Leading astronomers on Thursday, roved historic new guidelines under which distant Pluto is no longer defined as a planet.
After a tumultuous week of clashing over the essence of the cosmos, the International Asronomical Union stripped Pluto of the planetary status it has held since its discovery in 1930.
It is the first time that scientists have had a formal definition of what is - and is not - a planet.
Thursday's decision by the prestigious international group spells out the basic tests that celestial objects will have to meet before they can be considered for admission to the elite cosmic club.
For now, membership will be restricted to the eight "classical" planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Much-maligned Pluto doesn't make the grade under the new rules for a planet: "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a...nearly round shape, and has cleared the neighborhood around its orbit."
Pluto is automatically disqualified because its oblong orbit overlaps with Neptune's. Instead, it will be re-classed as "dwarf planets," similar to what have long been termed "minor planets."
The definition also lays out a third class of lesser objects that orbit the sun - "small solar system bodies," a term that will apply to numerous asteroids, comets and other natural satellites.