It was actually a misinterpretation, followed by a mis-translation. An Italian astronomer Giovanni Schiaparelli observed the planet Mars during its opposition (when the Earth and Mars were closest together) in 1877, and wrote about the "channels" on Mars; but the word "channel" in English is "canali" in Italian. So English reporters misinterpreted the word "canali" as "canal", and leapt to the conclusion that the "canali" of Mars were artificially-dug canals.
In reality, there are neither artificial canals nor natural channels; we humans are very good at seeing patterns in random data, especially if there are no patterns there. The "channels" were probably an optical illusion.
Mars
Neptune is a gas planet, thus it has no solid surface (that we've seen yet), thus no canals.
I believe it was Mars. It was promoted by Percival Lowell See related link
No, gas planets have no surface or core.
Of course. Planets with a gaseous surface are called Jovian planets or Outer planets because they are beyond the asteroid belt.
Planets are formed when the surface phase changes to a solid material. All planets were once stars.
Earth
No. The planet has no surface like the terrresterial planets do.
It is each planet's surface gravity.
Venus has the highest surface temperature of all the planets.
earth
Earth is the most massive terrestrial planet, so it has the highest surface gravity.