Our solar system is weird.
First of all, it doesn’t look much like other ones we’ve been finding. A lot of those have Jupiter-size giant planets orbiting very close in to their parents stars (“hot Jupiters”), closer even than Mercury orbits the Sun. By contrast, our Jupiter orbits the Sun much farther out, more than a dozen times Mercury’s distance from the Sun.
Worse, a lot of these other solar systems are compact. They have several planets orbiting close in to their star, and these planets tend to be “super-Earths,” bigger than our home world but smaller than Neptune. They probably have thick atmospheres, too. A good example of this is Kepler-11, which has six planets that orbit their star inside the size of Venus’ orbit.
So why are we so different than everyone else? The answer may be: Jupiter. A new paper has been released that points an accusatory finger at our solar system’s largest world. Ours may have looked a lot like all the others we’ve seen, but Jupiter came along and wiped it out, setting the stage for what see today: lower mass worlds like ours close in, and bigger ones farther out.
Here’s how this works. When the solar system was very young, just a few million years old, it was basically the Sun in the center surrounded by a huge disk of gas and dust. Jupiter formed probably not too far from where it currently is, a few hundred million kilometers out from the Sun … but it didn’t stay there.
Its gravity interacted with the material in the disk around it. The overall effect of this is to cause Jupiter to start moving inward, migrating toward the Sun. It continued to interact with the disk material, including with actively forming bodies that may have been many kilometers or even hundreds of kilometers in size. It would send them inward, crashing into the Sun. As much as 10–20 times the Earth’s mass worth of material could have been wiped out this way by the time Jupiter got to about 230 million kilometers from the Sun (very roughly where Mars is now).
Something stopped its inward movement at that point. The culprit here is Saturn; models have shown that Saturn and Jupiter would also interact gravitationally through a process called a resonance; Saturn repeatedly tugged on Jupiter, pulling it back out of the inner solar system, placing it where it is today.
When it was all done, there was far less material close in to the Sun than there was initially. The inner planets we see today, Mercury, Venus, Earth, and Mars, formed from whatever stuff was leftover, which wasn’t much.
The idea of Jupiter’s migration has been around a long time, but this new model of how it interacts with the disk explains a lot of the weirdness we see now—including why our planets are smaller than we tend to see in other systems (because of the paucity of material from which they formed). The inner planets are thought to have formed as late as 100–200 million years after the solar system got started, and this explains why, too. They formed after Jupiter bullied its way through the system.
It’s also consistent with the existence of hot Jupiters; in other solar systems where a massive planet like Jupiter forms, but no second, slightly less massive planet outside it like Saturn forms, there’s nothing to reverse the course of the bigger one. It keeps moving in until it destroys the inner disk; at that point it stops migrating and you’re left with a system with a big planet orbiting close in.
And here’s a very cool thing: We think super-Earths may form easily and quickly in solar system like ours, perhaps as rapidly as a million years. That may have even happened in our own solar system. But when Jupiter moved in it would have disrupted the orbits of those planets, dropping them into the Sun. If they once existed, they don’t now! Jupiter wiped the slate clean. Then our familiar planets formed later.
Imagine how different our solar system would look if Jupiter hadn’t formed, or Saturn hadn’t reined it in.
The beauty of this model, too, is that it doesn’t just explain what we see, it also makes predictions. For example, if we see an exoplanet system with lots of close-in super-Earths, we should not expect to see a Jupiter-size planet farther out. If it were there it should’ve wiped out the inner planets. If there is a Jupiter-size planet farther out, you should expect to find 1) a second massive planet outside the first, but slightly less massive than the first (if it’s more massive, then it becomes the one to control the situation), and 2) smaller planets like ours in the inner region, not super-Earths. Or maybe nothing at all, if all the material got wiped out.
We’re not quite at the stage yet where we can go through the exoplanets catalog and check that statistically, but we’re getting there. A new planet-finding orbiting observatory is in the works called TESS, which should yield huge numbers of such solar systems, allowing us to check the hypothesis. The Kepler mission, which discovered more than 1,000 planets, has been retooled and may also provide data to confirm or negate this study.
Oh, how I love this. This idea is still just a hypothesis, but it appears to be a good one, and better yet, it can be tested. And here’s the best part: By studying other solar systems, we learn more about ours. An example of one is a poor sample; you need many more to compare and contrast. The early discovery of hot Jupiters threw our ideas of planet formation for a loop, and then super-Earths messed with it more. But we use that data in planetary diversity to expand our models, refine them, and come to a better and greater understanding of ourselves.
Huh. Sounds like a pretty good lesson to me.