Which planet was once warm?

Answer 1

Short Answer:

Most or all planets had water.

Mars is the planet which almost certainly had significant liquid surface water for an extended period of time, perhaps for billions of years.

More information:

Perhaps all other planets, were likely formed with water but lost it.

Terrestrial Planets:

Mercury is just too close to the Sun to have ever had much water for very long, even at a time when the Sun was cooler than now.

Venus probably had water when it was formed and kept it for a few hundred million years, but then it was blown away by the solar wind. Water is a very light molecule and would disappear more easily than nitrogen, oxygen, carbon dioxide.

Earth has, of course.

Mars had water for a long time.

Gas Giants:

Jupiter, Neptune, Saturn, Uranus are gas giants and may have some solid rocky material but are primarily made from gas and various types of ice. Some of that ice is water ice, though that being below the gas surface is not well characterized.

Dwarf Planets:

Dwarf planets, like Pluto, are often made up of icy materials or at least have icy surfaces over rocky materials, but the Dwarf Planets have never been investigated at close range.

Answer 2

The most earth-like planet we know of right now is Mars

Answer 3

Mars

Answer 4

Mars