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  Rogue galaxies prompt rethink on Newton
Dwarf galaxies orbiting the Milky Way are breaking the laws of physics, prompting another modification of Newton's theory of gravitation.

Astrophysicist Dr Hemut Jerjen of the Australian National University and colleagues report their findings in two scientific papers this month.

"The only way out of this is to change Newton's laws," says Jerjen, whose research is published in The Astrophysical Journal and the Monthly Notices of the Royal Astronomical Society.

When astronomers first studied the dwarf galaxies that orbit the Milky Way, they found something intriguing.

The stars further away from the centre of the dwarf galaxy were moving faster than they should have.

Under Newton's gravitational law, the stars further out should have been moving slower than the ones closer to the centre.

This occurs, for example, in the Solar System where Neptune is the slowest-moving planet and Mercury is the fastest.

But astronomers found the stars in the dwarf galaxies were all moving at the same speed, regardless of how far away they were from the centre.

"They are moving too fast for classical Newtonian dynamics," says Jerjen.

Traditionally, astronomers have made sense of such anomalies by introducing the concept of "dark matter".

They suggested dwarf galaxies contained invisible matter, which explained the more rapid movement of the stars within them.

But now Jerjen and colleagues have made an observation that undermines a classical Newtonian explanation involving dark matter.
Dark matter given the flick

They found the dwarf galaxies were orbiting in a two-dimensional plane around the Milky Way, instead of orbiting randomly in a ball around it.

Previous research suggests these kind of dwarf galaxies are "tidal satellites", formed from the collision of two large galaxies.

Theoretical calculations predicted they could not contain dark matter.

"We are now in a dilemma," says Jerjen.

"We know from observing stars in these dwarf galaxies that the stars are moving too fast. Classically this was explained by dark matter."

"But because we know that these dwarf galaxies do not contain dark matter we have to come up with another solution."

Jerjen says the findings lend support to the Modified Newtonian Dynamics (MOND) theory, developed decades ago by Israeli scientist Mordehai Milgrom.

The theory proposes a new physical constant that only comes into play when one is observing large-scale objects that accelerate slowly, such as stars in tidal satellite galaxies.

"This theory was never really tested extensively," says Jerjen.

"What we've now found is empirical observational evidence that possibly this theory is a natural explanation of what we observe in these dwarf galaxies."

While classical Newtonian physics generally still applies to everyday life, various other modifications have been made to cope with particular circumstances.

Other modifications to Newton's theory of gravitation include Einstein's Special Theory of Relativity and General Relativity, which applies at astronomical scales, and quantum mechanics, which applies at the sub-atomic scale.
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