This is IN THE NEWS in VOA Special English.

This week, there were two big developments in astronomical science. One was a decision to name Pluto a "dwarf planet."

The other was the announcement earlier in the week that scientists have found direct evidence for dark matter. But they say they are still not sure what this mysterious matter is or where it comes from.

Scientists have theorized about dark matter for about seventy years. The idea is that the matter we see does not have enough gravitational pull to keep galaxies together. Visible matter has been estimated to represent only about five percent of the universe.

The new findings come from NASA's Chandra X-ray Observatory and other

telescopes.

A team of scientists observed a group of galaxies that formed when two galaxy clusters smashed into each other. They call it the "bullet cluster." It contains a bullet-shaped cloud of hot gas from a smaller cluster that passed through the hot gas from a larger one.

The bullet cluster is more than three thousand million light-years away. It formed in the last one hundred million years.

Scientists can observe what they believe to be dark matter only through its gravity. But the team says the crash was violent enough to break dark matter away from "normal" matter. Normal matter in galaxy clusters is mostly in the form of hot gas and stars.

They call it the strongest evidence yet that most of the matter in the universe is dark.

They say the observations cannot be explained by theories of gravity that remove the need for dark matter. These theories propose that gravity is stronger with huge galaxy clusters than the theories of Isaac Newton or Albert Einstein would suggest.

The findings will appear in the Astrophysical Journal Letters.

Now on to Pluto. This week the International Astronomical Union met in Prague, in the Czech Republic, with the goal to officially define a planet.

Scientists voted to set three requirements for a planet: It must orbit the sun. It must have enough mass so that its own gravity has formed it into a nearly round shape. And it must have cleared the area of other objects around its orbit. That is where Pluto fails: its orbit around the sun crosses paths with Neptune's.

American Clyde Tombaugh discovered Pluto in nineteen thirty. People have long debated about considering it the ninth planet in our solar system. Now Pluto will be called a "dwarf planet" along with at least two others: Xena and Ceres.

The changes divide astronomers. But supporters say recent discoveries of large objects in the outer solar system require them.

So the new model of our solar system has eight "classical" planets. The smaller, rocky worlds are Mercury, Venus, Earth and Mars. The four huge gas planets are Jupiter, Saturn, Uranus and Neptune.