Do we have to have visual contact with asteroids and planets? Because as far as i know very far objects are observed with infrared scanners, and that's sort of "seeking objects with heat". I'm probably wrong with this but i don't have physics degree and I wouldn't be asking if I knew.
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It depends. If they are relatively near by, then we can detect them using reflected sunlight. That, for example, is how Neptune and Pluto were imaged when discovered. They are both distant from the sun, and therefore cold. Neptune's average 'surface' temperature is about -200 degrees Celsius.
On the other hand, both Neptune and Pluto were actually discovered because of their gravitational effects on other objects in the solar system, and this method is also available for other objects. Sometimes we can work out that something must be there even if we can't see it. Several exoplanets (planets of stars other than our sun) have been found because of the way they move the stars they revolve round.
Another way of finding exoplanets is by observing transits. If the planet happens to pass between us and its star as it revolves around the star, then it will cut off a little bit of its star's light. That little deficit is now observable with modern, highly sensitive instruments, and more and more exoplanets are being found that way. So we don't have to have visual contact with objects to be certain they are there. It certainly helps, though!
On the other hand, both Neptune and Pluto were actually discovered because of their gravitational effects on other objects in the solar system, and this method is also available for other objects. Sometimes we can work out that something must be there even if we can't see it. Several exoplanets (planets of stars other than our sun) have been found because of the way they move the stars they revolve round.
Another way of finding exoplanets is by observing transits. If the planet happens to pass between us and its star as it revolves around the star, then it will cut off a little bit of its star's light. That little deficit is now observable with modern, highly sensitive instruments, and more and more exoplanets are being found that way. So we don't have to have visual contact with objects to be certain they are there. It certainly helps, though!
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Even asteroids have been primarily detected by the amount of sunlight they reflect. The main visible planets (Venus to Saturn) are clearly visible these evenings an hour or so after sunset. Mercury is setting before sunset these days, and Uranus is in that same area. Uranus can sometimes be spotted with unaided eye, although binoculars help a lot, as does knowing where to look. Neptune takes a telescope and a very good sky map.
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I have seen asteroids and planets with my eyes, so Yes, we have visual contact.