Miniature Habitable Zones
The current view among many astrobiologists is that, because there are so many environments where liquid water – and therefore the basic ingredients for life – might exist, there are many habitable zones in a solar system. There’s the traditional Goldilocks Zone, where solar heating keeps the planet at just the right temperature; there are orbits around gas giants, where tidal heating could keep water liquid and potentially habitable beneath the ice.
“The data point I seize on is more the number of potential habitable environments we have in our single solar system. I don't think that's a fluke,” said Curt Niebur, program scientist for NASA’s Europa Multiple Flyby Mission. “I think as we peer outward, we are going to find that in most solar systems we explore, either in person or via telescopes, that there is likely to be multiple habitable zones in every solar system.”
In fact, we’ve found more liquid water on icy moons in the outer solar system than in the temperate belt of the Habitable Zone. Some planetary scientists are even beginning to talk about the idea that gas giants, like Jupiter and Saturn, create their own habitable zones through their tidal heating of icy moons like Europa and Enceladus. And if McKinnon and his colleagues turn out to be right about what lies beneath Pluto’s Sputnik Planum, then there may even be little habitable zones far out in the frozen reaches of the Kuiper Belt.
“Sometimes it's around giant planets like Jupiter, sometimes it's on Earth-like planets, sometimes it's in the deep solar system like at Pluto,” said Niebur. “I think every one of those three cases is a Goldilocks zone, and I think that there are more Goldilocks zones out there remaining to be discovered.”
That means that we may not be giving gas giants enough credit as hosts for potentially habitable worlds. For one thing, they seem to be much more common – or at least easier to detect from Earth – than rocky planets, especially rocky planets that happen to orbit just the right distance from their stars, which means the odds are in favor of a gas giant winning the lottery of biochemistry.
“I think it's probably likely that gas giants are more common than terrestrial worlds, so just by sheer numbers, I think that they could either directly or indirectly provide far more habitable zones, far more Goldilocks zones, than terrestrial planets,” said Niebur.
That’s an eye-opening concept for astrobiology, but in practice it could be nearly impossible to draw a neat map of that type of habitable zone. Mapping a star’s Goldilocks Zone is pretty straightforward; the temperature of a planet depends on its distance from the star, as well as how much heat the star produces. Figuring out the region of potential habitability around a gas giant, on the other hand, requires a lot more information about the gas giant, its moons, and how they all interact.
The oceans of Europa, Enceladus, and Ganymede rely on tidal heating to keep them liquid, and those tidal forces come not only from the gravitational pull of the gas giants, but from gravitational interactions with other moons. For instance, every time Ganymede orbits Jupiter, Europa makes exactly two orbits, and Io makes exactly four. That means that the planets line up regularly, giving each other a gravitational tug that stretches their orbits out, making them more elliptical.
Thanks to orbital resonance, the tidal effects of the planet’s gravity are much more pronounced. In simple terms, that’s because the difference between “high tide” and “low tide” is exaggerated. That, in turn, keeps the moons’ interiors in motion – and warm.
That’s why Io is such a hotbed of volcanic activity, and it’s why Europa and Ganymede have enough geothermal heat to maintain liquid water so far from the Goldilocks Zone. Around Saturn, Enceladus is in a similar orbital resonance with its sister moon Dione, and that’s what keeps the plumes erupting from cracks in the moon’s icy crust.
Astronomers have a very good understanding of the dynamics that make the moons of Jupiter and Saturn so active, but beyond our solar system, there’s no way to spot tidally heated habitable zones – yet. To predict whether a moon might experience enough tidal heating to keep water liquid in its interior, astronomers would need to know how many other moons were orbiting the same planet and whether those orbits are in resonance with each other.
“The broader definition of habitable zones will also include some that we just can't observe with the missions that we're anticipating in the next decades,” said Lunine. “That includes icy moons around gas giants, which may be harboring life, or at least habitable oceans, that we can’t see yet.”