http://observers.france24.com/en/content/20090319-earth-seen-helium-balloon-30000-spanish-students
Teenagers did this test, if you look at the images it appears to be curved to me.
What you are seeing in those pictures is the rounded edge of the Sun's spotlight on the Earth.
the earth is a planet like any other rock planet (as opposed to the gas ones). it was formed by rock and dust collecting in orbit around a star and building up into a planet. the fact that earth has life is simply due to it forming with the right gasses in the atmosphere and the correct distance from the star(ive seen people make the connection between earth an special by using life. its just chance). every planet is formed in the same way. earth is a planet like any other and as such should be treated as such
We are familiar with Round Earth Theory, we do not need it explained to us. Please answer the question rather than recounting a theory.
first part: how the hell does a spotlight have a rounded edge that will produce such a curvature? you can see a slight curve in the 2nd pic fairly easily aswell, or are you blind?
second: i did answer the question by recounting the theory. it answers the question fairly well...
if your point was why do i think earth is the same as other planets? what makes it any different
what are the properties of a planet?
off the top of my head:
orbits a star
has a core, mantle etc; several layers within the planet
is large enough to support satelites (the moon, mainly)
is formed of rock, or has a rock core and a large gaseous outer "surface" (gas giants are believe to have a solid core anyway)
is sphericle (in general. not perfect due to mountains, craters, trenches etc)