I'm Shirley Griffith.
And I'm Steve Ember with the VOA Special English program EXPLORATIONS. The International Astronomical Union has given a name to a solar system object discovered last year. How the distant, icy world called Eris shook up astronomy and struck Pluto from the list of planets is a story worth telling.
On August twenty-fourth, astronomers made a big change to our model of the solar system. They voted to change the number of planets at a meeting of the International Astronomical Union in Prague, the capital of the Czech Republic. Astronomers debated several proposals, including expanding the definition of the word planet to include a total of twelve solar system objects.
But in the end, they decided to limit the definition of planet only to very large bodies.
They decided to call Pluto a "dwarf planet" -- not a real planet at all. Our new model of the solar system now has eight planets.
A discovery made last year seems to have forced this change. The discovery was a very distant object beyond Pluto's orbit. The new object was considered bigger than Pluto. It did not have an official name for more than a year after its discovery.
Many astronomers believed the new object, now called Eris, was the last evidence needed to prove that Pluto was not a planet. They said since Pluto was not even the largest object beyond the orbit of Neptune, it could not possibly be considered a true planet.
Astronomy has always used names from ancient stories, or myths. The names of all the planets, except Earth, come from Roman mythology. For example, Mercury was the flying messenger to a whole family of gods. True to its name, Mercury orbits the sun faster than any other planet — in about eighty-seven days.
Venus is the Roman goddess of love and is the brightest and most beautiful planet seen from Earth.
Mars, the red planet, is named for the Roman god of war. Jupiter, the second brightest planet, is the king of the Roman gods. Saturn is named for Jupiter's father. It takes almost thirty years to circle the sun — slowest among the bright planets.
In seventeen eighty-one, a planet beyond the orbit of Saturn was discovered. It was given the name Uranus, the god of the sky and father to Saturn. And more than sixty-five years later, the last large planet to be discovered was named Neptune, god of the sea and brother to Jupiter.
The word "nomenclature" describes a system of naming things in an area of science. Nomenclature in astronomy is especially careful to consider past traditions. This could be because study of the sky has taken place longer than written history. It could be that the apparent movement of bodies in the sky is linked to timekeeping. Whatever the reason, object names in astronomy have to stand the test of time.
In nineteen thirty, a young American astronomer, Clyde Tombaugh, discovered an object beyond the orbit of Neptune. The object was soon named Pluto, the Roman god of the underworld and death.
The name seemed to fit. Pluto is so distant that one orbit around the sun takes two hundred and forty-eight years. At that distance, the sun is much less bright than it is on Earth. It seemed Pluto was a dark, distant world, like the god of the underworld himself.
There was another reason why Pluto seemed to be a good name. The first letters of Pluto, P and L, are the first letters of the name of the man whose effort brought about Pluto's discovery. His name was Percival Lowell. He started Lowell observatory in Arizona where Clyde Tombaugh discovered Pluto. Lowell also did mathematical work suggesting the presence of a large planet beyond Neptune — Planet X.
Lowell died in nineteen sixteen. He had expected his Planet X to be massive enough to affect the orbit of Neptune. Today, we know Pluto's gravitational field is far too small to influence Neptune at all.
Planets have special signs, or symbols. Except for Uranus, these symbols all come from ancient tradition. The sign for the new planet also honored Percival Lowell. It was the letter P on top of the letter L.
But, Pluto's symbol is no longer that of a planet after the meeting of the International Astronomical Union. Instead, Pluto got a number. Planets do not have numbers in astronomical nomenclature. But other small objects orbiting the sun do.
The first such object was Ceres, which was discovered in eighteen hundred. At the time, astronomers did not know about small solar system objects. Ceres orbits the sun between Mars and Jupiter and is about nine hundred fifty kilometers across.
Ceres also has a number: one. Ceres was the first object of its kind to be discovered. But Pluto, which has only recently become a non-planet, is not so lucky. Pluto is about two thousand three hundred kilometers across — much bigger than Ceres. But it now has the hard-to-remember number of one-three-four-three-four-zero.
The huge size of Pluto's number is the result of new technology that has changed the way astronomers search the skies. Before computer technology, astronomers looking for new objects used a painfully slow process. They compared large photographs of exactly the same area of sky taken hours or days apart. Every star in these pictures is in exactly the same place on the photograph.
But a solar system object like a planet, asteroid or comet would move and appear in a different place on the second photograph. In this search method, an astronomer is looking for one small point that is misplaced among many thousands. That is how Clyde Tombaugh found Pluto.
Today there is a faster way. Computers search photographs taken by robotic telescopes for stars that have "moved." Now discoveries happen almost too fast.
Fewer than eight thousand small solar system bodies had been found by the end of nineteen ninety-six. But in September, Pluto got its number after more than one hundred thirty thousand earlier discoveries.
Eris was discovered in the modern way. The team of Mike Brown, Chad Trujillo and David Rabinowitz used pictures taken by the Samuel Oschin telescope at Palomar Observatory in California. This telescope operates like a huge camera pointing at the sky. In fact, the picture that identified Eris was taken on October twenty-first, two thousand three. The team did not recognize Eris until early last year.
But the astronomers did not think it was time to announce the discovery because more research was needed. News started to leak out however.
On July twenty-ninth of last year they were forced to announce a new object. Their early measurements showed that it was more than three thousand kilometers across and took five hundred and sixty years to orbit the sun.
The news was like fuel on a fire. Astronomers heatedly argued whether Pluto was a planet or not on television shows. Reports from the meeting of the International Astronomical Union in Prague described some of the arguments as almost violent. In the end, astronomers voted to define Pluto as something other than a planet.
The new object was known as Xena until September thirteenth when it was officially given the name Eris. Astronomers Brown, Trujillo and Rabinowitz had suggested the name one week before.
In mythology, Eris is the Greek name for the goddess of discord — or bad relations and trouble. Greek mythology tells us that she was not invited to the marriage of a king named Peleus and a sea goddess called Thetis. Eris was angry that she was not invited. She put in motion a series of events that caused the Trojan War. The Greek poet Homer tells the story of that war in the long poem, The Iliad.
But Eris has not finished causing trouble. In September of last year, a team using a telescope at the Keck Observatory in Hawaii discovered that Eris has a moon. It has been given the name Dysnomia, the daughter of Eris in Greek mythology.
And better measurements of Eris show that it is about two thousand four hundred kilometers across -- only a little larger than Pluto. In fact, Eris could even be smaller than Pluto.
The team that discovered Eris says many more discoveries in the distant area beyond Neptune are likely in the near future. Who knows what changes we will have to make to our model of the solar system next time?
This program was written and produced by Mario Ritter. I'm Shirley Griffith.
And I'm Steve Ember. You can read and listen to this program on our Web site, WWW.51VOA.COM. Join us again next week for Explorations in VOA Special English.