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I wonder how new planets and galaxies can be discovered from earth. I mean, there are not inifinately many directions to look with the telescope, right? Is it really so many different directions to look that there are still directions that noone has ever looked? Please excuse if my explaining is bad, I hope you understand. Thanks.

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    $\begingroup$ The angles we have to point telescopes remains constant over time, but we get better at zooming in. New discoveries are hidden in the blurry parts of images from the last generation. $\endgroup$ – Alan Rominger May 6 '16 at 18:44
  • $\begingroup$ We have done whole sky surveys (quite a few actually), but the deeper you look (i.e. the larger aperture instruments you have), the fainter objects you can see and that is not nearly exploited. Most of the universe, so far, is in the dark. There is also the matter that larger aperture telescopes can reveal more detail, so the resolution with which we can see individual objects can greatly increase. In total, there is billions, if not trillions of times more to see than what we have seen, so far. $\endgroup$ – CuriousOne May 6 '16 at 18:46
  • $\begingroup$ So if you have a telescope with old and bad zooming, there are no new things to discover? $\endgroup$ – john jonsson May 6 '16 at 18:48
  • $\begingroup$ There are infinitely many directions. And there are directions where nobody has looked - to a given depth. $\endgroup$ – Rob Jeffries May 6 '16 at 18:49
  • $\begingroup$ What kind of telescope are you talking about? A small amateur instrument? You can still make discoveries in the solar system with instruments as small as, I would say, ten to twelve inches in diameter. That opportunity will soon disappear, since the professional astronomy community is planning on having weekly whole sky surveys, or so, which kind of closes this niche, but still... $\endgroup$ – CuriousOne May 6 '16 at 18:49
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The principle way of detecting planets and discovering galaxies are completely different.

In general, distant galaxies are faint and their apparent magnitude is high. You need a collect a lot of photons from these objects to obtain an image with desired signal to noise ratio.

The total sky area is in the order of 40000 square degrees. This area has been scanned by a number of survey (viz. SDSS). But all surveys are magnitude limited, meaning they only look for object below a certain apparent magnitude (say 22 Mag).

That is why astronomers build larger and larger telescopes and preferably deploy them on space for better seeing. Newer telescopes allow us to push the magnitude limit to higher numbers and allow us to discover fainter objects.

Exoplanets are mostly detected by indirect methods. More details can be found here https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets. Same as above, advancements in technology will allow us to discover smaller planets which are currently beyond our limits.

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  • $\begingroup$ But we may also still discover new planets in our own solar system. At least dwarf planets are discovered fairly frequently. These are found much more like new galaxies are. $\endgroup$ – Thriveth May 7 '16 at 15:51

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