Strange things have occurred in the distant domain of the giant gaseous planets that inhabit the outer regions of our Solar System. Far from our Sun, the four worlds twirl in secretive splendor where they rule over their mysterious kingdom of cold darkness. Of the quartet of outer giant planets–Jupiter, Saturn, Uranus, and Neptune–the brightly banded behemoth Jupiter reigns supreme in its magnificence, as the largest planet in our Sun’s family. Indeed, Jupiter just missed becoming a star itself when our Solar System was young. If Jupiter had become a star, our Sun would have had a binary stellar companion, and we would not be here. In July 2018, astronomers announced that twelve new moons orbiting Jupiter have been discovered, and this treasure trove of moons consists of 11 “normal” satellites, and one they’re calling an “oddball”. These newly discovered Jovian satellites brings the giant planet’s total number of known moons up to a whopping 79–more than any other planet in our Sun’s family.
A team of astronomers led by Dr. Scott S. Sheppard of Carnegie Observatories (Pasadena, California) first discovered the moons in the spring of 2017 while they were on the hunt for very remote Solar System objects as part of the search for a possible massive planet lurking far beyond Pluto. In 2014, the same team of astronomers discovered the object with the most-distant known orbit in our Solar System. The astronomers were also the first to come to the realization that an unknown massive planet haunting the outer limits of our Sun’s sphere of influence, far beyond Pluto, could explain the similarity of the orbits of a handful of small and very remote objects. The distant giant planet, that may or may not exist, is sometimes popularly given the designation Planet X or Planet Nine. University of Hawaii’s (Manoa) Dr. Dave Tholen and Northern Arizona University’s (Flagstaff) Dr. Chad Trujillo are also members of this planet-hunting team of astronomers.
“Jupiter just happened to be in the sky near the search fields where we were looking for extremely distant Solar System objects, so we were serendipitously able to look for new moons around Jupiter while at the same time looking for planets at the fringes of our Solar System,” Dr. Sheppard commented in a July 16, 2018 Carnegie Science Press Release. Serendipity means that while you are searching for one thing, you find something else. Scientific serendipity occurs quite frequently.
Dr. Gareth Williams at the International Astronomical Union’s (IAU) Minor Planet Center used the team’s observations to calculate orbits for the newly discovered moons.
“It takes several observations to confirm an object actually orbits around Jupiter. So, the whole process took a year,” Dr. Williams explained in the same Carnegie Science Press Release.
Jupiter’s Distant Realm
Jupiter, the fifth planet from our Sun, is about 89,000 miles wide at its equator. This behemoth world is so immense that literally all of the other planets inhabiting our Solar System could be tucked inside of it. Indeed, 1,000 Earth’s could fit within this banded giant.
Jupiter has the same composition as stars. If it had been born about 80 times more massive, nuclear-fusion would have lit its stellar fires, and it would have become a star instead of a very large planet.
Jupiter’s mean distance from our Sun is approximately 5.2 astronomical units (AU). One AU is equivalent to Earth’s average distance from our Star, which is about 93,000,000 miles. This basically means that Jupiter circles our Sun a bit more than five times the distance between our planet and the Sun. When Jupiter is seen from Earth, it is usually the second brightest planet in the night sky–after Venus.
One day on Jupiter is only 10 hours long. This is because it rotates more rapidly than any other planet in our Solar System. In addition, Jupiter’s orbit is elliptical–that is, it is out of round. One day is equivalent to a single rotation (spin) of a planet.
This colorful banded behemoth is almost as big as a planet can be and still be designated a planet. Jupiter also has the same composition as our Sun–meaning that it is approximately 90 percent hydrogen and 10 percent helium. But there is an important difference between Jupiter’s composition and that of our Star. Unlike our Sun, Jupiter also harbors relatively small quantities of methane, water, ammonia, and rocky material. If any more material had been snared by the planet during its ancient formation, it would have been squashed tightly by the merciless pull of its own gravity–while at the same time its entire radius would actually increase by a small amount. Stars can grow to be much larger than Jupiter, but they also possess their own source of internal heat.
Even though Jupiter is composed mostly of gas, many planetary scientists propose that a liquid hydrogen ocean encircles Jupiter’s hidden core. Also, the Jovian atmosphere is made up mainly of hydrogen and helium. This suggests that there may be no solid ground on this immense planet, like we have on Earth, to weaken its turbulent and violent hurricane-like storms. Unfortunately, Jupiter’s heavy cloak of clouds block clear observations of its lower atmosphere.
The Many Moons Of Jupiter
Jupiter has more moons with reasonably stable orbits than any other planet in our Solar System. The most massive of the Jovian moons are the four Galilean moons, which were discovered in 1610 by the Italian astronomer Galileo Galilei (1564-1642) and the German astronomer Simon Marius (1573-1624), working independently of one another. However, the first claimed observation of one of Jupiter’s moons is that of the Chinese astronomer Gan De around 364 B.C.
Simon Marius discovered the moons only one day after Galileo. However, he did not publish his book describing this discovery until 1614. Nevertheless, the names that Marius gave the four moons are still being used today. No additional moons were discovered until E. E. Barnard observed Amalthea in 1892.
Discoveries of more and more Jovian moons were quickly made over the course of the 20th century, as a result of the use of telescopic photography. Himalia was discovered in 1904, Elara in 1905, Pasiphae in 1908, Sinope in 1914, Lysithea and Carme in 1938, Anake in 1951, and Leda in 1974.
When NASA’s Voyager space probes finally reached Jupiter in 1979, after a long and difficult journey through interplanetary space, 13 moons were discovered. This bevy of moon-worlds did not include Themisto, which was discovered in 1975. However, Themisto had gone missing, and was not rediscovered until the year 2000. This was due to insufficient initial observational information. The Voyager spacecraft found three more inner moons in 1979: Metis, Adrastea, and Thebe.
No additional moons were found for twenty years, generally during the 1980s and 1990s. However, between October 1999 and February 2003, astronomers discovered and ultimately named another 34 moons using sensitive ground-based detectors. By 2015, a total of 15 additional moons were found. Two more were discovered in 2016 by Dr. Sheppard’s team at the Carnegie Institution for Science, bringing the tally up to 69 Jovian moons.
Within this heavily populated collection of Jovian moons, the four Galilean moons stand out in the crowd. This is because the strange and diverse quartet are of special historical importance as they were the first objects discovered to orbit a body that was neither our own planet nor our Sun. From the end of the 19th century, a multitude of smaller Jovian moons were spotted, and have also received names that reflect either the lovers or daughters of the Roman god Jupiter–or his ancient Greek counterpart Zeus. The four fascinating Galilean moons are both the largest and most massive objects orbiting Jupiter, with the remaining known moons and the rings together accounting for only 0.003% of the entire orbiting mass.
The quartet of Galilean moons are, in order of their distance from Jupiter, going from the innermost moon to the outermost: Io, Europa, Ganymede, and Callisto. The four moons travel along nearly circular orbits around their banded planet, and are not greatly inclined with respect to Jupiter’s equatorial plane. The quartet of moons are all almost spherical as a result of their planetary mass. Irregularly shaped objects do not have sufficient mass for gravity to pull them into a spherical shape. Because the quartet of Galilean moons possess sufficient mass to become spherical, they would be classified as at least dwarf planets if they were in orbit around our Sun, instead of circling a planet.
There are four other regular Jovian satellites in addition to the quartet of Galilean moons. The other four regular satellites are much smaller than their Galilean counterparts, and are also closer to their enormous parent-planet. These four moons are the sources of the dust that creates Jupiter’s system of rings. The remaining Jovian moons are irregular satellites whose prograde and retrograde orbits are situated much farther from Jupiter and have both high inclinations and eccentricities. Many astronomers consider these irregular Jovian moons to really be captured objects that originally circled our Sun before they were caught in Jupiter’s irresistible gravitational embrace. Twenty-eight of the irregular moons have not yet received official names.
The Jovian moons are a diverse bunch. For example, the quartet of Galilean moons are all over 1,900 miles in diameter. Indeed, Ganymede, the largest of the four, is the ninth largest object in our Solar System, not counting the Sun and seven of the eight major planets. Ganymede is larger than Mercury, the innermost major planet in our Sun’s family. When not taking into account the quartet of large Galilean moons, the other Jovian satellites are all less than 160 miles in diameter. Indeed, most of Jupiter’s myriad moons barely exceed 3.1 miles in diameter. Their orbital shapes range from almost circular to extremely eccentric and inclined, and many circle their parent-planet in the direction opposite to Jupiter’s spin (retrograde motion). Retrograde orbits usually indicate a captured object. Orbital periods range from seven hours (taking less time than Jupiter does to spin around once on its axis), to about three thousand times more (equivalent to almost three Earth years).
The regular Jovian moons are believed to have been born from a primordial circumplanetary disk swirling around the newborn Jupiter. This ancient disk formed a ring around the giant planet that was composed of accreting gas and solid debris. Such disks are analogous to the protoplanetary accretion disk that circled our young Sun. This protoplanetary accretion disk ultimately gave rise to the planets and other objects that follow solar orbits. Similar protoplanetary disks have also been observed swirling around distant stars beyond our Sun.
Supercomputer simulations indicate that the circumplanetary disk surrounding the ancient Jupiter possessed a relatively high mass at any particular moment. As time passed, a large amount (several tenths of a precent) of the mass of Jupiter–that had been snared from the solar nebula–passed through the disk. Only approximately 2% of the proto-disk of Jupiter is necessary to explain the existing Jovian moons. Therefore, there may have been several generations of primordial Galilean-mass Jovian moons in Jupiter’s ancient past. Each generation of moons could have spiraled into the early Jupiter, because of drag from the circling disk, with new moons being born from the new debris snared from the solar nebula. By the time the current generation formed, which is possibly the fifth generation of Jovian moons, the disk had thinned out considerably to the point that it could no longer greatly interfere with the moons’ orbits. The present quartet of Galilean moons were nonetheless affected, falling into and thus being partially protected by an orbital resonance with each other. This orbital resonance still exists for Io, Europa, and Ganymede–Callisto has not as yet joined in on the dance. Ganymede’s larger mass suggests that it would have migrated inward at a more rapid rate than the smaller moons, Io and Europa,
Jupiter’s outer, irregular moons are often considered to be captured asteroids. In contrast, the protolunar disk was still sufficiently massive to be able to absorb much of their momentum and snare them into Jupiter-orbit. Astronomers think that many of them were shattered as a result of mechanical stresses during their capture–or, possibly, after their capture as a result of collision with other small bodies, thus creating the Jovian moons that we see today.
On July 17, 2018, the Internatonal Astronomical Union (IAU) confirmed that Dr. Sheppard’s team had discovered ten more moons in orbit around Jupiter, bringing the total number up to 79.
Moons, Moons, And More Moons, Including An “Oddball”
Nine of the newly discovered Jovian moons are members of a distant outer swarm of moonlets that orbit it in the retrograde (opposite) direction of Jupiter’s spin rotation. These distant retrograde moons are divided into at least three separate and distinct orbital groups. Many astronomers think that these moons are the tattle-tale relics of a trio of once-larger original moon-worlds that fragmented during violent smash-ups with other moons, as well as with asteroids and comets. The newly discovered retrograde moons require approximately two years to circle around Jupiter.
A duo of the newly discovered moons are members of a closer, inner collection of moons that circle in prograde (same direction) as Jupiter’s rotation. These two inner prograde moons sport similar orbital distances and angles of inclinations around their enormous parent-planet. This means that they may also be fragments of a doomed larger moon that was blasted to pieces during a collision with another object. This duo of moons take less than a year to orbit their planet.
“Our other discovery is a real oddball and has an orbit like no other known Jovian moon. It’s also likely Jupiter’s smallest known moon, being less than one kilometer in diameter,” Dr. Sheppard explained in the July 16, 2018 Carnegie Science Press Release.
This new little “oddball” moon is more distant and more inclined than the collection of prograde moons. This is the reason why it takes about one and a half years to orbit its parent-planet. Therefore, in contrast to the closer-in prograde Jovian moons, this newly-discovered little “oddball” moon-world sports an orbit that crosses the outer retrograde moons.
For this reason, head-on smash-ups are much more likely to occur between the “oddball” prograde and the retrograde moons, which are traveling around Jupiter in the opposite direction.
“This is an unstable situation. Head-on collisions would quickly break apart and grind the objects down to dust,” Dr. Sheppard added.
The differing orbital moon groupings, that astronomers observe today, may have formed in the distant past due to similar collisions. Dr. Sheppard’s team proposes that the tiny “oddball” prograde moonlet could be the last lingering fragment of a once-larger prograde-orbiting moon, that formed some of the retrograde moon collections, during past head-on smash-ups. The astronomers have also suggested the name Valetudo for this weird little moon-world, after the Roman god Jupiter’s great-granddaughter, the goddess of health and hygiene.
Attaining a new understanding of the complex interactions that shaped a moon’s mysterious orbital history can teach astronomers more about the long-lost secrets of our ancient Solar System. For example, the discovery that the smallest moons within Jupiter’s differing orbital collections are still abundant, indicates the smash-ups that created them happened after the era of planet formation. During this primordial era, our Sun was still encircled by a rotating protoplanetary accretion disk composed of gas and dust from which the planets were born.
Because of their small sizes–only one to three kilometers–these moonlets would be greatly influenced by ambient gas and dust. If these moon-building raw materials had still been lingering when Jupiter’s first generation of moons blasted into one another to create its current clustered groupings of moons, the drag exerted by any remaining gas and dust on the smaller moons would have been adequate to cause them to perform a death-spiral inward toward their murderous parent-planet. It is for this reason, that their existence suggests to astronomers that they were probably born after this gas and dust had already dissipated.