Space

Proposed planetary system classifications suggest ours is the rarest

An artist's impression of the TRAPPIST-1 system, an example of a "similar" system according to a new study
NASA/JPL-Caltech
An artist's impression of the TRAPPIST-1 system, an example of a "similar" system according to a new study
NASA/JPL-Caltech

Astronomers have classified planetary systems into four distinct categories, based on the sizes and arrangements of their planets. As it turns out, the architecture of our own solar system is the rarest kind.

Decades of telescopes dedicated to the hunt for worlds around stars other than our own Sun have yielded more than 5,300 of these exoplanets so far, contained in 3,910 planetary systems. With that much data astronomers have been able to classify these planets into different groups based on their characteristics – there are rocky planets, gas giants, Super-Earths, mini-Neptunes and water worlds, among others.

But can planetary systems themselves be classified in similar ways? And if so, how does our own solar system stack up on a cosmic scale? Answering those questions was the goal of a new study by scientists in Switzerland, who examined data from all 853 systems known to contain multiple planets.

From this analysis, the team settled on four main classes that planetary systems fall into, based on the sizes and arrangements of their planets: Similar, Ordered, Anti-ordered and Mixed. Similar systems, the most common arrangement, are those where the planets are all about the same size – for example, the TRAPPIST-1 system, which contains seven roughly Earth-sized rocky planets. Ordered systems are those where the inner planets are small and rocky, and give way to the gas and ice giants in the outskirts. Our own solar system falls into this group, and the team says it’s the rarest configuration.

Anti-ordered systems are the inverse – the bigger planets appear closer to the star and get smaller the further out you go. And finally there are Mixed systems, which don’t seem to have any rhyme or reason to the arrangement of their planets.

So how do planetary systems end up in these different configurations? Like many things, the team says it’s a mix of “nature and nurture” – it partly depends on the initial conditions that the system is born from, including the mass of the disk of dust and gas that forms the planets, and the abundance of heavy metals in the host star. It also partly depends on the dynamics of the planets during the system’s lifespan.

“From rather small, low-mass disks and stars with few heavy elements, 'similar' planetary systems emerge,” said Lokesh Mishra, lead author of the study. “Large, massive disks with many heavy elements in the star give rise to more ordered and anti-ordered systems. Mixed systems emerge from medium-sized disks. Dynamic interactions between planets – such as collisions or ejections – influence the final architecture.”

The more we can learn about other planetary systems, the better we can understand our place in the universe.

The research was published in two studies in the journal Astronomy & Astrophysics.

Source: University of Bern

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5 comments
Ric
I’m skeptical that our detective technologies are advanced enough to determine relative abundance of these different types. A detectability bias might be at play here at this stage of the game. Still interesting that strides are being made in making these kinds of distinctions. I suspect in the end it will be discovered to be a bit (or a lot) more complex.
rgbatduke
I agree, Ric -- I was thinking of posting exactly the same thing. We can't even detect Earth-scale rocky planets under most circumstances, for example for G-type stars, especially if there ARE gas giants around dominating the tiny orbital or brightness variations that are used to infer the existence of planets at all. It isn't like we can actually see them for nearly all of the cases where we can "detect" them. And range/resolution is a huge issue. One wonders whether the new resources coming on line will be used primarily to continue the search for MORE exoplanets at the limits of detection, or to study in increasing detail nearby stars including ones that might have a category not yet discussed -- JUST a few rocky planets, Earth/Venus scale -- in the Goldilocks zone around suitable e.g. Sol-like stars. My recollection is that we can only detect them if their mass is 2-3 Earth masses minimum at this point, and often can only detect them from eclipse variations as they orbit which requires a coincidence in orbital plane.

I should work out the math a bit more -- using visible-ish light (wavelength "around" 1 micron), an Earth-sized planet at 1 AU from a star 15 LY (1.5 \times 10^{17} meters) away has an angular separation of 1.5 \times 10^11/1.5 \times 10^17 = 10^{-6} radians. Rayleigh's Criterion then tells us 10^{-6} = 1.22 \lambda/D => D = 1.22 \lambda/10^{-6} = 1.22 meters, so Hubble-scale telescopes using visible light "can" resolve this within the diffraction limit. However, the relative strength of the light -- hmm, if the planet were as bright as the star, it would be emitting a millionth of the light. But it isn't -- it would be reflecting light averaging at most a few hundred W/m^2 as opposed to "a lot of damn light" on a star's surface. I recall recent articles where they managed to block out the direct light from the stars and actually resolve a planet nearby, but I THINK it was still a large one, not Earth scale, and maybe a lot brighter as well?

As you say, strides are being made, but it is good to be skeptical for some time now. As is the case in SETI whining, it is an act of hubris to suppose that our existing technology is up to the task of detecting detailed structure of entire planetary systems, let alone detecting the miniscule RF radiation we naively "expect" from nearby civilizations within the paltry diameter we can convince ourselves that we can meaningfully search. But we are getting there.
Username
If you're going to say it's the rarest maybe you should give percentage of each type.
fluke meter
I guess it seems to me 'ordered' perhaps should have meant largest to smallest as presumably the sun is the largest body in the system and order would be from largest to smallest.. I suspect we considered our order to be the right or natural order and thus we are ordered and the other way around inverse - alas..
fluke meter
In addition - its kind of funny to think of this as research - it sort of sounds like a spreadsheet sort and math for a high school intern..