What features do all planets share in common?

Answer 1

= Answer: = In ancient times, "planet" merely meant "wanderer" in reference to the celestial objects that appeared to wander around the sky differently than the fixed background stars. With that definition, anything that wandered the sky was a planet, including the sun and moon. With the many astronomical discoveries over time leading to better understanding of the cosmos, planets typically referred to those objects which orbit the sun, and presumably analogous objects which undoubtedly orbit distant stars. The term was still very ambiguous, though, so in 2005, the International Astronomical Union (IAU) made a list of three characteristics that an object must have to be considered a planet, thus giving the word "planet" an unambiguous definition for the first time. Those characteristics are:

* A planet is any object in orbit around the Sun with a diameter greater than 2000 km. * A planet is any object in orbit around the Sun whose shape is stable due to its own gravity. * A planet is any object in orbit around the Sun that is dominant in its immediate neighborhood.

Any analogous object which fits these characteristics but is in orbit around a star other than the sun would be considered an Extra-Solar planet. As more discoveries are made, the definition may require further fine-tuning. But for now, those are the things which make a planet a planet. It is also important to note that there are identifiers, or words which specify other attributes of a planet. For example, there are major and planets which gives more information regarding their size. There are also primary, secondary, and tertiary planets, specifying whether the object is orbiting the star, another planet, or a natural satellite.

Answer 2

• All planets orbit the Sun rather than another planet. (Otherwise, they would be a satellite.)
• All planets have enough mass to have been pulled into hydrostatic equilibrium: in other words, a sphere.
• All major planets have cleared their orbit of debris, meaning that any objects that still share their orbit are there because of the planet's gravitational dominance.

That third definition was added by vote of the International Astronomical Union in 2006, to legally settle the definition of a planet. It led to Pluto being demoted, and Ceres promoted, to the status of a minor planet.

Answer 3

Within our solar system . . .

-- All are nearly spherical.

-- All rotate.

-- All revolve counter clockwise (from above Earth's north pole) around the Sun in elliptical orbits with small eccentricities.

-- All of their orbits are inclined only slightly to the plane of the ecliptic.

-- All rotational axes are inclined to their orbital planes by different amounts.

None are perpendicular.

-- All except two of the known planets have one or more moons.

-- All except one of the known planets have gaseous atmospheres.

-- All except one of the known planets have no life on their surfaces.

Answer 4

They are round (or at least a spheroid) and their gravity is strong enough to pull neighboring objects and become the dominant object around its orbit (sorry, Pluto).