This definition would include Ceres, 2the largest and most massive object in the asteroid belt between Mars and Jupiter. Ceres is less than half the diameter of Pluto."
Astronomy Magazine's website has a news story , "The Great Planet Debate", about the meeting August 14 - 16 at The Johns Hopkins University Applied Physics Laboratory where Scientists debated about what makes a planet a planet.
Being Scientists, their definition of "round" is probably a little more complex than yours or mine: round is the shape of bodies that are in "hydrostatic equilibrium," where the pressure from an essentially fluid interior is balanced against gravity and centrifugal force at the body's surface to round the object. This makes planetary objects fundamentally different from small, irregularly shaped asteroids and comets."
If you want to find out more,
- Visit this page, which has links to the papers contributed to the meeting (topics such as Ceres and Pluto: Dwarf Planets: A New Way of Thinking about an Old Solar System John Ristvey and Lucy McFadden) visit this page, where there is a retelling of the opening debate:
"Tyson, director of the Hayden Planetarium in New York and host of Nova ScienceNow, and who is in the camp that Pluto is not a planet, began his opening statements with "It's simple. The word 'planet' has lost all scientific value." He went on, saying "planet" doesn't tell you much and you have to ask all sorts of questions such as is it big or small, rocky or gaseous, in the habitable zone or not, etc. "If you have to ask twenty questions after I say I've discovered a planet, the word has lost its utility.""
- Visit this page, which highlights the fine points of defining a planet:
"Labeling planets based on their dynamics around the sun distinguishes the planets as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
>But if Earth were orbiting the sun out in the Kuiper Belt, based on a dynamical definition and the mass of Earth, it would not be a planet, Sykes and other scientists pointed out."
- Visit this page, which follows the debate's progress:
"Sykes did not abandon his call for a new definition based solely on roundness, but agreed that the important issue is not Pluto's status or how many planets there are,broadening our perspective on the solar system. "It's not a battle over which list you want to have or what numbers you want to have, but what's the basis for looking at things [from different perspectives].""Audio of part of the debate is posted here, with these links: Listen Now and Listen Now