By observing them regularly using telescopes and by sending orbital or lander probes to them.
Telescope
They have sent a prob thingie to take pics about pluto. They do that to a lot of planets.
they learn by going to a good school and study the stars and planets
yes
to learn more about it!
to make sure that every one know there are more planets
Why is it difficult for astronomors to learn about mercury
They are gas planets
An improvement in microscope increases its magnifying power by which the scientists can see cell's components clearly and their study becomes easier thus they can learn more.
why are you searching it up? you should have known
Scientists learn a few different things. Scientists learn new things very day.
Telescopes, Space missions, probes all help scientists discover more about the moon.