There’s one less planet in the solar system today now that Pluto has officially been demoted.

Once known as the smallest planet furthest from our sun, Pluto, discovered in 1930, was reclassified Thursday as a “dwarf planet” by the International Astronomical Union General Assembly.

The assembly, meeting in Prague, ruled that a planet must meet three specific criteria: have enough mass and gravity to gather itself into a ball; it must orbit the sun; and must reign supreme in its own orbit having “cleared the neighborhood” of other competing bodies.

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune meet those requirements. Pluto, however, shares the outer solar system with thousands of other Pluto-like objects.

Jim Hill, director of the Rainwater Observatory at French Camp, said debate over whether Pluto was actually a planet or not started around 1980 and gained momentum after other icy objects, similar to Pluto, began to be discovered.

As telescopes advanced, Hill said it became apparent to much of the scientific community that Pluto is not even as large as it was originally thought to be. It appeared larger than it is because of the light reflected from the icy, frost-covered former planet.

“Now we know it’s about one-five hundredth of earth’s mass,” Hill said Thursday. “The fact is that if Pluto were discovered in the last 25 years they never would have classified it as a planet anyway.”

In a press release issued by the IAU Thursday, the organization may classify other dwarf planets at a later date.

Pluto was discovered at the Lowell Observatory in Flagstaff, Ariz., by astronomer Clyde Tombaugh on Feb. 18, 1930. And, Thursday’s reclassification of Pluto was not a welcome change to everyone.

The Associated Press quoted Tombaugh’s 93-year-old widow, Patricia Tombaugh, Thursday on her reaction to Pluto’s demotion, which she described as disappointing and confusing.

“I don’t know just how you handle it. It kind of sounds like I just lost my job,” Tombaugh said from Las Cruces, N.M. “But I understand science is not something that just sits there. It goes on. Clyde finally said before he died, ‘It’s there. Whatever it is. It is there.’”

Clyde Tombaugh died in 1997.

NASA reported Pluto’s downgrade would not affect its $700 million New Horizons spacecraft mission, which this year began a 9 1/2-year journey to study Pluto.

Alan Stern, head of the mission, said he was ‘‘embarrassed’’ by Pluto’s undoing and predicted that Thursday’s vote would not end the debate over its status as a planet. Although 2,500 astronomers from 75 nations attended the conference, only about 300 voted on the resolution.

“It’s a sloppy definition. It’s bad science,” Stern said. “It ain’t over.”



The Associated Press contributed to this report.