we settle different regions of space and end up with extended periods of relative isolation.

And now consider that this entire timeline is but 400k years. That's not even a blink in the eye of the cosmos.

It took billions of years before complexity started. We don't even know if that is a long time or a short time. We may be among the first.

Absolutely. The point I was making there is that imagine there was another perfect clone of our solar system that was identical in every single way and its timeline was 99.995% identical to ours. Intuitively you'd think that they'd probably be at, more or less, the same point in their development. But that 0.005% is the difference between between glorified monkeys and widespread civilizations launching objects and people into space. And if we add another 0.005% to where we are now, everything we know leads us to believe that the change will be even more extreme since our current technological trajectory remains on a very sharp exponential curve upwards with no reason to expect it to end. As an aside these numbers are also based on the age of the Earth. Looking at the age of the universe would probably be more reasonable and would lead to even more extreme results.

Hm, fair enough. We just don't know a great many things. :)

I think it might work quite well actually. They could certainly not work with any of the biological systems as ours, such as proteins and fats etc.

But I think it is likely that there would be internal subdivisions in a complex life-form. This is however likening, branches in trees, to cells in an organism, to crystals in a rock.

Not convincing.

We know Kepler results are heavily biased for (1) systems edge-on to us (2) with big planets (3) very close to their star.

The only conclusion to draw is that planets that are all clustered close to their star tend to similar sizes. Then only Mercury is unusual. Even there, it's not unique.

Stars with planets distributed more widely, like ours, have been systematically filtered out of the results. We have no idea how common they are. It might be that they are rare, and that would make us unusual. In that case, distribution would be interesting and relative sizes would be unremarkable.

If in fact they are common, there is no reason to expect sizes in those cases to be uniform.

A system that condensed from a cloud with less intrinsic rotation seems more likely to have its planets clustered close. Less intrinsic rotation could also result in more uniformity.

> We know Kepler results are heavily biased for (1) systems edge-on to us (2) with big planets (3) very close to their star.

This article was written by an astronomer working with Kepler, and I wonder why you apparently seem to believe that they are so naive that they forget about the bias in their own research tools.

Furthermore, the article explicitly mentions this issue, and provides an explanation for why they believe it does not apply, making me wonder if you read it at all:

> I concocted (on my laptop) imaginary planetary systems in which the sizes of planets orbiting a given star were random. Could some sort of bias in Kepler’s method of finding planets—which favors the detection of large planets close to their stars—contrive to make the planets in each of my imaginary systems appear to fit the pattern? The answer was no: in more 1000 trials with randomly assigned planet sizes put through a virtual Kepler’s detection scheme, a pattern of similarly-sized planets in the same systems never emerged.

Unless you have a reason to dispute this method of verification of course, but then you should share that.

I personally see no problem: the claim is implicitly that since the measured data does not match the predictions of the model, the model is wrong. The model + known bias of Kepler is used in simulation to generate expected statistical output of measured data sets. The generated data does not match the measured data. Hence, either the model of Kepler's bias is wrong, or the model of what planetary systems to expect.

Since we based the planetary system model on our own solar system, if that model is wrong then that makes our own solar system special.

>This article was written by an astronomer working with Kepler, and I wonder why you apparently seem to believe that they are so naive that they forget about the bias in their own research tools.

Exactly. I actually did some research on Weiss.

So she studied astronomy at Harvard where she got her BS, then went on to Cambridge for her masters, and then a PhD from Berkeley. Now she works with NASA.

Odd that all these people who clearly didn't even read the article seem to find supposedly obvious bias and flaws in the research of someone who is clearly among the top young researchers/experts in this field.

> Since we based the planetary system model on our own solar system, if that model is wrong then that makes our own solar system special.

These two statements are not equivalent; the model may be wrong in its assumption of uncorrelated planet sizes and star type, for example.

The author demonstrated that a particular model is not consistent with the selected systems. That is not evidence strongly in favor of a single other model (uniform sizes = normal) it is evidence very weakly in favor of every other possible model.

It’s a paper:


I’m sure if you contacted the journal, authors and reviewers about these obvious errors they’d add your name to the list of reviewers and force this so-called scientist to reevaluate their criteria for publishing worthiness.

The paper was actually linked from the original article. Good job scientific american!

I was wondering, how can Kepler detect planets like Saturn that take decades to orbit, let alone planets like Uranus or Neptune? Wouldn't we need to be looking for 50 years at least in order to have enough observations to drawn meaningful conclusions?

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