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Saturn overtakes Jupiter as planet with most moons


SaturnImage copyright NASA/JPL-Caltech/Space Science Institute
Image caption Roll over Jupiter, Saturn is the new moon king

Saturn has overtaken Jupiter as the planet with the most moons, according to US researchers.

A team discovered a haul of 20 new moons orbiting the ringed planet, bringing its total to 82; Jupiter, by contrast, has 79 natural satellites.

The moons were discovered using the Subaru telescope on Maunakea, Hawaii.

Each of the newly discovered moons in orbit around Saturn is about 5km (three miles) in diameter; 17 of them orbit the planet "backwards".

This is known as a retrograde direction. The other three moons orbit in a prograde direction - the same direction as Saturn rotates.

Two of the prograde moons take about two years to travel once around the ringed planet.

The more-distant retrograde moons and one of the prograde moons are further out, each taking more than three years to complete an orbit.

Image copyright CARNEGIE INSTITUTION FOR SCIENCE

"Studying the orbits of these moons can reveal their origins, as well as information about the conditions surrounding Saturn at the time of its formation," said Dr Scott Sheppard, from the Carnegie Institution for Science in Washington DC, who led the team.

The outer moons appear to be grouped into three distinct clusters based on the inclinations of the angles at which they orbit the planet.

The retrograde and prograde moons may have once been part of at least three larger bodies. These were broken apart by collisions either between distinct moons or with outside objects such as passing asteroids in the distant past.

One of the newly discovered retrograde moons is the furthest known moon around Saturn.

"These moons have fairly inclined orbits to Saturn and are pretty far out, so we don't think they formed with the planet, we think they were captured by the planet in the past. If an asteroid happens to be passing by, you can't capture it today because you can't dissipate its energy," Dr Sheppard told BBC News.

However, in the Solar System's youth, when Saturn was in the process of forming, a cloud, or "disc", of dust and gas surrounded the planet. This helped dissipate the energy of passing objects. But in most cases, these bodies ended up spiralling into the planet and becoming part of it.

"We think these moons interacted with that gas and dust. These were comets or asteroids that happened to be passing by," Dr Sheppard explained.

"Most objects would spiral into the planet and help form the planet itself. But we think these objects were captured right when the gas and dust started dissipating. So these objects were captured into orbits around the planet instead than falling into the planet. We think these are the last remnants of what formed [Saturn]."

Image copyright SPL
Image caption The observations that led to the discovery were made with the Subaru telescope

Dr Sheppard told BBC News that Jupiter had been the planet with most known moons since the late 1990s.

The finds were made by applying new computing algorithms to data gathered between 2004 and 2007 with the Subaru telescope. These algorithms were able to fit orbits to potential moons identified in these old data.

"We thought they were moons of Saturn, but we weren't able to get full orbits to determine this," said Dr Sheppard.

"By using this new computer power, I was able to link these 20 objects that we thought were moons to officially find orbits for them and find that they really were moons."

The original observing team included Dr Sheppard, David Jewitt of University of California, Los Angeles (UCLA), and Jan Kleyna of the University of Hawaii.

Dr Sheppard said more moons were probably waiting to be found around Saturn. But astronomers would need larger telescopes - such as those set to come online in coming decades - to discover these smaller satellites of around 1km in size.

The team has initiated a contest to name the moons. They have to be named after giants from Norse, Gallic or Inuit mythology, corresponding to the three different clusters.

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