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?
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