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The three kilometre wide moon S/2010 J 1 is circled in green. Credit: University of British Columbia The three kilometre wide moon S/2010 J 1 is circled in green. Credit: University of British Columbia

Tiny moons around Jupiter

Sen— The largest planet in the Solar System has been hiding a couple of minuscule moons, which have only recently been confirmed.

Jupiter now has 67 known satellites. The two new additions, dubbed S/2010 J 1 and S/2010 J 2, were discovered in September 2010 using the Palomar 200 inch Hale Telescope and the Canada-France-Hawaii telescope (CFHT). The moons were spotted when routine observations of previously known moons were being performed.

Follow up observations were required to confirm that these are indeed satellites of Jupiter, and not just random passers by. Once the orbits were known, the objects were designated moons of Jupiter by the International Astronomical Union in June 2011. More observations late in 2011 resulted in a more detailed knowledge of the orbits, which in turn allowed the astronomers to predict the future orbits.

A more in-depth knowledge of the orbit also lets astronomers look into the past to see where the moons were years ago. It turns out that one of the moons, S/2010 J 1, had already been observed in 2003 also using the CFHT. This observation was part of project hunting for new moons around Jupiter, however observations were too sparse to confirm an orbit.

"We had actually already reported measurements of the first moon from Feb. 27 and 28, 2003 to the Minor Planet Center eight years ago", said Brett Gladman from the University of British Columbia. "But observations over several months are required to prove that the object is orbiting Jupiter, and this moon was too faint for the 2003 surveys to consistently track."

S/2010 J 2 was not visible in the 2003 images, however this is to be expected as it is the faintest moon to be discovered orbiting Jupiter. S/2010 J 2 is also the smallest known Jovian moon, with a diameter of only two kilometres. "It was exciting to realise that this is the smallest moon in the Solar System that was discovered and tracked from Earth," said Mike Alexandersen, also from UBC. S/2010 J 1 is only slightly larger, at around three kilometres wide.

It is thought that all Jovian moons greater than three kilometres in size are already known, however there may still be more moons smaller than this that have yet to be found.

S/2010 J 1 orbits Jupiter every 2.02 years, while its larger companion orbits the giant planet every 1.69 years. The two moons are thought to be from two different “families” of satellites, with S/2010 J 1 being a member of the Carme group and S/2010 J 2 belonging to the Ananke group. A family of satellites will all have similar orbits and sizes, are believed to have formed from asteroid colliding with larger moons in the past.

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