The Top 8 Worlds In Our Solar System For Life (Synopsis)

“If I had to describe myself to an alien I’d say I was bigger than the average human, enjoy a drink or two with a good meal and have a bigger head than most. I’d also say I’m really handsome — especially if they were a female alien.” -Dwayne Johnson

The fact that Earth has not only life on it, but had the potential for life just from the raw ingredients that the Universe birthed us with should give us tremendous hope for the future. Not only might we find life like us around other stars on a planet not too different from our own, but our own backyard might offer some possibilities -- either in the same way or a different one -- from how things unfolded here.

Image credit: ESA, HEXOS and the HIFI consortium; E. Bergin. Image credit: ESA, HEXOS and the HIFI consortium; E. Bergin.

In our Solar System, the top 8 candidates (probably) are:

1.) Europa
2.) Enceladus
3.) Mars
4.) Titan
5.) Venus
6.) Triton
7.) Ceres
8.) Pluto

Image credit: NASA/JPL/SSI, Cassini orbiter. Image credit: NASA/JPL/SSI, Cassini orbiter.

Come and find out what makes each of these worlds special, and what their prospects for life might be!

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Finding and identifying life on any of these places is likely to be hard. A probe which landed almost anywhere on our planet's solid surface wouldn't need highly sophisticated equipment to identify life, plants and animals would be imaged by even a crude camera. Yet on these other worlds, life might well only exist deep underground, and its likely to be sparse single celled organisms which live in a low energy environment. Finding and confirming it is likely difficult. As a case in point Mars, we have a few decades of landers plus rovers, and yet we can still only speculate about whether is has or had life.

By Omega Centauri (not verified) on 14 Oct 2015 #permalink

Ethan points out that Enceladus may be very easy, by sampling the plumes from orbit. Life by-products and tiny lifeforms could easily be there.

"The fact that Earth has not only life on it, but had the potential for life just from the raw ingredients that the Universe birthed us with should give us tremendous hope for the future"

Only in the sense filling a can with the raw ingredients, with as much heat as it wants, has the potential for life.

if all the raw ingredients in the universe were divvied up into similar cans, how many would spring into life. Reasonably...none.

By Chris Mannering (not verified) on 15 Oct 2015 #permalink

People are very interested in knowing if life can spring forth from raw ingredients + conditions + time. The experiments we've done show it doesn't happen easily, or we're missing something. One thing we can't directly test is millions of years incubating.

If you conclude life is unique or life is abundant, right now either is largely a matter of faith. We simply don't know.

It depends on what you want to call life.

Basic organic chemistry is VERY easy to get going. The basic chemicals of life are abundant. Self-replicating structures are commonplace and unremarkable.

If you want to start at something recognisably a modern bacteria, that is more difficult. The points for it are really just a lack of any reason to think it doesn't happen, than for any reason for it happening or not.

"if all the raw ingredients in the universe were divvied up into similar cans, how many would spring into life. Reasonably…none."

Except that is not a reasonable request.

Life doesn't want or need that to happen and arbitrary shenanigans being unable to get life is no more remarkable than finding that if I look where I left something, I will be able to find it nearly 100% of the time.

Organisation arises from opportunity.

The form that organisation takes doesn't lead to it still working if you chop it up into the original pieces. And why would you expect it to? But why is that proof organisation doesn't happen?

I follow your thinking through the first 5 sentences, Wow. However, at the sixth sentence I start having trouble understanding what you mean -

"The points for it are really just a lack of any reason to think it doesn’t happen, than for any reason for it happening or not."

I don't know what that means. I think my difficulty is because I'm not sure what the various pronouns ("it") refer to.

I'm also confused by the "arbitrary shenanigans" sentence, though not so much. It seems you're saying that the failures of experiments to "create life" are not surprising. There's more there, but I don't understand how you are applying the metaphor 'looking for something where you left it'.

The final paragraph says it's not surprising that destroying something makes it not work. Agreed, but I don't see how that relates to experiments to create life from the building blocks that are naturally present.

Not trying to say you are wrong, just that I don't understand your points. Perhaps your thoughts outpace your typing, I don't know.

You mention "Self-replicating structures are commonplace and unremarkable."

The Scripps and MRC Labs researchers seemed to drop custom RNA into an amino acid soup and it replicated and did neat things. But I don't see where the RNA formed from the soup. Have you seen anything that connects the amino acid soup to RNA? Very interesting.

"I don’t know what that means. I think my difficulty is because I’m not sure what the various pronouns (“it”) refer to."

The it in all of the bit you quote is the "it" that you call life. Which is undefined because, like the opening sentence says, depends on how you want to define it.

There's no reason to suppose it would be any harder on another random planet than ours to start off, and it started here VERY quickly geologically speaking. So no reason to think life would be common, just no reason to think it anything else.

"But I don’t see where the RNA formed from the soup. Have you seen anything that connects the amino acid soup to RNA?"

It was RNA here on earth.

But it's not the only construction that works with carbon, oxygen and hydrogen.

The contents of RNA are all in that "acid soup". They already replicate by adding those bits together, and just one RNA strand that can replicate would do it.

So it would only require finding an "RNA" *once* for it to be commonplace very VERY quickly.

Remember, there aren't any predators yet. Only competition for the building blocks. And if you're the only one able to replicate at the moment,you can steal all that stuff before anything else gets in on the act.

The RNA I've seen that replicates is pretty big, so unlikely to form by chance. It's a neat proof of concept, but shouldn't there be some much smaller self-replicators out there?

I wonder how small a self-replicator has to be to have a good chance of forming. 100 base pairs? More? Seems like a reasonable stab at setting the threshold size could be made using undergrad statistics.

Very interesting, but this is one of the articles that lack a constrained context. Who are "we" in the text, is it astrobiology consensus or is it a personal opinion? The use of "I wouldn’t rule it out" implies it is the latter.

Well then, here is my own opinion as interested in astrobiology:

There are two main theories for life emergence, soup theory and battery theory. To base an opinion on chemical building blocks or complex products is not looking at the basic Water-CHNOPS-Energy-Time rectangle but providing the soup theory with one of its two ingredients - concentrated organics - and hope for the other - concentration or other gradients for evolution.

I would look for constraints on both theories, in which case a list ordered in prior likelihood would be:

1) Mars. It has the soup provisions (organics seen by Curiosity) as Earth but also the battery provisions (silica producing alkaline hydrothermal vents seen by Spirit) as Earth. Also, it has the first peer review published putative fossils. [… ]

2) Enceladus. Again both soup (organics in the ocean seen by Cassini) and battery (alkaline hydrothermal vents seen by Cassini) provisions.

3) Europa, Ganymede (IIRC) et cetera of the remaining tidal heated ocean with crust contact ice moons. Likely soups, likely vents but as opposed to Enceladus we don't know yet.

4) Triton, Pluto, Ceres et cetera of the ice debris populations. They too may have oceans with crust contact, but the period of habitability is lower. [… ]

By Torbjörn Larsson (not verified) on 16 Oct 2015 #permalink

@OC: "Finding and identifying life on any of these places is likely to be hard."

Yes and no. Conceptually it is, but there are shortcuts, see below. Also note that when you discuss life detection you are concentrating on individual life, while life is a process among populations. It can therefore take longer time (need to see evolution over generations) but also have simpler constraints (observe traits unique to life, such as chirality).

While fossils are rare, we have already seen candidates on Mars but need more surveys (to exclude geological alternatives) and sample return (to make the required microanalysis in the observation criteria list). [ Link in previous comment.]

And the LIFE detection experiment would use a statistical tree analysis to observe or exclude (or deliver an indeterminate result) by diving through Enceladus's plumes. Only if all of 3 simple criteria - chirality, isotope ratios from metabolism, fatty acid synthesis based on acetyl - are fulfilled would it yield a positive answer.

By Torbjörn Larsson (not verified) on 16 Oct 2015 #permalink

"The RNA I’ve seen that replicates is pretty big, so unlikely to form by chance"

Yes. The RNA we have have to compete with everything else, so it evolved just as much as everything else did. So the RNA you see today is more complicated.

And evolution is anything BUT chance. Is it "chance" that makes raindrops fall straight down? Or is it the overseer god making it do that? Or maybe it's the inevitable consequence of the forces in action on the water droplet.

And, frankly, nothing cares whether you think something likely or not. You have to work hard to understand the phenomenon before you can make a judgement call on the likelihood of something happening.

"I wonder how small a self-replicator has to be to have a good chance of forming. "

Any catalysed reaction (say on clay soils, they are good structured catalysts formed naturally and commonly) is self-replication in action. And that can be as small as a benzine ring.

@Chris: "Only in the sense filling a can with the raw ingredients, with as much heat as it wants, has the potential for life."

You are referring to the idea of spontaneous generation, something that was rejected over a century ago.

Evolution shows that these processes incorporate contingency (here geophysical growth processes) and differential reproduction (small, survivable changes over generations within populations). If your can is a young, hot planet it will have the potential for life as we have tested well with our existence on Earth.

@Carl: Concluding that life emerges easily is not a matter of religious unwarranted faith but of well grounded belief, it is a hypothesis.

Same as we could predict from the properties of the planetary emergence process observed in our system that there would be many habitable planets (because watery disk formation was coupled to star formation, and planet formation was rapid) - now well tested by Kepler - we can predict from the properties of the life emergence process observed on our planet that there will be many inhabited planets (because ocean vent formation was coupled to planet formation, and life formation was rapid).

As a note, estimates of life formation is that it likely happened in 10-100 kyrs, not millions of years. We are testing the pathways, same as we test planet formation theories in the lab, not on whole systems but on the processes and outcomes.

"The RNA I’ve seen that replicates is pretty big, so unlikely to form by chance."

You don't have to form long RNA strands by chance. The beauty of battery theory is that there were perfect thermophoresis reactors around over 4.3 billion years ago, alkaline hydrothermal vents in a slightly acidic ocean. They are the only known systems that produce increasing strand length products (happens by the flow physics of pores) and it was recently shown that PCR of RNA happens in these conditions.

Those vents also produced pyruvate, and glucose/pentose from that, a prerequisite to form nucleotides. So it seems to fit a geophysical growth process contingency pathway. What hasn't been shown is that metal atoms (most likely, since that is how enzyme replicases work) can do the PCR, but it should be relatively easy to test.

"Have you seen anything that connects the amino acid soup to RNA?"

Soup theory doesn't specifically predict such a connection. But we see it from evolution:

The nucleotide bases are all metabolic elaborations of amino acids, and in purines it starts out with the sugar being thusly elaborated. We can also see in the ribosome evolution that it likely started out as producing random dimer peptides as RNA cofactors before the evolution of longer peptides and ordered such - the triplet code. Conversely they could recently show the last amino acid (tryptophan IIRC) to be included in the code, showing RNA genetics predated modern peptide use.

More basically, the quaternary base pair code predated the triplet code, and it balances the genetics of RNA (fewer bases improves replication) with is enzymatic activity (more bases improves ribozymes).

"I wonder how small a self-replicator has to be to have a good chance of forming. 100 base pairs."

Half that nowadays, 56 bp.

As per above, the then chemistry wasn't chiral. (And we can see from frozen in traits of the ribosome that chirality likely evolved when it was necessary in order to establish the triplet code.) Cross-chiral RNA replicases are "just right" because they can more easily detach under PCR heating while they are more reactive to opposite chirality strands.

Mind that a thermophoresis PCR reactor with pores should achieve 60 bp in minutes, and then theoretically cycle through the 2^60 ~ 10^18 permutations of a purine binary code in a matter of minutes given exponential production of longer strands from smaller pieces and sufficient nucleotide production. The cross-chiral replicase pairs would survive the reactor onslaught of strands in some outer colder pores, and evolve from there.

I don't think this is a conceptual problem any longer. Those I know of all fell the last year (such as the unsubstantiated claim that non-enzymatic metabolic-like pathways weren't possible, the largely untested claim that cross-chiral RNA replication wasn't possible, or the more substantiated claim that strand elongation wasn't).

If you ask "is it likely", well, yes, something(s) was likely - it happened fast - and while soup theory is simple (two components to protocell replicators) battery theory has a lot of shared traits between Hadean geophysical systems and modern cells. (Such as the correct cell potential difference, et cetera.) I'll bet my 2c on this for the time being.

By Torbjörn Larsson (not verified) on 16 Oct 2015 #permalink

A note on comments and references, I have a comment with reference links - that I later refer to - waiting in moderation. And frankly putting them up is somewhat time consuming.

So if I seem to make claims without support and you can't check yourself, ask for a reference. (And I may have forgotten to check for such at some claims, so it is good to have mistakes pointed out.)

By Torbjörn Larsson (not verified) on 16 Oct 2015 #permalink

What (i) find interesting about this type of discussions among reasoning human beings is about how life started all it's own and is to here and now to a point where we can comprehend it Billions of years later, and yet each and everyone of us with reasoning and thinking capacity makes mistakes all the time.
Even in this thread with thinking educated individuals there are probably mistakes made in grammar and miscommunication.
However, non thinking entities RNA, DNA, Peptides Ect..
Do their dance to make us so we can observer our mistakes.

By Ragtag Media (not verified) on 17 Oct 2015 #permalink

"However, non thinking entities RNA, DNA, Peptides..."

Republicans, Deniers. Or is that just a repeat?

Speaking of mistakes, I did forget about two conceptual problems that I just recently have understood. It is what Benner calls the "tar/asphalt problem" and "water problem":

"Chemistry's 'water problem' refers to the fact that nucleoside phosphates do not form in water because hydrolysis is a more thermodynamically stable reaction. Decades ago, scientists addressed this problem by using formamide as a solvent instead of water.

The 'asphalt problem' refers to the degradation of carbohydrates."

[… ]

Here is my analysis:

I am very sympathetic to the idea that "battery" theory could moot Benner's two showstoppers. I listened to a recent webinar of his, and I think one of the constraints was on time scale, 'tar' formation within years. But alkaline hydrothermal vents are thermophoresis PCR reactors capable of RNA strand reproduction on much smaller time scales.

The question is if, say, solute metal atoms can do the PCR catalysis. Solute metal atoms is doing it without tar formation or other side reactions in the Keller et al non-enzymatic pathway to pentose, supplemented with greigite producing the necessary pyruvate substrate, all within alkaline hydrothermal vents. [Ibid] Seems both Woese and Benner got that one wrong.

The remaining conceptual problem is the 'water problem'. I am not much of a chemist unfortunately, but the first non-committing step in purine metabolism is making PRPP out of Keller's ribose 5-phosphate. It is the pyrophosphate on the PRPP that drives the later committing step. That metabolism is obviously not using drying or formamide to drive out the water and enforce the desired reaction thermodynamics, but an Mg-ATP polyphosphate. (The Mg ion balances charges during reactions.)

Is it really out of question that the Keller et al glycolysis/gluconeogenesis pathway - that is reversible in the core vent - could produce inorganic polyphosphates by substrate level phosphorylation in the glycolysis pay off phase in the colder boundary where the pathway should tend towards being irreversible? Inorganic polyphosphates are known to be able to substitute for ATP in cellular metabolism. (Possibly a PRPP polyhosphate analog could do it too.) Wouldn't it be a simpler phylogeny if that ability evolved first rather than as a later supplement?


- The "tar problem" seems to be mooted by Keller et al's non-enzymatic pathways. [ See e.g. here:… ]

- The "water problem" is solved by modern metabolism. I don't see how Benner can exclude that this solution was found early on, and the above pathways seem to offer exactly that solution.

By Torbjörn Larsson (not verified) on 18 Oct 2015 #permalink

@Torbjörn Larsson - thank you! Very excellent information, I didn't even know about the battery theory! Lucky for me, it doesn't come up in my life very often (only once so far!).

It sounds like we see a very plausible methodology for the origin of life - I'm amazed it hasn't made more of a splash in the news. Maybe publishers are waiting for some definitive experiment... oh, who knows why some things go "viral" and others not.

Off topic, I'm not sure why people keep thinking I'm talking religion when I use the word "faith". I've reviewed all my posts where I use the word, and I'm using it correctly. I suppose the word has been strongly co-opted by religion, and those here in this science blog are sensitive to the science/religion divide of the last century or three.

"It sounds like we see a very plausible methodology for the origin of life – I’m amazed it hasn’t made more of a splash in the news."

It's complicated, doesn't stir anything up with an audience, has no money in it, and those who need to know will never want to listen.

Sadly true, the media has found it more profitable to entertain people and seek the lowest common denominator. But there are some educational shows (e.g. Nova?) that might cover it.

I want to believe that someone has covered it or is working on it. I'm exposed to a narrow window of news; maybe I've just missed it.

How easy is it for life to form? An interesting story just posted on, "Life on Earth likely started 4.1 billion years ago, much earlier than scientists thought".

Pretty neat! If life forms easily, the next questions are "how hard is it to get multicellular life?", and "what about animals?". It's a great time to be alive, I love this age of exploration we live in!

Eventually, our descendants will need a way to move between the stars in a single lifetime. One can only hope!

@Carl #22: The Big Questions are always the most fun to speculate about, aren't they? The hard part, and what distinguishes the speculators from scientists, is asking "what do we need to observe or measure to answer this question?"

For the particular new result you allude to, the actual data is a measurement of enhanced C-12 to C-13 in a sample of graphite which was incorporated into a 4.1 billion year old zircon found in the Jack Hills formation in Australia. While it is true that biological processes on Earth do preferentially bias the C-12/C-13 ratio, there are also well known _non_-biological processes which do so as well; the authors of the actual research paper discuss these processes and what additional measurements would be needed to either observe them or rule them out. Little of that was discussed in the associated news articles, and of course wasn't even hinted at in the headline.

You can play with thinking like a scientist yourself. How hard is it to get multicellular life? Look into the various features which cells in a multicellular organism have, and are different from related features in single-celled organisms (protists). Which of those features are really "new," and which are connected (either in function or in development) to features of protists? Is the change of function a plausible outcome, or would it be fatal to a developing cell? Does a particular change of function require (or induce) other changes along the way? How could you test this?

Is any of this potentially observable in the fossil record? Or do we need to draw inferences about the development of early life by comparison with extant organisms?

By Michael Kelsey (not verified) on 20 Oct 2015 #permalink