Compared to other planetary systems, how typical is the Solar System? Until recently this question couldn’t be given a satsfactory answer because no other planetery systems were known. Astronomers’ theories of things like planet formation tended to be overfitted to the Solar System simply because there wasn’t enough data to come up with much else. But since the mid-2000s, several thousand exoplanets have been discovered, and many of these belong to one of more than 800 known multiplanetary systems. With these discoveries it is now possible to consider the Solar System (that is, the multiplanetary system based on the star Sol) in a more general context.

QUESTION: Broadly speaking, do other multiplanetary systems look like the Solar System?

ANSWER: Yes.

A striking feature of the Solar System is its orderliness. All the planets orbit the Sun in what can be roughly imagined as a more-or-less flat plane, and they all move in the same direction. They don’t orbit perpendicular to each other, they don’t go in opposite directions, they don’t pull each other into funny loop-de-loops, nothing like that. The planets all get along nice. Are other systems configured this way, or do they have other arrangements?

In fact, the flat, orderly plane seems to be typical of multiplanetary systems. This is something that could have been predicted a priori. A system having a more complicated choreography, with different planets orbiting in opposite directions, perpendicular to each other, etc, wouldn’t be expected to be very stable. Such an unstable configuration could cause two planets to bang into each other, further increasing instability, and eventually the whole system would collapse. Chances are we wouldn’t be able to observe such a system, and thus observable systems tend to be stable.

QUESTION: Are the planetary categories of the Solar System universal?

ANSWER: Mostly.

The Solar System is made of up eight planets¹ grouped into three categories:

Rocky / terrestrial (Mercury, Venus, Earth, Mars)
Gas giants (Jupiter, Saturn)
Ice giants (Uranus, Neptune)

A theory of planet formation overfitted to the Solar System would predict that all planets must fit into one of these categories. A priori one might expect that there would be a extreme diversity in the composition of planets, and that such a classification would be crude and limited. But in fact, many known exoplanets do more or less match one of the categories, so such a theory would not be wildly mistaken.

One group of planets with no Solar System analogue are the so-called neptinis, also known less evocatively as sub-Neptunes. These are planets that are roughly midway in volume between Earth and Neptune. Planets of such size are believed to be Neptune-like rather than Earth-like because a rocky planet can only get so big before it gets compressed by its own gravity. A puffy ice planet, on the other hand, has much less mass and can therefore expand significantly. Actually, most ice planets are smaller than Neptune and Uranus, and thus the “giants” are the outliers in the category of ice planets.

QUESTION: Is the ordering of the planets typical?

ANSWER: No.

In the Solar System, the rocky planets are closest to the star, then the gas giants, and the ice giants are the furthest away. A theory overfitted to the Solar System might predict that this ordering should always appear. But all sorts of different orderings have been discovered, and so such a theory would be totally wrong. The most prominent kind of counterexample are the inner Jupiters. These are Jupiter-like gas giants that orbit their stars at a distance more like Mercury.

Inner Jupiters are not uncommon among known exoplanets, but it would be imprudent to assume that they are common generally. This is because of a selection bias. Known exoplanets are, obviously, known, which is to say that they have somehow been observed. There are a few ways to detect exoplanets, but by far the most successful has been the transit method. When a planet passes directly between its star and an observer (that is, when it transits), the star will briefly be darkened, and this can be measured. The darkening will be more pronounced the larger or closer to the star the planet is. Thus a large planet close to its star will be an ideal candidate for getting observed, and so such planets are likely over-represented among known planets.

Footnotes

¹ The order of the planets can be remebered using a handy mnemonic device: Mary’s “Virgin” Explanation Made Joseph Suspect Upstairs Neighbor.