When Giant Planets Collide

Giant planets like Jupiter are expected to exist around a significant fraction of stars in the galaxy. In fact, stars containing multiple giant planets in orbit around them are likely to be fairly common as well. Examples include our Sun with 2 giant planets (Jupiter and Saturn) and HR 8799 with 4 known giant planets. The process of planetary system formation around stars is a chaotic one where planets can merge with one another, fall into their host stars, get thrown out, etc. In planetary systems containing multiple giant planets, collisions between giant planets can occasionally occur. It is estimated that a few giant planet collisions are expected to happen in the galaxy each year.

Figure 1: An artistic illustration of the giant planet HR 8799 b. Credit: NASA, ESA and G. Bacon (STScI).

The outcome of a collision event depends on how two giant planets approach each other, their internal structure and the ratio of planet masses. There are 3 possible outcomes following the collision of two giant planets: (i) merger into a single object, (ii) grazing contact or (iii) total destruction of the planets. For the 3rd outcome to occur, the relative approach velocity during a collision event needs to be larger than the surface escape velocity of the giant planet. This is only valid for a class of giant planets called hot-Jupiters which orbit very close to their host stars. The orbital velocity of a hot-Jupiter around its host star is much larger than its surface escape velocity and this provides the kinetic energy needed for a total destruction following a sufficiently “head-on” collision. Giant planets found further from their host stars are moving too slow for total destruction of the planets to occur in a collision event.

Jupiter’s internal structure is believed to consist of a dense core that is surrounded by a thick layer of liquid metallic hydrogen and an outer layer of molecular hydrogen. Such an internal structure is probably typical for giant planets. The most interesting outcome following the collision of two giant planets is the 2nd outcome which involves a grazing collision. After colliding, the giant planets do not merge but continue moving away from each other. During the collision process, some fraction of material will be ejected. The ejected material will be heated to tens of thousands of degrees as kinetic energy gets converted into heat. If the geometric intersection of the giant planets is deep enough, liquid metallic hydrogen can get ejected. Since liquid metallic hydrogen is stable under high pressure, its ejection into the zero pressure environment of space may transform the metallic phase back into the dielectric phase with an energy release of 290 MJ/kg in the process. As a result, a collision event between giant planets is expected to release a prodigious amount of optical and near UV radiation.

Figure 2: The graph here shows a solid curve which represents a giant planet with an internal structure similar to Jupiter’s. The vertical axis represents the relative mass loss ΔM/M and the horizontal axis represents the penetration radius, where r/R = 1.0 denotes the surface of the giant planet. To the left of where ΔM/M ~ 1/80 (horizontal dotted line) intersects the solid curve, the grazing collision is sufficiently deep to penetrate the region of liquid metallic hydrogen. To the left of where ΔM/M ~ 1/20 (horizontal dotted line) intersects the solid curve, the collision is sufficiently “head-on” for the collided giant planets to merge. Between ΔM/M ~ 1/80 and ΔM/M ~ 1/20, liquid metallic hydrogen can be ejected from the collision event. For comparison, the dashed curve represents a giant planet with an internal structure similar to Saturn’s.

Giant planets have strong magnetic fields which gradually become weaker with age. Since giant planet collisions tend to occur in young planetary systems, the giant planets involved are expected to have magnetic fields that are many times stronger than giant planets that are billions of years old. As giant planets collide, the destruction of their powerful magnetospheres produce intense bursts of radio waves that are large enough to be detected by modern radio telescopes.

It should be realized that the properties of metallic hydrogen are still largely unknown because metallic hydrogen can only form under extreme pressure and has yet to be reliably produced in laboratory experiments on Earth. Nevertheless, it is hypothesized that metallic hydrogen may be metastable over billions of years, similar to diamond - a metastable form of carbon. If a metastable form of metallic hydrogen exists, fragments from possible giant planet collisions during the formative period of our solar system may still linger around. Additionally, ejected fragments of metastable metallic hydrogen from other planetary systems can occasionally enter our solar system. The existence of these fragments may be determined by looking out for unusual types of meteorites.

References:

1. V. I. Dokuchaev and Yu. N. Eroshenko (2012), “Collisional Destructions of Giant Planets and Rare Types of Meteorites”, arXiv:1202.5920 [astro-ph.EP]

2. V. I. Dokuchaev and Yu. N. Eroshenko (2013), “Observational Signatures of the Giant Planets Collisions”, http://dx.doi.org/10.1016/j.pss.2013.01.007