April 26, 2021 — The solar system’s two largest planets, Jupiter and Saturn, received worldwide publicity on December 21, 2020, as they glided closer than they’ve been since 1623. Visible around the globe, “The Great Conjunction” placed the two planets only 0.1 degree apart from one another.
Typically, however, Jupiter and Saturn have been known to “keep their distance” from one another. And, understanding why these two planets have so much space between them was the focus of an Icarus journal article earlier this month. Born Eccentric: Constraints on Jupiter and Saturn’s Pre-Instability Orbits encompassed the analysis of supercomputer simulations by an international team of researchers – thanks to allocations from the National Science Foundation’s (NSF) Extreme Science and Engineering Discovery Environment (XSEDE).
Comet at the San Diego Supercomputer Center at UC San Diego and Bridges at the Pittsburgh Supercomputing Center were used to run more than 6000 simulations to better understand the space between Jupiter and Saturn. The simulations’ development and analyses was led by Carnegie Institution of Washington Postdoctoral Fellow Matthew Clement, who teamed with astronomer Sean Raymond of the Laboratoire d’ Astrophysique de Bordeaux and several researchers from the University of Oklahoma, Rice University, and the Southwest Research Institute.
“We are fairly certain that the giant planets, including Jupiter and Saturn, were born closer together than they are today and one challenge to determine how and why they are now so far apart is to better understand how Jupiter’s orbit became so eccentric and elliptical (non-circular),” said Clement. “Historically, simulations that reproduce Jupiter’s orbital shape tend to push Saturn too far out in to the outer solar system, beyond where Uranus is today, so with our study, we used initial conditions consistent with hydrodynamical models of the giant planets forming in gaseous proto-planetary disks to more consistently generate Jupiter and Saturn-like orbits.”
While previous studies have assumed that Jupiter and Saturn were born in what is known as a 3:2 mean motion resonance (Jupiter went around the Sun three times for every two Saturn cycles), Clement’s research considered an initial 2:1 resonance (two Jupiter orbits for every one of Saturn’s). Thus, the planets formed further apart.
“This is the best way to explain the planets’ modern orbital dance,” he said. “Interestingly, perhaps the best observed photo-planetary disk, known as PDS-70, a system of planets in the process of growing, seems to be dominated by two giant planets similar to Jupiter and Saturn in our own solar system also in a 2:1 resonance.”
Understanding how Jupiter and Saturn acquired their orbital shapes and mutual spacing may not seem like that big of a deal, but it turns out that the interplay of these two gas giants’ orbits drive a good amount of the solar system’s evolution as a whole. Jupiter itself makes up about two-thirds of all the total mass in planets, asteroids, and comets in the solar system. Meanwhile, Saturn comprises the majority of the rest of the material.
“The orbital dance that Jupiter and Saturn perform today drive a myriad of dynamical effects in the solar system, and likely affected the Earth’s growth in the past,” explained Clement. “This helps us understand why Earth is a nice temperate and water-rich place where we can live, while Mars and Venus are quite inhospitable to life as we know it.”
Understanding Jupiter and Saturn in this manner also helps us compare our own system of planets to the large contingent of discovered exoplanets. Clement said that if we were observing our own solar system from afar, with current techniques, we would only be able to detect Jupiter and Saturn – not any of the other planets. However, when we look at the population of planets detected so far with masses similar to that of Jupiter and Saturn, their orbits look nothing like those in the solar system.
Some systems host Jupiter-like planets on very short orbits, closer to the Sun than Mercury (the so-called hot Jupiters). Others host Jupiter and Saturn-like planets on more distant orbits (like those of the actual Jupiter and Saturn), however their orbital eccentricities are extremely high (only comets have orbits this extreme in our solar system). There are also a few systems with four or more giant planets on wide orbits with low eccentricities like our giant planets, but they are in a chain of resonances. So, the solar system exists in the curious “middle ground” between those last two types of systems.
“Our work essentially tries to understand why we appear to be the ‘missing link’ between these two types of systems, and our results indicate that this is because of Jupiter and Saturn formed in the 2:1 resonance rather than a more compact chain like the 3:2,” said Clement. “Because this is such a highly chaotic process, we would not have been able to take our project to this scale of thousands of simulations without Comet and Bridges.”
“The computational resources available to us through XSEDE were key to the success of our project, which I learned about through our campus champion Floyd Fayton,” concluded Clement. “XSEDE really opens up all kinds of possibilities in terms of being able to investigate complex problems and new ideas – these resources provide an invaluable contribution to my field of science.”
Key funding for this research was provided by the National Science Foundation grant AST-1615975, NSF CAREER award 1846388, the NASA Astrobiology Institute solicitation NNH12ZDA002C and cooperative agreement number NNA13AA93A, and NASA grant 80NSSC18K0828. Time on Comet and Bridges were awarded via XSEDE allocation TG-AST200004.