The Solar System is a disappointment. It does contain an inhabited planet with forests and oceans and frisbees and beer (Earth). But it only has one. There are a couple worlds with some potential, but they are pretty disappointing too. Jupiter’s moons Europa and Ganymede have liquid water, but it’s hidden beneath miles and miles of ice. Saturn’s moon Titan has an atmosphere and lakes, but they are made of liquid ethane. Mars seems to have had liquid water on its surface long ago but now it’s just a cold sterile desert. Pathetic.
Where are our alien neighbors? Who are we going to space-trade with and have space-wars with or play space-ball games against? We’ve all read and watched enough science fiction to know that these things are awesome. So why not us? Well, stars are ridiculously far apart and interstellar travel won’t happen for a long long time, so that is out. Our only hope for alien action is with other planets in our own planetary system.
Let’s try a thought experiment to re-imagine the Solar System. The goal is to maximize the number of habitable worlds. Notice that we are talking about worlds, not just planets. Some of the best candidates for life might actually be large moons or what we now call minor planets; these are objects like Pluto that are a little too puny to be called planets.
The game has two rules:
- We can only use what is actually in the Solar System. Planets, moons, or minor planets are fair game. But we only get one Earth, one Jupiter, one Mars, and so on.
- We must preserve the Solar System’s orbits, both of planets and minor planets around the Sun, and of moons around planets. We can’t just add orbits where we want.
Let’s take stock of what we have to work with. First, here are the planets and minor planets. You can tell just how much smaller the minor planets are.
And here are the most interesting moons.
Among these bodies, which have the potential for life? This is a tricky question because a) we don’t know what other kinds of life could be out there, and b) even for life like ours, we don’t know the exact conditions that are needed on a planet to make it habitable.
So let’s simplify things again. Let’s say that any life in our new Solar System needs liquid water. That means that they need water (as water or ice), a source of heat and the right amount of heat (to produce water, not just ice). The worlds should therefore orbit the Sun in the liquid water habitable zone, the orbital belt inside which water on planets like Earth would be liquid. For the Sun, the habitable zone stretches from about 0.9 to 1.6 times the Earth-Sun distance of 1 AU (although this is debated). Dry, “Dune”-like planets might be habitable a little closer to the Sun. And planets with crazy orbital tilts or very thick atmospheres might be habitable a little farther out.
There exist other forms of heat of course. Jupiter’s Galilean satellites — Io, Europa and Ganymede in particular — are constantly stretched and flexed by Jupiter’s powerful gravity and are thus heated from the inside out. Io is the most volcanically-active body in the Solar System. Europa and Ganymede have subsurface oceans of liquid water that are made possible by this internal source of heat.
Let’s take a look at our best candidates for life:
- Earth. A no-brainer.
- Venus. Present-day Venus is a hellish planet: its surface is hot enough to melt lead and is completely dry. However, we think that Venus formed basically as an Earth twin with a similar water content and maybe also had oceans and continents early in its history. In time, Venus got too hot and dried out. But the young Venus may be a very good candidate for life, on a cooler orbit than its current one (located at 0.72 AU).
- Mars. There is evidence that liquid water once flowed on the Red Planet’s surface, and there may even have been a global ocean. Mars may have been habitable in the distant past, and it is possible that its surface could remain habitable for billions of years under different conditions.
- The large moons of the gas giant planets, in particular Jupiter’s moons Ganymede, Europa and Callisto, Saturn’s moon Titan, and Neptune’s moon Triton. These moons are made mostly of rock and water, but since they are found so far from the Sun they are pretty much just iceballs with rocky cores. (Except for heating from Jupiter’s gravity). In warmer conditions, it’s easy to imagine them as little ocean-covered worlds.
- The minor planets, especially the largest ones: Pluto, Eris, Sedna, and Ceres. Like the large moons, these minor planets are thought to be made mostly of rock and water ice and so should make nice little water worlds. However, their low gravity could make it hard to retain an atmosphere for billions of years in warmer conditions.
The gas giants were excluded because they have no solid surfaces. Mercury was excluded because it has very little water. Io was excluded because it is dry and volcano-ridden (although that is probably just from too much heating).
Now, we need to cram as many habitable worlds into a narrow belt of orbits where those worlds can actually be habitable. The trick in doing this is to use moons to our advantage. We can replace the Earth’s Moon with a better candidate for life. We can also put Jupiter in the habitable zone and leave its 4 orbits open to habitable worlds! So here is the new, better Solar System I came up with:
I put Mars on Venus’ present-day orbit, the hottest orbit that we think life might tolerate. Mars took this spot because drier planets have an easier time remaining habitable on close-in orbits. Earth is on its current orbit — why mess with success? — but Saturn’s moon Titan was swapped for the Moon. Jupiter is at 1.5 AU (Mars’ current orbit), the most distant orbit in the habitable zone. Jupiter’s two most promising moons, Europa and Ganymede, stayed put, but its innermost moon Io was swapped out for the minor planet Eris, and the outer moon Callisto was replaced by Venus. The amount of heat injected by Jupiter’s gravity depends on the planet’s size, so a small, water-rich body like Eris has a shot at not getting fried or drying out too quickly. And early Venus — not the present-day hellhole Venus but the young one with oceans — could be quite habitable on a cool orbit exterior to the other moons.
So there you have it. The Solar System could have seven habitable worlds instead of just one. Imagine all the interplanetary or inter-moon disputes, vacation destinations (little ocean worlds!), and alien encounters you could have in my new, better Solar System! Much more interesting and full of life than the real Solar System!
Let me know your thoughts: is there an even better Solar System than the one I came up with?
Of course, the real range in orbital configurations and planetary characteristics is much wider than what is contained in the Solar System. By packing planets that are best-suited for life into tighter and tighter orbital configurations in the habitable zone, it might be possible to make an even better planetary system (for life).
UPDATE (June 12 2003): I checked the orbital stability of the new Solar System and found it to be stable for at least 50 million years. However, what is interesting is that some planets’ orbital shapes would be different. In particular, Earth’s orbit would be more stretched out; this is measured using the orbital eccentricity, where a value of zero is a circle. In the real Solar System, Earth’s eccentricity bounces up and down between zero and 0.06, meaning that its orbit remains nearly perfectly circular at all times. The image below shows that in our new Solar System the Earth’s eccentricity would fluctuate more strongly. Over a million years of evolution the eccentricity bounces up and down between about 0.02 and 0.2. The more stretched out a planet’s orbit, the more light it intercepts from the star. So, as the new Earth’s eccentricity fluctuates, so too does the effective brightness of the star, although only by about 2% in this case. Still, it’s worth noting that when a planet’s orbit changes in time it can have a big effect on the planet’s climate (for example, by triggering periodic glaciations).
So, by changing the ordering of the orbits in the new Solar System we would inherently be changing the long-term orbital dynamics of the system. It remains to be seen how that would affect the planets’ and moons’ climates. If anyone wants to run climate models of the new Solar System let me know!
[Disclaimer: For the inquisitive among you, I should note a few caveats to this analysis. First, I have not taken into account the fact that you probably need a minimum mass of about 0.1 times Earth's mass to retain at atmosphere for billions of years in the habitable zone. I have now checked that the system is dynamically stable (see update above). Of course I cannot guarantee that such a system could ever form, although it doesn't look too crazy compared with the extra-solar planetary systems we see.]