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Jupiter's family of moons grows by two

Tiny dancers The largest planet in our solar system is home to two of the smallest moons, Canadian astronomers have discovered.

The discovery, to be published in the Astronomical Journal, pushes the number of Jovian moons to 67.

Named S/2010 J 1 and S/2010 J 2, the two moons are only three and two kilometres in size respectively.

S/2010 J 1 orbits Jupiter once every 2.02 years, at an average distance of 23.45 million kilometres from the planet, while S/2010 J 2 takes 1.69 years at a distance of 21.01 million kilometres.

The moons were discovered using ground-based telescopes in September 2010. Follow-up observations between October 2010 and January 2011, allowed the astronomers to accurately plot their orbits and confirm they are moons, not passing asteroids.

The researchers then used the brightness from each moon to determine their diameter.

"It was exciting to realise that this [S/2010 J 2] is the smallest moon in the solar system that was discovered and tracked from Earth," says lead author Mike Alexandersen, a PhD student at the University of British Columbia, Canada.

Alexandersen and colleagues traced the path of the moons, and compared it with data collected from a 2003 survey by Professor Brett Gladman, also of University of British Columbia.

"We had actually already reported measurements of the first moon [S/2010 J 1] to the Minor Planet Center eight years ago", says Gladman, "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."

How small can you go?

Professor Mike Brown, an astronomer at California Institute of Technology who was not involved in the study, says spotting tiny moons around Jupiter isn't that difficult.

"The telescopes used are pretty modestly-sized compared to the largest ones that astronomers use," says Brown. "With our biggest telescopes you could go for even smaller."

He says while there is no limit to how small a moon around a planet could be, external forces could prevent smaller ones staying in a stable orbit.

'"When things get small, their orbits get affected by exterior forces, such as sunlight and collisions. At some point you're too small to have a long term stable orbit," says Brown.

"I've never thought to try to calculate that number for Jupiter, but I would guess we have quite a ways to go before we get so small things don't have semi-permanent orbits."