Formation of Binary Giant Planets

Giant planets seem to be ubiquitous around Sun-like stars. Our Solar System has two giant planets - Jupiter and Saturn. Both planets are primarily composed of hydrogen and helium. Jupiter and Saturn have 318 and 95 times the mass of Earth, respectively. Beyond Saturn, the planets Uranus and Neptune are generally classified as “ice giants” because they have much smaller masses and differ considerably in composition compared to Jupiter and Saturn. The orbits of Jupiter and Saturn form a 5:2 orbital resonance. For every five times Jupiter circles the Sun, Saturn would circle the Sun twice. On the whole, the orbits of Jupiter and Saturn are stable over the entire age of our Solar System.

In a planetary system with two giant planets, such as our Solar System, energy and angular momentum are conserved between the two giant planets, and the planetary system is stable. Instability only occurs if the orbits of the two giant planets bring them very close to one another. Exoplanet discoveries over the years have revealed a remarkable diversity of planetary systems. A number of studies have shown that planetary systems with three or more giant planets tend to be unstable. For such a planetary system, perturbations by the additional giant planet(s) tend to destabilise the system.

Figure 1: Artist’s impression of a pair of binary giant planets.

Figure 2: Artist’s impression of a giant planet.

When a planetary system consisting of three or more giant planets is destabilised, it can lead to a number of interesting outcomes. Ochiai et al. (2014) show that gravitationally bounded pairs of giant planets (i.e. binary giant planets) can form via planet-planet scattering during the destabilisation of a planetary system with three giant planets. In their study, N-body simulations of planetary systems with three Jupiter-mass giant planets were performed. The N-body simulations show that as much as ~10 percent of the planetary systems result in the formation of binary giant planets.

During the destabilization of a planetary system with three giant planets, the possible outcomes are - ejection of a planet, planet-planet collision, planet-star collision, formation of a hot-Jupiter and formation of a pair of binary giant planets. A hot-Jupiter forms when a giant planet is thrown inwards to its star whereby planet-star tidal interactions can circularise the orbit of the giant planet into a close-in orbit around the star, leading to the formation of a hot-Jupiter. As for binary giant planets, such a pair could form when two giant planets pass sufficiently close to one another that enough tidal dissipation occurs between them to form a gravitationally bound pair.

In their N-body simulations of planetary systems with three giant planets, Ochiai et al. (2014) used four sets of 100 simulation runs corresponding to the four different initial stellarcentric semimajor axes - 1, 3, 5 and 10 AU for the innermost giant planet. In the nomenclature, “stellarcentric semimajor axis” refers to the average distance of the giant planet from its host star and 1 AU is a unit of measurement equal to the average Earth-Sun separation distance. For the two outer giant plants, their semimajor axes are, respectively, factors of 1.45 and 1.9 times the semimajor axis of the innermost giant planet. The four sets of 100 runs follow the evolution of the planetary system over a period of 10 million years.

The results from the 400 simulation runs show that the formation rate of binary giant planets is ~10 percent and nearly independent of the stellarcentric semimajor axis. Binary giant planets generally form near their initial orbits because the period when they form is normally during the early stages of orbital instability. Regardless of the initial stellarcentric semimajor axes, the distribution of the semimajor axes of the binary giant planets (i.e. average distance between the two giant planets in the binary) show a peak at 2 to 4 times the combined planetary radii of the two giant planets in the binary. Also, the 400 simulation runs show that ejection rates increase and collision rates decrease as stellarcentric semimajor axis increases.

Figure 3: Distribution of the semimajor axes of the binary giant planets obtained from the 400 simulation runs. For each pair of binary giant planets, the semimajor axis is expressed as a ratio to the combined planetary radii of the two giant planets in the binary. Ochiai et al. (2014).

Figure 4: Results obtained from the 400 simulation runs for the four different initial stellarcentric semimajor axes - 1, 3, 5 and 10 AU. The colours represent binary giant planets (red), planet-planet or planet-star collisions (light green), hot-Jupiters (blue), ejections (magenta), and three giant planets still remaining after 10 million years (light blue). Ochiai et al. (2014).

Binary giant planets are expected to be stable over the long-term. If the stellarcentric semimajor axis of a pair of binary giant planets is larger than ~0.3 AU, the system is stable for ~10 billion years, which is similar in duration to the main-sequence lifespan of a Sun-like star. Interestingly, binary giant planets can have moons with wide orbits that circumscribe both planets. A loosely bound moon around one of the two giant planets has a roughly 20 percent chance of surviving the formation process leading to a pair of binary giant planets. Additionally, binary giant planets can also capture large moons into orbit around them, much like how Neptune captured its large moon Triton. Current planet detection methods might be able to detect binary giant planets.

Reference:

Ochiai et al., “Extrasolar Binary Planets. I. Formation by Tidal Capture during Planet-Planet Scattering”, ApJ 790:92 (10pp), 2014 August 1