Astronomers over the centuries have always believed that the planets in the solar system aren't the only ones. It's hard enough seeing other stars with any clarity. It's impossible to actually see extrasolar planets – planets orbiting other stars. Astronomers and scientists can only guess at their presence by doing complex calculations.
They are sure that they must be there because when the star is formed, there's plenty of material in the area that's left over that needs to go somewhere. And they do their calculations.
They study a distant star carefully to see if there is a regular and periodic dimming in the light coming from the star. If they do find that it happens, they imagine that it's because there's a planet passing in front of it and it dims the light of that distant star, as it were. If there is a massive enough planet going around the star – kind of like how Jupiter goes around the sun – that'll make the star wobble. When they detect a slight wobble like that, they guess that there must be planet making it happen.
They are sure that they must be there because when the star is formed, there's plenty of material in the area that's left over that needs to go somewhere. And they do their calculations.
They study a distant star carefully to see if there is a regular and periodic dimming in the light coming from the star. If they do find that it happens, they imagine that it's because there's a planet passing in front of it and it dims the light of that distant star, as it were. If there is a massive enough planet going around the star – kind of like how Jupiter goes around the sun – that'll make the star wobble. When they detect a slight wobble like that, they guess that there must be planet making it happen.