Banded, big, and beautiful, enormous Jupiter reigns supreme as the King of Planets in our Sun’s enchanting family. This distant world, famous for its crimson hurricane-like storms and many moons, sports the hefty mass of 2.5 times that of all the other major planets in our Solar System combined. Indeed, Jupiter is so massive that its barycenter with our Sun is situated above the Sun’s surface at 1.068 solar radii from the Sun’s center. But, beneath its extremely heavy blanket of gas, Jupiter hides a tragic secret. At its very core, Jupiter has a broken heart. In August 2019, a team of astronomers announced that they may have determined how Jupiter’s heart was broken. This majestic world may still be reeling from a colossal head-on collision, that it suffered in its youth, with a still-forming protoplanet. The sad event occurred in the early days of our Solar System, about 4.5 billion years ago. This new theory could explain mysterious readings obtained from NASA’s Juno spacecraft, according to a study published in the journal Science. Juno is a space probe in orbit around Jupiter. It was built by Lockheed Martin and is operated by NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
A team of astronomers from Rice University in Houston, Texas, and China’s Sun Yat-sen University, propose that an ancient head-on collision between young Jupiter and a doomed still-forming world can explain previously surprising gravitational readings. The puzzling readings suggest that Jupiter’s core is less dense than expected.
“This is surprising. It suggests that something happened that stirred up the core, and that’s where the giant impact comes into play,” commented study co-author Dr. Andrea Isella in an August 14, 2019 Rice University Press Release. Dr. Isella is an astronomer at Rice.
Dr. Isella went on to explain that leading theories of planet formation propose that Jupiter began as a dense rocky or icy youthful planet that later collected its extremely thick atmosphere from the primordial disk of gas and dust that gave birth to our Star. What was left of the ancient protoplanetary accretion disk eventually provided the material that formed the planets, moons, and other objects inhabiting our Solar System.
Big, Banded, And Beautiful
Jupiter is the fifth major planet from our Sun, and it is separated from the quartet of inner solid planets–Mercury, Venus, Earth, and Mars–by the Main Asteroid Belt, that is situated between it and the outermost of the inner planets, Mars. Jupiter is the innermost of the four outer giant gaseous worlds that also inlude Saturn, Uranus, and Neptune. In the cold perpetual twilight of the outer Solar System, beyond Neptune–the major planet farthest from our Star–there is a region populated by myriad frigid comet nuclei. This very remote area of our Solar System is called the Kuiper Belt.
Both asteroids and comets are the relics of a bygone era, when our Solar System was first forming. The dust within the protoplanetary accretion disk, swirling around our young Sun, was well-endowed with a natural “stickiness”. The tiny motes of dust bumped into one another and merged, forming ever larger and larger objects–from pebble size, to boulder size, to mountain size, to moon size, to planet size. The primordial planetary building blocks, named planetesimals, collided and merged, thus forming the planets. The asteroids that primarily inhabit the inner Solar System are close kin to the rocky and metallic planetesimals that collided and merged to form the quartet of solid inner planets. Likewise, the icy planetesimals bumped into one another and ultimately grew into the giant gas-laden planets inhabiting our Solar System’s outer limits. Jupiter’s core was built up by these colliding and merging icy planetesimals, that crashed into one another in a newborn Solar System that was a violent place–a “cosmic shooting gallery” where horrific collisions frequently occurred between rampaging objects.
Jupiter sports a hefty mass that is one-thousandth of the mass of our Sun, and it has been known to sky-watchers since antiquity. Because of its enormous size, it was named after the King of the Roman gods, Jupiter (Greek Zeus). When Jupiter is observed from Earth, it can be bright eough for its reflected light to cast shadows on our planet. Indeed, it is (on average) the third brightest natural object in Earth’s night sky after the Moon and Venus.
Jupiter is primarily made up of hydrogen, with about 25% of its mass composed of helium–although helium accounts for only about one tenth of the number of molecules. The atmospheric proportions of hydrogen and helium are thought to be close to the theoretical composition of the primordial solar nebula. If Jupiter really does have a rocky core, it is composed of elements heavier than hydrogen and helium. Although this enormous King of Planets has been known since ancient times, it remains a “riddle, wrapped in a mystery, inside an enigma.” This heavily gas-wrapped behemoth reveals its many secrets very slowly.
Like the other three gas-enshrouded giant planets, Jupiter does not have a well-defined solid surface. Because it has a very rapid rotation rate, Jupiter’s shape has evolved into an oblate spheroid–that is, it sports a slight but noticeable bulge around its equator.
The Jovian outer atmosphere is observed to be segregated into several bands at differing latitudes. This results in turbulence and stormy weather along their intersecting boundaries. A very noticeable result is Jupiter’s famous Great Red Spot, which is an enormous vortex storm similar to hurricanes on Earth. This crimson storm has been observed since at least the 17th-century, when it was first detected by telescopes. There is also a faint planetary ring system circling Jupiter and an extremely strong magnetosphere. At last count, Jupiter is known to have 79 moons, including the four large Galilean moons, discovered by Galileo Galilei in January 1610.
In recent decades, astronomers have discovered almost 500 planetary systems that host multiple planets. Normally, these systems include a few planets boasting impressive masses that are several times heftier than Earth’s (super-Earths), hugging their stellar parents in sizzling orbits that carry them closer to their stars than Mercury’s orbit around our Sun. One theory proposes that Earth and its neighboring planets may have been born from fragments of planets that met their doom after collisions with Jupiter. This could have destroyed super-Earths that may have been basking near our Sun.
Astronomers think that Jupiter migrated from a colder and more distant birthplace when our Solar System was young. This primordial wandering resulted in what theorists refer to as the grand tack hypothesis. According to this theory, gravitational tugs and pulls caused a series of collisions between the doomed super-Earths as their orbits began to overlap. Scientists from Lund University in Sweden found that Jupiter’s migration lasted for about 700,000 years, and that it occurred approximately 2 to 3 million years after Jupiter’s birth as an ice asteroid, far from our Sun in our Solar System’s deep freeze. The treacherous journey towards our Star followed a spiraling course. This means that Jupiter continued to circle our Sun–but in an increasingly tighter and tighter orbit. Jupiter’s migration is thought to have been the result of gravitational forces originating from surrounding gases in our Solar System. The ancient Solar System was a violent place, with numerous crashes occurring between youthful objects. The primordial objects met up with one another, usually with tragic consequences.
Jupiter’s Broken Heart
Dr. Isella was skeptical at first when study lead author, Dr. Sheng -Fei Liu, initially proposed that the puzzling data could be explained by a catastrophic giant impact that shattered Jupiter’s core. The impact would have resulted in mixing the denser contents of the Jovian core with layers that were less dense.situated above them. Dr. Liu is a former postdoctoral researcher in Dr. Isella’s group. He is currently a member of the faculty at Sun Yat-sen in Zhuhai, China.
“It sounded very unlikely to me, like a one-in-a-million probability. But Shang-Fei convinced me, by shear calculation, that this was not so improbable,” Dr. Isella commented in the August 14, 2019 Rice University Press Release.
The team of astronomers ran thousands of computer simulations, and they found that a fast-growing Jupiter could have disrupted the orbits of nearby “planetary embryos” (protoplanets) that were still in the early stages of formation.
Dr. Liu continued to explain that the calculations included estimates of the probable angles. In all of the cases modeled, Dr. Liu and his colleagues found that there was at least a 40% chance that the migrating young Jupiter would devour a planetary embryo within its first few million years. In addition, head-on collisions would be more probable than grazing strikes for the young Jupiter as it continued its march inward towards our Star.
Dr. Isella went on to comment in the August 14, 2019 Rice University Press Release that the collision scenario became more compelling after Dr. Lui ran a 3D computer model that showed how a collision would affect Jupiter’s core.
“Because it’s denser, and it comes in with a lot of energy, the impactor would be like a bullet that goes through the atmosphere and hits the core head-on. Before impact you have a very dense core surrounded by atmosphere. The head-on impact spreads things out, diluting the core,” Dr. Isella explained in the same Rice University Press Release.
Impacts that occurred at a grazing angle could result in a doomed protoplanet becoming gravitationally trapped–eventually sinking down into Jupiter’s core. Smaller planetary embryos, approximately as massive as Earth, would fall apart in Jupiter’s thick atmosphere.
“The only scenario that resulted in a core-density profile similar to what Juno measures today is a head-on impact with a planetary embry 10 times more massive than Earth,” Dr. Lui noted in the August 14, 2019 Rice University Press Release.
Dr. Isella continued to explain that the calculations suggest that even if this impact happened 4.5 billion years ago, “It could still take many, many billions of years for the heavy material to settle back down into a dense core under the circumstances suggested by the paper.”
Dr. Isella, who is also a co-investigator in the Rice-based NASA-funded CLEVER Planets project, added that this study has implications that reach beyond our own Solar System.
“There are astronomical observations of stars that might be explained by this kind of event,” he commented.
“This is still a new field, so the results are far from solid, but as some people have been looking for planets around distant stars, they sometimes see infrared emissions that disappear after a few years. One idea is that if you are looking at a star as two rocky planets collide head-on and shatter, you could create a cloud of dust that absorbs stellar light and reemits it. So, you kind of see a flash, in the same sense that now you have this cloud of dust that emits light. And then after some time, the dust dissipates and that emission goes away,” Dr. Isella added.