This is chapter 6.5 in the Solar System’s story. It’s a semi-philosophical discussion about the factors that influenced whether Earth could become habitable, focusing on the origin of Earth’s water.
Well, what’s so special about our Solar System? It’s true that our Solar System is indeed unusual among known planetary systems. But not that unusual. Among Sun-like stars our system is about a 1% outlier, meaning that you’d need to sample about 100 yellow Suns to find a system like ours (in practice, a Sun-Jupiter system like ours). Taking into account the fact that most stars are red, not yellow, the Solar System ends up as a 1-in-1000 case.
But there is one thing that makes our Solar System completely unique: life! Earth is the only planet in the Universe that we know harbors life — including intelligent life capable of solving problems, building technology, and making dumb movies.
You might be screaming: THIS IS RIDICULOUS! FOR ALL WE KNOW EVERY OTHER STAR HAS PLANETS WITH LIFE!
Of course, I can’t claim to know that life does NOT exist around other stars. But we’ve got to start somewhere. Life exists on Earth, and no one has found convincing evidence for life on another planet in our Solar System, despite centuries of searching (see Percival Lowell‘s Martian canals).
So I will make the assumption that Earth represents an ideal habitable planet — that it has the right conditions for life, whatever those are, and that the other planets in our Solar System do not.
What exactly are the criteria for a life-bearing planet? Just how “lucky” is Earth?
There are two ways that people address this question. Either there is a very long list of requirements for life, or a very short one.
The book Rare Earth came up with a loooong list of boxes to check for a planet to host complex life, nicely summarized in this image:
Each of these criteria has a reason for it. For example, plate tectonics are a key driver of the carbonate-silicate cycle, which acts as Earth’s long-term thermostat. The Moon stabilizes Earth’s spin axis (at least at its current spin rate). And it might not be a great thing for a planet to be right next to a supernova explosion. These all sound like pretty important for hosting *complex* life.
But are these criteria vital, or simply a reflection of what happened on Earth? Let’s try a thought experiment: imagine explaining the reason for your success after you made the key shot to win the big game (in whatever sport you choose, although my words hint at basketball). When asked how to replicate your success, it’s easy to tell others to do exactly what you did. But was it really important that you wore two pairs of socks on each foot and ate 3 burgers the night before? Or was it simply that you practiced a lot and prepared mentally, then happened to be in the right place at the right time? In other words, with a single example of “success,” it’s no simple matter to disentangle the factors that actually made a difference from coincidental occurrences and randomness.
What does this have to do with the Solar System’s story?
We need to know which specific things our models must reproduce. If the list is short, then the story can be relatively simple, but if the list is long then the story must be pretty complex. It’s like ordering an ice cream — there’s a big difference between asking for a chocolate ice cream cone and a three-scoop, mint-chocolate-chip plus speculoos plus lemon sorbet in a waffle cone with chocolate sauce and gummy bears and Reeses pieces and purple sprinkles, whipped cream and a cherry on top!
In my opinion, that long list of Rare Earth factors includes mostly circumstantial, anthropocentric factors. For instance, while the Moon does stabilize the Earth’s spin axis in the current configuration, Earth’s spin would not need stabilizing if our days were much shorter, as they would be if the Moon hadn’t slowed down Earth’s rotation (through tides; see here). And Jupiter does indeed protect Earth from comet impacts, but those comets would not be at risk of hitting Earth if their orbits had not already been changed by Jupiter (see here). Also, supernova explosions are not as big a deal for atmospheres as you might think, and the concept of a “galactic habitable zone” has been debunked (see here). The question of whether life really requires a yellow star is debated, but there are currently no slam-dunk showstoppers that would absolutely prevent life on planets around even the tiniest red dwarf stars like in the iconic Trappist-1 system.
In my mind, the minimum requirements for life are simply a rocky planet with liquid water and an energy budget within a Goldilocks-esque range (not too hot, not too cold; orbiting in the habitable zone). And even these minimal requirements are up for debate, as it has been argued that life could exist in Venus’ clouds or on free-floating planets in interstellar space. But remember: I am assuming that Earth life is the target, so a rocky planet on an orbit like Earth’s is a given.
In Chapter 6 I laid out the different ideas for how the rocky planets were sculpted. Here is a (perhaps over-stimulating) visual recap of the five successful models that can match the inner Solar System:
Each of these models succeeds in forming a rocky planet with an orbit similar to Earth’s. That checks most of my minimal habitability list boxes right there, except the one about liquid water.
So, how did Earth end up with water?
Despite what you learned in school, Earth doesn’t actually have all that much water. Even though oceans cover about 3/4 of our planet’s surface, water only makes up about one part in a thousand of Earth’s mass (including a few “oceans” trapped in rocks inside the mantle). All of the oceans put together don’t add up to much:
When trying to figure out the source of Earth’s water, there are two pieces of information that can help us “rewind the clock”. First, there is good reason to think that the building blocks of the planets were dry close to the Sun, but contained some water farther out. Bring predominantly rocky, Earth likely formed mostly from dry, inner material but with a sprinkling of wet, outer stuff. Given Earth’s relatively small water budget, that sprinkling could have been as little as 1% of Earth’s total mass.
Second, water from different sources has different chemical fingerprints. Deuterium is “heavy” water — a water molecule in which one hydrogen atom has an extra neutron. The ratio of Deuterium to normal water is called the D/H ratio, and it is measured to have quite a big range in different Solar System materials. The D/H of Jupiter’s atmosphere is very low, while that of some comets is very high, and Earth’s is in between. The source of Earth’s water must naturally match Earth’s D/H ratio.
A good match to Earth’s water comes from carbonaceous chondrite meteorites (see chapter 2). These meteorites contain about 10% water by mass and represent pieces of C-type asteroids that live in the outer parts of the main belt.
[Technical note: it was recently discovered that Enstatite chondrite meteorites — which may sample the main building blocks of Earth — have more water than once thought and with the same D/H as Earth. They may represent a source for some of Earth’s water, although C-type planetesimals probably delivered most.]
There is good reason to think that C-type asteroids were not born where we find them today. Their distribution overlaps with other types of asteroids: C-types dominate the outer main asteroid belt, and S-types dominate the inner main belt. But C- and S-types are different from a chemical standpoint (see the meteorite dichotomy discussed in chapter 2). We don’t think they formed side-by-side in the asteroid belt, because if they had there should be plenty of meteorites that lie in between the two groups, from a chemical (isotopic) standpoint. Those in-betweeners don’t seem to exist.
Current thinking is that the C-types formed farther from the Sun, past Jupiter. They were gravitationally scattered inward during the giant planets’ growth and migration (see chapter 5). This animation shows a cartoon of that process (actual simulation on YouTube here).
[The animation also shows the S-types being scattered outward from closer to the Sun. While I think that’s a reasonable idea (see here), it’s less well-accepted than C-types being scattered inward.]
You might count the blue arrows in the animation and think, “a lot of C-types end up in the asteroid belt but only a few end up closer to the Sun, where the rocky planets were growing.” Well, in reality it’s a little more complicated. The balance between whether scattered water-rich planetesimals end up in the asteroid belt or the rocky planet region depends on details like the density of the gas disk when this was happening (see this paper). The giant planets didn’t grow all at once, so there may have been many episodes of C-type scattering, some of which implanted more objects in the asteroid belt and others that scattered more toward the rocky planets to deliver water.
Once the C-type asteroids were scattered into the inner Solar System, each of the models of rocky planet formation can explain how a fraction of that material ended up colliding with the growing Earth and delivering water. I won’t go into the details because they simply reflect the dynamics of the models that I already explained in chapter 6 (see the overly-busy gallery above). We don’t know exactly how much water was lost during Earth’s growth, but each of the models can at least deliver Earth’s current water budget of a few oceans.
Now you might be wondering: I thought that Jupiter was not a Rare Earth factor. But now you’ve shown that it’s responsible for water on Earth. Does that mean that Earth would be much drier (and possibly uninhabitable) if it had formed in a system with no Jupiter?
In my opinion, the answer is NO: Earth would instead be much much wetter (not drier) if Jupiter was absent. Actually, it would probably be a ‘super-Earth’ on a much hotter orbit.
When I was in graduate school, it was hard for anyone to imagine how water was delivered to Earth. There was an apparent paradox about the process, that was explained to me like this. Life needs liquid water, and on Earth water is liquid above zero degrees Celsius (or 32 degrees Fahrenheit, in grandma units). Planet-forming disks have very low pressures such that water cannot exist as a liquid. Instead, water is a gas (vapor) unless it is colder than about -100 degrees Celsius. The line separating rocky material from water-rock mixtures is called the snow line (or “frost line” in the image above), and these arguments suggest that it must have been much farther from the Sun than the Earth. How, then, did Earth ever get any water?
In the past two decades there has been major progress in our understanding of what planet-forming disks look like (see chapter 1) and how the planets’ building blocks form (see chapters 2 and 3). Two key realizations are 1) that disks can be very cold — the snow line moves inward in time and often comes closer than Earth’s orbit; and 2) that large dust grains (“pebbles“) are continuously drifting inward through the disk. Put those two pieces together and it’s easy to imagine a scenario in which the inward-drifting snow line swept past Earth’s orbit while Earth was just a baby planetary embryo. What would have happened next? Pebble accretion past the snow line moves so fast that Earth would have grown very quickly. It would have become extremely water-rich and would almost certainly have started to migrate inward and ended up as a close-in ‘super-Earth‘ like those found around a large fraction of stars (but not the Sun, of course). Even if the snow line didn’t cross Earth’s orbit, pebble accretion in the rocky planet zone may still have been efficient enough that Earth would probably have grown much faster and ended up as a super-Earth. Not to mention that other growing ice-rich planetary embryos from past the snow line may have migrated inward into the terrestrial planet region.
This means that Jupiter may well have stopped Earth from becoming a super-Earth. When Jupiter’s core became massive enough, it blocked the inward flux of pebbles and starved the rocky planets. From then on, planetesimals and planetary embryos could only grow by bashing into each other, rather than by grabbing on to pebbles.
This is how Jupiter’s core may have stopped Earth from growing and held back the floodgates of water-rich bodies. Later, when Jupiter grew to its full size it threw the rocky planets a few bones by scattering some planetesimals inward. The rocky planets ended up with their ‘sprinkling’ of water but no more.
[It’s worth noting that this picture is not universally accepted — the pebble-driven model for terrestrial planet formation instead uses pebbles to deliver water, although Jupiter’s growth would already provide water for free.]
It’s hard to imagine Earth growing much larger without becoming much wetter and migrating inward. This is simply because a more massive planet requires a higher density of solids, and a higher density of solids implies faster growth, which triggers orbital migration and promotes mixing between different regions of the disk (and therefore lots of water-rich material ending up in the rocky planet zone). By blocking both the inward flux of pebbles and the inward migration of outer planetary embryos, Jupiter kept the rocky planets relatively low-mass and dry.
So, should Jupiter be a Rare Earth factor?
It’s hard to say. Would it be bad for life if Jupiter had not formed and Earth was much wetter? Answering that question requires imagining the geological cycles on a different “Earth,” which is fascinating but tricky business.
And of course, the biggest wild card is biology. Life needs liquid water to exist, at least on Earth. But liquid water is not a sufficient condition; life must actually originate, or be seeded somehow. Exactly how that process unfolded on Earth, and exactly what conditions were required, is a whole fascinating field of study in itself.
The TL;DR version of this post: The factors needed for life to originate on Earth remains uncertain, although water was essential. Earth’s water was likely delivered by planetesimals scattered inward by Jupiter. Yet, ironically, Earth would probably be much wetter with no Jupiter.
- The Solar System’s story (with links to all chapters)
- Where did Earth’s (and the asteroid belt’s) water come from? In this blog post from 2017, I summarize the main idea — which still holds — that all of the water in the asteroid belt and on Earth can be thought of as a simple byproduct of Jupiter’s growth (based on this paper). There’s a bit more detail than in this post, and fewer Rare Earth digressions.
- Sources of Earth’s water: models and constraints. This is a technical review paper that I co-wrote with Karen Meech (discoverer of ‘Oumuamua, among her many accomplishments).
- The Problem With “The Rare Earth Hypothesis” — A Cool Worlds video providing a nice discussion of the origin of life and Rare Earth factors
- The Rare Earth hypothesis (Wikipedia). Side note: the authors of Rare Earth were both professors in the Astrobiology program at the University of Washington while I was in graduate school there. They are both wonderful people, and each is a giant in their respective scientific fields. One of them (Don Brownlee) was on my PhD thesis committee, and I loved talking with him about all sorts of things. My criticism of Rare Earth factors is not intended as a personal criticism — I greatly admire them and their book. Rare Earth has stimulated a massive amount of thinking about this issues and was a big step forward for the field of astrobiology and the search for extant life.
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