We may find habitable planets around other stars, but our chances are slim. Water is the main issue, and the need for water imposes a few constraints on the kind of planet that could support us:

  • it would have to be dense, to hold on to its atmosphere;
  • it would need a magnetic field, to keep solar winds from blowing the atmosphere away;
  • it would need to be in the ‘Goldilocks Zone’ around its star, where there is just enough sunlight for liquid water on the surface.

Most of the planets that astronomers have discovered do not meet those requirements. While the Goldilocks Zone of our own star holds our rocky and watery Earth, most stars have a ‘hot Jupiter’ spinning around at that distance. These gas giants don’t have liquid water, so it seems that our most common neighbors are doomed to be uninhabitable.

Yet, some of those ‘hot Jupiters’ may have a surprise for us: chances are good that they have captured a rocky, planet-sized moon. Those moon-worlds are likely much more common than another Earth, due to the sheer number of ‘hot Jupiters’. And, they have a few benefits, which dramatically increase their chances for supporting life. I am not expecting that life would already be on such a moon — at the very least, moon-worlds are the most likely places for humans to colonize.

Where Do Moon-Worlds Come From?

Most ‘hot Jupiters’ begin their life much farther from their star, and slowly ‘migrate’ closer. During that migration, smaller planets’ orbits are perturbed, and these rocky worlds are ‘kicked out’ of the solar system, traveling on long elliptic or parabolic trajectories. None of the original orbits remain… yet, those worlds can swing back toward the ‘hot Jupiter’ and become captured as a moon. Even though re-capturing planets this way is rare, it can create a larger number of habitable worlds than a solar system like ours. For example: suppose that only 1 in 10 of the ‘rocky planet’ solar systems has a planet in its ‘Goldilocks Zone’, while just 1 in 200 ‘hot Jupiters’ re-captures a planet as its moon. Meanwhile, we spot 1,000 ‘hot Jupiter’ solar systems, and only find 10 ‘rocky planet’ solar systems. In this example, even though most ‘hot Jupiters’ don’t have a moon-world, we would still find 5 solar systems with moon-worlds orbiting a ‘hot Jupiter’, and only 1 solar system with another Earth. I think our chances are even better than that.

If a ‘hot Jupiter’ with a planet-sized moon is more likely, then we are likely to find one that is much closer to us than an Earth-like world. Being closer makes a huge difference for our chance of colonizing such a planet: we could carry more equipment with less fuel, get there faster, and encounter fewer hazards and uncertainties along the way. Jovian moons are the ideal ‘stepping stones’ for human colonies.

More Goldilocks

A planet-sized moon around a ‘hot Jupiter’ would see a number of benefits.

  • Its Jupiter would catch comets and icy asteroids, increasing the rate of impacts on the moon, which replenishes its water supply.
  • The Jupiter could also help to shield the moon-world from the solar wind, because of its enormous magnetosphere.
  • The interaction between the moon-world and its Jupiter would churn the moon-world’s core, helping to spew volcanic gasses that renew the atmosphere. This tidal flexing is what warms the waters of Europa, and churns the volcanoes of Io.
  • Finally, a moon-world would most likely become tidally locked to its Jupiter, and the side of the planet that always faced its Jupiter would experience a daily eclipse when its Jupiter passes in front of the star. This would create a cooler region, increasing the range of temperatures on the planet’s surface.

Those last two factors greatly expand the ‘Goldilocks Zone’ available to moon-worlds; they might support a watery surface even when they are orbiting ‘too close’ or ‘too far’ from their parent star. If the orbit would normally be ‘too far’, and water would normally freeze, tidal heating allows a moon-world to maintain a liquid ocean. A moon-world orbiting at the distance of Mars might stay warm enough for liquid water, because its Jupiter would churn magma that releases heat and greenhouse gasses.

And, if the orbit would normally be ‘too close’, where a water-world would boil away, a moon-world might still sustain life. If the moon-world is tidally locked, then the face that points toward its Jupiter would be cooled by long, daily eclipses — potentially making it cold enough to retain liquid water. The side of the planet that faced away from its Jupiter may be too hot, evaporating water quickly during the day, yet the Jupiter-facing side could still experience condensation and have pools or lakes. And, because the Jupiter would keep catching comets, those lakes would be replenished for hundreds of millions of years.

Longer Habitation Timelines

Some planets may only be habitable for a brief period in their history — Mars was likely habitable, shortly after it formed. When a planet is habitable for a longer period of time, then it is more likely that we will be looking at it during its habitable period. Many planets may have liquid water for the first tens of millions of years after their formation, but few of the planets that we see are so young. A moon-world, by retaining and replenishing its water for a longer period of time, has a much better chance of being found during its habitable phase.

Stars can also change their brightness, pushing or pulling their ‘Goldilocks Zone’, and turning habitable planets inhospitable. Because a moon-world can experience a greater range of temperatures on its surface (the outward, warmer face, and the inward, cooler face) and can support volcanism for longer periods of time (due to its Jupiter’s tidal flexing), it has a better chance of remaining habitable, even when the star’s intensity changes. An early ‘heat wave’ could have sterilized Earth; a moon-world would have retained glaciers or lakes on its inward face, and survived. This resilience dramatically increases our chances of finding a moon-world that is still habitable, long after initial planet formation.

Nearly Habitable

This scenario may sound outlandish, yet it could be our best chance for a habitable world: a rocky moon-world could be artificially watered by colliding an icy moon with it. If we were to send a spacecraft to another star, it would need to accelerate towards its destination, and then decelerate when it gets there. The spacecraft would spend a huge amount of energy slowing down. The propulsion energy that we spend slowing the spacecraft down can be directed at an icy micro-planet orbiting our destination star, causing it to wobble out of its usual trajectory. By coordinating that wobble, we could cause the icy micro-planet to collide with a moon around a ‘hot Jupiter’, giving it a fresh dose of cooling water. We would create a habitable moon when we arrive, by pumping the brakes on our spacecraft!

Planets which are already Earth-like may be a rarity, and rocky moon-worlds that are currently habitable may be a bit more common — however, rocky moons that could be terraformed with an icy impact are likely abundant. Also, these moons wouldn’t suffer from a narrow habitation timeline — they might be uninhabitable for their entire history, only becoming livable when we make them so. Then, we would have millions of years to capture moisture and alter the surface, before facing any risk of water escaping on the solar wind. Because these moons are more common than already-habitable Earths, the distance between them is small. They are ideal ‘stepping-stones’ to other stars.

With advanced robotics, our chances for colonization improve greatly. A robotic spacecraft could nudge icy micro-planets and rocky moons over the course of decades, churning satellites into orbit around a ‘hot Jupiter’ until a suitable world was formed. Jupiters tend to have many moons, so even small satellites could be collided to form a larger moon-world. We could build an Earth-sized moon from chunks of all the other satellites. It might take hundreds of years for the robotic craft to shape a planet, but it would still be cheaper and faster than flying to a far-distant Earth.

Spotting Candidates

We currently detect distant planets one of two ways: the wobble they exert on their star, as they orbit, or the dimming of their star, when they transit. If a ‘hot Jupiter’ passes in front of its star, that star dims in a predictable fashion. If that Jupiter had a large moon, then that moon will cause the Jupiter’s orbit to alternately slow down and speed up. We would see a periodic lag in the pace of the dimming of the star. Essentially, we can detect a wobble in the dimming-rate — a combination of both techniques! This method would only detect moon-worlds that orbit in our plane of vision, passing directly between their star and our observatories. And, it would only detect moon-worlds that were large enough and close enough to cause their Jupiter to wobble. Yet, these would be our best targets for exploration and colonization.

If we planned to capture many icy micro-planets and use them to water a moon-world, our robotic spacecraft would need to orbit far away from the ‘hot Jupiter’, in the band where comets and asteroids settled. From that high perch, a spacecraft could nudge these icy bodies ever so slightly, causing them to spin inward on a collision course. Like tipping a boulder down a hillside, this would require the minimal energy to propel so much mass. This is also the best strategy for watering worlds here in our solar system; one of Saturn’s ice moons would chill and moisten Venus faster and cheaper than a brine-mining operation on Mars!

Realistically, if Earth-like planets are rare, then the distance to them may be so vast that it is forever uneconomical to make the journey in a single leap. Instead, we may need to travel to a series of much-closer ‘hot Jupiters’, watering their rocky moons with captured comets so that we can rebuild and re-launch from there. I suspect our autonomous spacecraft would ‘land’ on numerous tiny rocks, to gather materials for vast solar cells, and repeatedly nudge those rocks together, to form a habitable moon. Humans would only ‘wake up’ when a world had been built. And, from there, the spacecraft would be re-fueled, to make the next leap.

We have already found plenty of stars with ‘hot Jupiters’. Most of those solar systems support an outer ring of icy clods, and they must have many rocky moons. If we can nudge those proto-planets together, we many not need to travel far at all.

Written by

Easily distracted mathematician

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