|Do the planets grow in size?
NO, not really. Once a planet has formed then
its mass is pretty constant. The only addition is
by meteoritic debris which is about 200 tons per
day.... that may sound like a lot but when you
note that the mass of the earth is
6,000,000,000,000,000,000,000 tons then us see
that even over all of geologic time of 4.5 billion
years, the addition of mass has been about less
than 1,000,000,000,000 tons. That is a VERY VERY
VERY small fraction of the mass of the Earth!
They do!But very, very slowly: our solar system
is full of material left over from its formation
(mainly, asteroids and comets). As these objects
fly around, sooner or later they hit a planet,
increasing its mass. This process occurred
relatively quickly when the solar system began to
form, but as the planets started to accumulate all
of the material near their orbit, the process
slowed down. It still occurs today (every time an
asteroid hits the Earth, the Earth's mass
increases a little bit), but the rate of growth of
the size of the planets from these contributions
During the birth of our solar system, planets
grew in size for many millions of years as
asteroids, comets, and dust collided to form the
planets. However, over time the solar system
became more organized, and collisions with planets
became much more rare. The planets reached their
modern masses quite a long time ago (sometime
around 4.5 billion years ago). Since then, the
planets have both lost mass (mostly gases in the
upper atmosphere), and gained mass (accumulation
of meteorites and dust). In general, these
additions and subtractions of material have not
led to a significant change in planet size.
The other factor affecting planet size is
temperature. As our planet and others like it
radiate away their internal heat, they begin to
cool and contract. You can imagine earth as a
balloon that you take from a warm house and set
out in the cold. As the internal balloon heat
radiates away, the balloon will shrink.
Similarly, the planets are expected to shrink as
they cool off in the vacuum of space. Again, this
is a very slow process (taking billions of years),
and the magnitude of change is probably difficult
Generally speaking, no - except during planet
formation. Every time an asteroid or comet hits a
planet, generally speaking the mass of the
asteroid or comet is added to that of the planet.
However, because this happens, planets tend to
"gobble up" most of the asteroids and comets in a
solar system early in the solar system's history,
so by now the mass remaining in the solar system
contained within asteroids and comets amounts to a
pittance compared to the significant planets in
the solar system.
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