Neptune but only on a very good night i think
In perfect conditions with perfect eyesight you can see the Sun, the Moon, six planets and about 1900 stars. Sometimes also a comet from time to time.
It may surprise you to know that only about 6,000 stars can be seen without a telescope at any one time in the night sky.
In outer space, you can see the Earth, as well as planets, moons, the sun, and other stars, without being hindered by the Earth's atmosphere. It is much clearer, and having the Hubble space telescope in Earth orbit allows it to see much more than any optical telescope could ever see on the Earth's surface.
Most planets can be seen at various times in the night. Mercury can only be seen just before sunrise and just after sunset. Venus, Mars, Jupiter and Saturn can be seen with the naked eye. You will need a telescope to see Uranus or Neptune. Uranus is visible with the naked eye, but it has to be perfect viewing conditions without any light pollution.
Some comets can be seen without any mechanical help if they are large enough or come close to the earth. Any sort of telescope will help you see more comets if you know where to look and what to look for.
There is no such thing as asteroid showers, perhaps you mean meteor showers.
Yes, Mercury, Venus, Mars, Jupiter and Saturn had been discovered several millennia before the invention of the telescope.
The number of planets never seen could be millions, billions, or trillions. We just have never seen them.
"microscopic" Naked = The average human eye, without the use of any artificial aids like, glasses, microscope, telescope etc.
No.
Mercury is the most unlikely to be viewed because of its proximity to the sun, but it can be viewed with special equipment, like a light filter. All outer planets can be viewed with a telescope depending on the planets position relative to Earth.
Something like 4,000 to 6,000 .