This is a great question. There is a possibility of there being another planet like Earth in the universe. All celestial bodies, in fact all bodies in general, have gravity. In order for a planet to have the same gravity as Earth it would have to be the same mass and have the same size since the force of gravity depends on the mass and size of an object. As for air and water the planet would have to be about the same size, and temperature as Earth in order to have water and an atmosphere similar to ours. The temperature of a gas and the gravity of a planet are the important factors that control what gases stay on a planet and what ones escape.

For example, in our atmosphere, hydrogen gas is too light for the earth to retain and it escapes out of our atmosphere. Earth is too small to have enough gravity to hold on to it. Jupiter, however, is much larger than Earth and so has a stronger force due to gravity. Jupiter will hold hydrogen gas in its atmosphere. The third thing we will consider is the temperature of the planet. We, as earthlings, are very fortunate that our planet is the distance from the sun that it is. Earth is just far enough from the sun to have a temperature range that allows water to exist in all three fundamental states, solid (ice), liquid water, and gas (steam). Biologists and astrobiologists agree that in order for conditions on a planet to be suitable for higher forms of life water must exist in these three forms. So for review, if there is a planet out there in the universe, that is the same distance from a star like our sun (for temperature), and is the same size and density (density is mass divided by volume) than it should resemble our home planet. Wouldn't it be great if astronomers found a planet like ours?