King of the Planets now King of the Moons

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  Last updated August 2, 2018 at 10:59 am

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Twelve new moons of Jupiter – including one ‘oddball’ on a collision course.





A treasure trove of a dozen newly discovered moons of Jupiter has been confirmed, taking the total to 79. The largest planet in the Solar System is now also the one with the most moons, for now.


Of those dozen new moons, eleven behave in a somewhat normal way. One, however, has a very different path leading the researchers to describe it as an “oddball” and warn that could lead to tragedy for the moon.


The discovery of so many moons may seem an oversight on the part of astronomers, after all Jupiter has been in our sights since the first records of ancient civilisations.


However, the latest moons are truly a different category of faintness as their small sizes, just one to three kilometres, across reflect the tiniest amounts of Sunlight. It’s really only thanks to ever improving technology that objects so tiny, and so faint, were even able to be seen.


But what makes their discovery by the Carnegie Institution for Science’s Scott Sheppard and his team all the more astounding is they weren’t even looking for them.


Fortuitous finding


The faint point of light is seen to move relative to background stars between two exposures, indicating that it is close to Jupiter, but to be so faint while still being this close means that it must be tiny. Plotting the position of this moon over time allowed astronomers at the International Astronomical Union’s Minor Planet Center to calculate the surprising orbit of Valetudo. Credit: Carnegie Institution for Science


The team were searching for the recently hypothesised Planet X (or more accurately Planet Nine after Pluto’s demotion), a massive planet orbiting on the very outskirts of our Solar System.


A planet so far out would reflect very little of the faint sunlight at such great distances, and would appear to be moving very slowly. Hunting for this world requires long exposures with large telescopes to see faint objects, over extended periods of time to spot the slight change in the position of this faint point of light against the essentially unmoving background stars.


“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,” said Sheppard.


The perfect instrument for such a search is NOAO’s Blanco 4-meter telescope at Cerro Tololo Inter-American in Chile, with the newly attached Dark Energy Camera (DECam) able to see incredibly faint objects over just 3 square degrees, or the area of 15 Full Moons, worth of sky at once.


It just so happened that when the researchers studied their images, Jupiter was close enough to the survey areas that the motions of the previously undiscovered moons were detected as well.


They then followed up their accidental finding using even larger and more sensitive telescopes, such as the 6.5 metre Magellan Telescope.


The new collection of moons orbit in a retrograde motion, in the opposite direction to the inner moons and the planet’s spin itself. However one oddball, Valetudo, cuts across this and spins in a prograde direction, like a car driving against the traffic in a freeway. Credit: Roberto Molar-Candanosa/Carnegie Institution for Science


11 moons and an oddball


Two of the newly discovered moons are part of an existing inner collection that orbit in the same direction as Jupiter’s rotation, termed prograde. They both have similar orbital distances and are thought to potentially be fragments of a larger object that broke apart.


In contrast, nine of the outer swarm of moons all orbit in an opposite direction to those inner moons of Jupiter, and indeed the planet itself, in a retrograde fashion. They appear to be grouped in three distinct orbital groups, suggesting that they are all that remain of three larger bodies long since smashed to pieces through earlier collisions with asteroids, comets or other moons.


The twelfth moon, however, has been described as an “oddball” by the researchers. Known as Valetudo, it is in a prograde orbit of Jupiter like the inner group, and is tiny at less than 1 kilometre in diameter. However, its orbit crosses the orbit of the outer retrograde group like a car driving against the traffic on a freeway.


Just as with the car, sooner or later this moon will have a head-on collision with another and be ground into dust.


How these moons came to be in such an unsustainable, and dangerous, position is unclear but does suggest that they are in fact all that remains of ancient moons that formed an even larger outer group.


The team think this small “oddball” prograde moon could be the last-remaining remnant of a once-larger prograde-orbiting moon that formed some of the retrograde moon groupings in past head-on collisions.


Although Jupiter is for now the Solar System’s titleholder with the most number of moons, that situation can change dramatically as Valetudo the oddball weaves a dangerous path through its newly discovered brethren.


The research has not yet been published in a journal, and is being released in an International Astronomical Union Minor Planet Electronic Circular.


Video courtesy of Roberto Molar-Candanosa/Carnegie Institution for Science


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Education Resource:


King of the Planets now King of the Moons




About the Author

Alan Duffy
Associate Professor Alan Duffy is an astronomer and physicist at Swinburne University of Technology, Melbourne. He's also lead scientist for Australia's Science Channel. You can find him on Twitter @astroduff.

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Science and technology is as much a part of our cultural fabric as art, music, theatre and literature. They play a significant role in our daily lives, yet, in a world dependent on science, we often take them for granted. Australia’s Science Channel believes every citizen has a right, and a responsibility, to be informed, and our mission is to create programs to bring that about.


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