Sometimes, we get papers and press releases where the cool news and the news they are trying to push forward are not entirely the same thing. Today, we received a pair of press releases about a new paper in Astronomy and Astrophysics concerning the relationship between the formation of large rocky worlds and Jupiter-like planets at middle-distances in our solar system. The headline states that “Earth-like Planets Often Come with a Bodyguard,” and then goes on to discuss new computer models being created at the Max Planck Institute for Astrophysics.

If you’re a long-time member of our audience, you know that computer models currently are pretty bad at describing planet formation, and this title and the article lede intrigued me because it implied that something we’ve long suspected is true can be reproduced in models, which would be awesome.

And then I read things in detail and realized, nope, what they have is a review of observations that shows that in the Venn diagram of solar systems layouts, 30% of systems observed to have large rocky worlds have a cold Jupiter, and all solar systems with a cold Jupiter have a large rocky world. Put another way, if a cold Jupiter is present, a large rocky world is always present, too, but there are also systems – in fact, there are more than twice as many observed systems – that have a known large rocky world and no matching cold Jupiter.

And the simulations discussed in the stories lede – they do not match what is being seen. Nope. Not at all. They ran 1000 simulations of planetary systems that evolved in a protoplanetary disk around a Sun-like star. Instead of all the cold Jupiters having a rocky world, only 30% of the simulated cold Jupiters had one. In the simulations, instead of 30% of the rocky worlds having a cold Jupiter, only 10% of them did. 

They reason, and here I quote the press release: One explanation has to do with the rate at which gas planets gradually migrate inward. Planet formation theory seems to predict higher rates than observed, leading to an increased accumulation of gas giants on orbits of intermediate distance. In the simulations, these ‘warm Jupiters’ interfere with the inner orbits and cause more super-Earths to be ejected or even collide in gigantic collisions. With a slightly lower tendency of the simulated gas planets to migrate, more of the super-Earths would remain, which would be more compatible with the observations.

Here’s the thing though: these simulations have been done over recent years not weeks, and our understanding of solar system formation is changing so rapidly that these folks are missing new information on planets co-evolving at the same time as stars, with similar timescales for initial formation. Beyond having a mismatch in timelines, the simulations also – like all simulations – can’t accurately reproduce planetary migration because we don’t completely understand planetary migration.

This is a cool note of “Hey if you have a cold Jupiter, you’re going to have a large rocky world.” This implies something fundamental about how forming gas giants influence the formation of smaller bodies. The fact that only 30% of large rocky worlds have gas giants also implies there are either multiple formation mechanisms for large rocky worlds, or that there are mechanisms for eradicating cold Jupiters from view, or maybe both. This is cool.

This is just not the focus of the press release. 

The reality is that folks most likely got funding to do extensive modeling of solar systems, and they absolutely did that to the best of their ability with the incomplete data we have. Having completed their work, they needed to publish their results, and they have results. But when your simulations and reality don’t entirely match… well, this is the kind of press release you get. It has cool results, just not the ones they were probably hoping for. Ideally, you want the simulations and reality to match, but science progresses through both successes and things that go sideways. It all contributes, and now we know these particular models are missing some key ingredients needed to describe our reality. And our reality is kind of cool.

More Information

MPIA press release 

“The New Generation Planetary Population Synthesis (NGPPS). III. Warm
Super-Earths and Cold Jupiters: A Weak Occurrence Correlation, But with a
Strong Architecture-Composition Link,” M. Schlecker et al., 2020, in Astronomy & Astrophysics (preprint on arxiv.org)