“There’s nothing that cleanses your soul like getting the hell kicked out of you.”
There’s no doubt that we lucked out when it came to the formation of our Solar System.
Our inner Solar System, where temperatures are ideal for liquid water and life-as-we-know-it, is full of rocky planets and devoid of any gas giants for many hundreds of millions of miles. But, as we know all too well from the last twenty years of finding exoplanets, this isn’t the only way it could have turned out.
In fact, of the some 2,300 planets found around other stars so far, the vast majority of them are either very, very hot, very, very large, or both.
This could just be due to the fact that those types of planets are the easiest ones to find with the technology and techniques we have available to us right now, and we may wind up finding more solar systems like ours. What seems most important, at least as far as the search for extraterrestrial life (or possibly habitable exoplanets) goes, is looking for rocky planets where the temperature is right for liquid water.
And every star has a region around it where that’s possible.
But although we’ve found a small but important number of stars with rocky planets that live in their habitable zones, there’s one type of exoplanet system that never has a rocky planet in their habitable zones.
Not only that, but they — at least so far — never have any other planets in their solar systems! What gives?
Over in our Solar System, we’ve got our rocky worlds interior to all of our gas giants, with an asteroid belt interior to the giant planets, and a kuiper belt exterior to our four Jovians. But in systems where there’s a Hot Jupiter, or a gas giant very close to its parent star, we don’t find any other planets anywhere near the star itself.
Why is that?
When you bring a large-mass planet into close quarters with a much smaller one, something’s got to give. Because of the way gravitation works, you’re going to have a three-body gravitational interaction, which is one of the most difficult theoretical problems in all of physics.
But universally, what’s going to happen is that there’s going to be a transfer of momentum and angular momentum, and one of three things is going to happen. Either:
- The large planet will gravitationally capture the smaller one, turning it into a moon,
- The large planet will fling the small planet either violently into its Sun or violently out of the Solar System, or
- The large planet will collide with the small one, effectively eating it.
In other words, these Hot Jupiters are like the psychotic eldest child that murders its younger siblings! We see that there are no rocky planets among the gas giants in our Solar System for exactly this reason. But how does this work, and why does this prevent solar systems with Hot Jupiters from having habitable planets outside of them? This is not obvious, so let me first set up the situation for you to help you get a handle on it.
As far as we understand it, all solar systems start out the same way in the very early stages: there’s a central region where a star forms, accompanied by a proto-planetary disk, which will coalesce and form into planets.
But over time, planets clear the space around them. They do this, believe it or not, in the same way that star clusters all eventually end up dissipating: simple gravity. Watch this simulation of a star cluster to see what I’m talking about, and pay particular attention to the first 25 seconds of the video.
Notice how an occasional star just sped off from the star cluster in a random direction, even early on in the simulation? That’s what gravitational encounters between more than 2 bodies do, pretty much all of the time! The physical process is called violent relaxation, which is a wonderful term that (somehow) no enthusiast has created a wikipedia entry for!
Just as the structure of the star cluster changes because of these gravitational ejections, the structure of the young solar system changes because of its gravitational interactions. These large planets can migrate, depending on how they absorb or remove each tiny mass they encounter. Some encounters move them closer towards their parent star, others move them farther away. In the case of Jupiter and Saturn, they were once significantly closer to the asteroid belt than they are today; in the case of Hot Jupiters, they clear out at least the entire habitable zone!
According to research led by Eric Ford, the reason we haven’t found any of these habitable, rocky planets around stars with Hot Jupiters — even though we’ve found 63 of them around other stars — is because Hot Jupiters perturb the orbits of these planetesimals in the young solar systems sufficiently so as to completely clean them out, leaving no possible rocky worlds behind!
In our Solar System, our four gas giants completely cleared out the regions between about 4 Astronomical Units and about 30 Astronomical Units, where one of them is the distance between the Earth and the Sun. But if you put a Jupiter-sized planet anywhere in the inner Solar System, then all of the rocky planets wouldn’t be here!
And that’s why systems with exoplanet bullies in them — Hot Jupiters, to be specific — have no Earth-like planets in them, or rocky worlds with the proper temperatures for water. That’s just one more example of the power of gravity!