Which giant planet formed first?
Short answer: Jupiter
Long answer: Still Jupiter, but let’s dive in and take a more detailed look.
Birth of a Gas Giant
A long time ago in a solar system very near you, just 1 or 2 AU past the snow line, enough surrounding planetesimals were accreted to become an Earth-like body containing about ten (10) Earth masses of metal and rock. This, in turn, gave this massive body enough gravitational attraction to pull vast amounts of hydrogen, helium and ices near its orbit, creating the first planet in our solar system: Jupiter. Impacts from the infalling gases and ices heated Jupiter up, so much so that for a short time, it outshown the protosun, if viewed from equal distances. Jupiter lacked the total mass to become a star, needing to be seventy-five (75) times more massive to achieve the necessary compression and heat in its core to sustain fusion.
Jupiter plowed through the surrounding gas and dust, losing energy and began to spiral inward, possibly reaching as far as Mars’s present orbit before it cleared its neighborhood and stopped growing. The formation of the next planet, Saturn, probably reversed Jupiter’s inward spiral towards the Sun. Together, these first two giants cleared their orbits of gas a debris and as a result of their gravitation interaction with each other, began the slow spiral dance outward to their present resonant orbits. Eventually, Jupiter revolved around the Sun twice for every revolution of Saturn.
Recent Theoretical Developments
As noted above, theoretical work strongly suggests that Jupiter took shape early in the solar system’s history, but the planet’s precise age had remained a mystery. A recent study published in the journal Proceedings of the National Academy of Sciences this past June suggests the nascent gas giant was already that big enough (20 times more massive than Earth), within the first one million years of solar system history. (Wall, 2017)
Nice model of solar system formation proposes that in the outer solar system Jupiter formed first, followed by Saturn, and then by Neptune and Uranus, which were then flung out to their present orbits by gravitational forces from Jupiter and Saturn.
Snow line: The boundary, no closer than roughly 3 AU, between the orbits of Mars and Jupiter, beyond which these gases, primarily hydrogen and helium, and ices of water, carbon dioxide, methane and ammonia, persisted. AKA frost or ice line.
Planetesimals: Boulders with dimensions of a kilometer or so, large enough for their mutual gravitational attractions to enhance the rate at which they collide.
Resonance: Where the periods of two orbits are related by whole numbers because the resulting gravitational interactions between the two bodies reinforce each other.
Comins, N. F. (2015). Chapter 4 Formation of the Solar System. In Discovering the Essential Universe (6th ed., p. 103-5,110). New York, NY: W.H. Freeman and Company.
Striking Gemini images point Juno spacecraft toward discovery. (2017, June 30). Retrieved September 20, 2017, from https://phys.org/news/2017-06-gemini-images-juno-spacecraft-discovery.html
Redd, N. T. (2016, November 28). How Was Jupiter Formed? Retrieved from https://www.space.com/18389-how-was-jupiter-formed.html
Wall, M. (2017, June 12). Ancient Jupiter: Gas Giant Is Solar System’s Oldest Planet. Retrieved from https://www.space.com/37173-jupiter-oldest-planet-solar-system.html