Jupiter: The Banded, Big, And Precocious “King Of Planets”
By Judith E Braffman-Miller
Banded, big, and precocious, the gas-giant Jupiter is our Solar System’s resident planetary behemoth with a mass that is one-thousandth that of our Sun–and two and a half times that of all the other planets in our Solar System combined. Jupiter is like a star in composition, and if it had grown about 80 times more massive, its nuclear-fusing fires would have ignited–and a star would have been born, instead of a planet. Our Solar System emerged approximately 4.56 billion years ago when a relatively small, but extremely dense blob, embedded within the swirling, undulating folds of a beautiful, giant, dark, and frigid molecular cloud, collapsed under the relentless and merciless weight of its own gravity–giving birth to our Sun, and its stellar siblings. But even though our Solar System is our planet’s home, our Sun and its family have managed to keep some enticing mysteries to themselves–just waiting to be solved. In June 2017, an international team of astronomers announced they have found that Jupiter is an old-timer–the first-born planetary offspring of our parent-Star, the Sun.
By examining tungsten and molybdenum isotopes on iron meteorites, the team, composed of scientists from Lawrence Livermore National Laboratory (LLNL) in Livermore, California, and the Institut fur Planetologie at the University of Munster in Germany discovered that meteorites are made up from two genetically distinct nebular reservoirs that coexisted–but, nevertheless, remained separated between 1 million and 3 to 4 million years after our Solar System had begun to emerge from its natal nebula.
The giant, dark molecular cloud from which our Sun and its family emerged is only one of many that haunt our Milky Way Galaxy. These billowing, frigid clouds are mainly composed of gas and dust, and they serve as the nurseries of bright new baby stars. When a small dense blob, that is embedded within the swirls of a molecular cloud, finally experiences gravitational collapse, most of its material gathers at the center, and nuclear-fusion ultimately lights its fire–and a new baby star is born. The remaining material circles around the young protostar, evolving into what is termed a planetary accretion disk. This rotating disk of gas and dust circles its newborn star. Once, very long ago, such a disk circled around our primordial Star, and the extremely tiny particles of naturally “sticky” dust floating around within it bumped into each other and “glued” themselves together to form ever larger and larger objects. At last, an enormous populations of planetesimals evolved, and eventually built up the major planets.
“The most plausible mechanism for this efficient separation is the formation of Jupiter, opening a gap in the disc (a plane of gas and dust from stars) and preventing the exchange of material between the two reservoirs. Jupiter is the oldest planet in the Solar System, and its solid core formed well before the solar nebula gas dissipated, consistent with the core accretion model for giant planet formation,” explained Dr. Thomas Kruijer in a June 17, 2017 LLNL Press Release. Dr. Kruijer is lead author of the paper appearing in the June 12, 2017 online issue of Proceedings of the National Academy of Sciences. Formerly of the University of Munster, Dr. Kruijer is now at LLNL.
The core accretion model is the most widely accepted scenario for the birth of Jupiter-like planets. According to this model a rocky core develops as a result of the coagulation of planetesimals until it is massive enough to accrete a heavy gaseous envelope. At first, this envelope is in hydrostatic equilibrium, with the lion’s share of the luminosity originating from accreting planetesimals–the building blocks of planets in a primordial planetary system. When the core finally reaches a critical mass, however, hydrostatic equilibrium is no longer possible. At this point, an era of rapid gas accretion occurs.
Our Star’s First Born Planet
When Jupiter was born, it could have become a star. However, it did not fulfill this promise–and it failed. The energy hurled out by the crashing material caused Jupiter’s interior to become extremely hot–and the larger Jupiter grew, the hotter it became. At last, when the material stolen from the turbulent, surrounding disk was depleted, Jupiter may have shown the impressive diameter of more than 10 times that which it currently sports. It also possessed a central temperature of a broiling 50,000 Kelvin (the Kelvin scale is an absolute scale of temperature, where zero equals -459.4 degrees Fahrenheit). The primordial Jupiter was extremely bright, and it had a luminosity approximately 1% as great as that of our Sun today.
If Jupiter had been born more massive, it would have grown hotter and hotter and hotter, as it shrunk in size–until its nuclear-fusing stellar furnace ignited, giving birth to a brand new blazing baby star. If this had happened, we would not be here today. Our own Sun, like most other stars, would then have had a binary companion. According to this model, our own planet, as well as the rest of our Solar System, probably could not have formed.
Jupiter is the fifth planet from our Sun, and it rotates more rapidly than any other planet in our Solar System, as it orbits our Star at a mean distance of 5.2 astronomical units (AU). One AU is equivalent to the average Earth-Sun separation of 93,000,000 miles. This means that Jupiter’s distance from our Sun is a little more than five times the separation between our planet and our Star. When observed from Earth, Jupiter is usually the second brightest planet in our night sky–only the planet Venus is brighter.
Jupiter takes about 12 Earth-years to finish a single orbit around our Sun. This means that one year on Jupiter is equal to a dozen years on our own planet. The temperature of the Jovian clouds, that skim the very top of Jupiter’s thick atmosphere, is a frigid -234 degrees Fahrenheit. In dramatic contrast, the temperature near the planet’s core may reach a broiling 43,000 degrees Fahrenheit–a temperature that is even hotter than the surface of our Sun.
If an Earthling could stand on the top of the Jovian clouds–which, of course, is impossible–the intense force of gravity would be the equivalent of approximately 2.4 times the force of gravity on our own planet. This means that a person who weighs 100 pounds on Earth would weigh about 240 pounds standing on the clouds of Jupiter.
The Jovian winds are also fierce. This very windy planet’s powerful gales roar between 193 miles per hour to more than 400 miles per hour. The surface of Jupiter is banded with thick white, yellow, red, and brown clouds. It is also circled by three slender gossimer rings, that were spotted for the first time back in 1979 by NASA’s Voyager 1 spacecraft. The delicate, faint ring system is primarily composed of very fine particles of dust.
Jupiter’s magnetic field is also very powerful. Far beneath Jupiter’s thick, heavy shroud of impenetrable clouds, there may exist a vast ocean of rare metallic liquid hydrogen. As Jupiter rotates, the churning, swirling, liquid metal ocean creates the most powerful magnetic field in our Solar System. At the tops of the blanketing clouds (tens of thousands of miles higher than where the field originates), Jupiter’s magnetic field is about 20 times more powerful than Earth’s magnetic field.
Composed primarily of liquid and gaseous matter, Jupiter sports an average density of 1.326 grams per cubic centimeter, which is the second highest density of our Solar System’s quartet of giant gaseous outer planets. Jupiter’s density is lower than those of the quartet of much smaller, solid inner planets: Mercury, Venus, Earth, and Mars. Of the four outer gaseous giant planets, Jupiter and Saturn represent the gas-giant duo, while Uranus and Neptune are ice giants. The two ice-giants are smaller than Jupiter and Saturn, and have relatively less hydrogen and helium and relatively more ices. Uranus and Neptune also have thinner gaseous envelopes than the duo of gas-giants, although they are still quite dense. The two ice-giants also contain smaller solid cores. In this distant realm of the four giant planets, inhabiting our Solar System’s outer limits, our Sun’s heat and light are relatively weak.
Jupiter is thought to have a dense core composed of a mixture of elements, surrounding a layer of liquid metallic hydrogen with some helium, and an outer layer made up predominantly of molecular hydrogen. The Jovian core is frequently described as rocky, although its detailed composition remains unanswered. Also, unanswered are the properties of the materials that exist at the temperatures and pressures found at such depths.
Jupiter’s upper atmosphere is approximately 88 to 92% hydrogen and 8 to 12% helium by percent volume of gas molecules. One helium atom has approximately four times as much mass as a hydrogen atom, and for this reason the composition changes when described as the proportion of mass contributed by differed atoms. As a result, Jupiter’s atmosphere is about 75% hydrogen and 24% helium by mass, with the remaining one percent of the mass composed of other atomic elements. Jupiter’s atmosphere contains trace quantities of water vapor, methane, ammonia, and silicon-based compounds. In addition, there are also traces of ethane, hydrogen sulfide, carbon, oxygen, neon, phosphine, and sulfur. The outermost layer of the Jovian atmosphere contains frozen ammonia crystals. In contrast, the inner layer contains denser materials–by mass it is approximately 71% hydrogen, 24% helium, and 5% assorted other elements. Through measurements made in the infrared and ultraviolet bands of the electromagnetic spectrum, traces of benzene and other hydrocarbons have also been observed. The atmospheric proportions of hydrogen and helium are close to what scientists think composed the primordial solar nebula.
In June 2017, astronomers announced that they have discovered a duo of new moons orbiting Jupiter–and that they also found 5 “lost” little moons belonging to this gaseous planetary behemoth. The astronomers made this serendipitous discovery while searching for the elusive, and possibly non-existent, Planet X. Planet X is a theoretical Neptune-sized planet that is believed to be lurking beyond the orbit of the ice dwarf planet Pluto, in a region so remote that scientists are only now first beginning to explore it. This distant, frigid, twilight domain is called the Kuiper Belt, and it is the home of icy comet nuclei–as well as other undiscovered objects haunting this distant realm. Pluto, the little world with a big heart, was the first denizen of the Kuiper Belt to be detected back in 1930.
Dr. Scott Sheppard, a Carnegie science researcher at the Department of Terrestrial Magnetism in Washington, was hunting for objects in the dark night sky along with his colleagues, Dr. David Tholen of the University of Hawaii and Dr. Chadwick Trujillo of Northern Arizona University, when they aimed their telescopes toward Jupiter and discovered the duo of new, mile-wide little moons.
Jupiter has 53 officially named moons, and more than a dozen “provisional” satellites still awaiting confirmation before they receive names from the International Astronomical Union (IAU) Currently, the two new moons are known only by the designations of S/2016 J 1 and S/2017 J 1, and they were discovered in March 2016 and March 2017, respectively.
Jupiter has been visited on several occasions by robotic spacecraft–most notably during the early Pioneer and Voyager missions and later by the Galileo orbiter. The New Horizons Probe, on its way to Pluto and its moons in the Kuiper Belt, made use of Jupiter’s powerful gravity to get a boost in speed and also to bend its trajectory as it made its way to that small icy world.
NASA’s Juno spacecraft is the most recent probe to visit Jupiter, and it is currently in orbit around the gas-giant. It was built by Lockheed Martin and is operated by NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. Juno was launched from Cape Canaveral Air Force Station in Florida on August 5, 2011, as part of the New Frontiers program. It entered a polar orbit of Jupiter on July 5, 2016, and began to explore the many mysteries of this colorfully banded, enormous world. After Juno has completed its mission, the doomed–but highly successful–spacecraft will be intentionally evicted from orbit. Juno will then meet its fate, plunging into the atmosphere of the distant world that it had studied for so long.
Juno’s mission is to measure Jupiter’s polar magnetosphere, composition, and gravity field. It will also go on the hunt for clues explaining how this enormous, gaseous planet was born, and whether it really does have a rocky core. In addition, Juno will measure the amount of water existing within the deep atmosphere, as well as Jupiter’s mass distribution, and fierce deep winds.
Juno is the second spacecraft to enter Jupiter orbit. It follows the nuclear-powered Galileo orbiter, which visited Jupiter from 1995 to 2003. However, unlike all of the previous spacecraft sent to explore the four outer planets, Juno is powered only by solar arrays, which are commonly used by satellites in orbit around Earth, performing their missions in the welcoming warmth and brilliant light of the inner Solar System.
Jupiter: The Banded, Big, And Precocious “King of Planets”
Jupiter is by far the most massive planet in our Sun’s family, and its existence had a dramatic effect on the dynamics of the solar accretion disk. Therefore, knowing Jupiter’s true age is of great importance in gaining a scientific understanding of how our Solar System evolved from its earliest days to the present. Even though models predict that Jupiter was born relatively early in our Solar System’s history, until now the precise date of its birth had not been determined.
“We do not have any samples from Jupiter, in contrast to other bodies like the Earth, Mars, the Moon and asteroids. In our study, we use isotope signatures of meteorites (which are derived from asteroids) to infer Jupiter’s age,” Dr. Kruijer noted in the June 17, 2017 LLNL Press Release.
The team of planetary scientists demonstrated, by using the technique of isotope analyses of meteorites, that Jupiter’s solid core formed within a “mere” 1 million years after the birth of our Solar System–making Jupiter the oldest planet in our Sun’s family. Because of its rapid formation, Jupiter served as an effective barrier against inward transport of material across the disk. This can potentially provide the explanation for why our Solar System is barren of super-Earths. Super-Earths are exoplanets that sport masses greater than that of our own Earth.
The team of planetary scientists found that the Jovian core grew to approximately 20 Earth-masses within 1 million years. This rapid formation was then followed by a more leisurely rate of growth, whereby Jupiter’s core grew to become about 50 Earth-masses. This later period of more prolonged growth lasted at least 3 to 4 million years after our Solar System formed.
Previous theories proposed that the formation of gas-giant planets, such as Jupiter and Saturn, involved the growth of large solid cores of approximately 10 to 20 Earth-masses. The formation of these very hefty cores was followed by the accumulation of large amounts of gas onto these cores. From this, the team of planetary scientists concluded that gas-giant cores must have formed before the dissipation of the natal solar nebula–the gaseous circumstellar disk encircling the early Sun–which probably occurred between 1 million years and 10 million years after our Solar System’s birth.
In their study, the team of scientists confirmed the earlier theories, but they were also able to date Jupiter’s formation much more precisely. The planetary scientists were able to date Jupiter to within 1 million years by using the isotopic signatures of meteorites.
Although this very speedy accretion of the cores has been modeled before, it had not been possible to date their formation.
“Our measurements show that the growth of Jupiter can be dated using the distinct genetic heritage and formation times of meteorites,” Dr. Kruijer continued to comment in the June 17, 2017 LLNL Press Release.
Most meteorites originate from small Solar System bodies located in the Main Asteroid Belt between Mars and Jupiter. These small bodies are thought to have formed at a much wider range of heliocentric distances, as indicated by the distinct chemical and isotopic compositions of meteorites and by dynamical models. This suggests that the gravitational influence of the gas-giants resulted in the scattering of small bodies into the Main Asteroid Belt.
Other authors include Dr. Christoph Burkhardt, Dr. Gerrit Budde and Dr. Thorsten Kleine of the University of Munster.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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