Early this morning going out on the balcony, I looked up on a star chart app to verify it was Jupiter I was seeing. Then I noticed the alignment of Mars, Saturn, and Pluto on the app.

Never being able to identify it before, I stared at where Pluto should be and I'm pretty sure I saw it.

My only question is - since it's said that planets shine and stars twinkle, it did seem that Pluto was flicking a bit. Is this normal? Something to do with the relatively low luminosity and greater length of space?

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    It wouldn't have been Pluto, it's far too small and faint to be seen without a really powerful telescope. It was most likely a star that just happened to be in roughly the same place. – user10106 Mar 15 at 16:50
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    I agree with the previous statement. There's absolutely no chance to see Pluto with your naked eye. One can't even see objects of Plutos size in the asteroid belt (which is much closer) with the naked eye. – AtmosphericPrisonEscape Mar 15 at 17:12
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    Thanks for setting me straight. Good thing I didn't go into the real world bragging yet. – Jason P Sallinger Mar 15 at 17:47
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    Pluto is so small that it fails to fully occult stars. That's small enough to twinkle. But I still think you fooled yourself. – Joshua Mar 15 at 21:12
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    If Pluto was visible to the naked eye, it would have been known since antiquity. But it was not, neither were Uranus or Neptune which are closer, larger, and brighter. – swbarnes2 Mar 16 at 17:00
up vote 98 down vote accepted

Pluto is something like magnitude 14. The limit for the human vision is somewhere between magnitude 6 (widely accepted) and 8-ish (highly trained observers with perfect vision in ideal conditions using special techniques - and it's a bit controversial anyway).

There's zero chance that was Pluto. It was definitely a fixed star.

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    The difference is somewhere between 250 to 1000. It means, it is not a little bit fainter to be visible, it should be 1000 times lighter to be visible like a very faint star. – peterh Mar 15 at 19:30
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    It surely wasn't Pluto indeed. You cannot be sure it was a star though (e.g. satellite, plane, ...) – Eric Duminil Mar 16 at 20:37
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    But a plane must move faster on the sky than a planet, does it not? – mathreadler Mar 18 at 8:06
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    @mathreadler: Planes that are far away but flying in your exact direction can be surprisingly bright and stable in the sky. – Eric Duminil Mar 18 at 10:42
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    Obviously stars are fixed, or they'd all come crashing down on us. – Vince O'Sullivan Mar 19 at 8:27

As Florin correctly stated, it can't have been Pluto. You have probably looked at it and you have even gotten its light in your eyes. That little itty bitty shine just has no chance to make your retina do anything (edit: Interesting link in the comments. Might be that people actually can sense single photons. Doesn't help at all to see Pluto though).

Stellarium is a nice tool in order to check which star you might have confused with Pluto. This is what it has to offer:

Stellariun screenshot showing Saturn, Mars and Pluto

The red crosshair is where Pluto should be and Stellarium doesn't bother to color a single pixel because Pluto is about 14.28 mag. 5 mag difference mean 100 times dimmer, so Pluto is at least 10000 times less bright than many of the stars that you see in this image, let alone the two planets (Mars is about 26 times closer than Pluto atm.).

My guess is that you have seen Pi Sagittarii (HIP 94141) which would be 2.85 mag and lines up nicely with the planets. Unless I'm wrong, it's roughly 37000 times brighter than Pluto

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    That little itty bitty shine most definitely can make your retina do something: nature.com/news/people-can-sense-single-photons-1.20282 - your brain just won't pick it up when there's other, more intense signals anywhere. – Michael Borgwardt Mar 16 at 8:29
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    @MichaelBorgwardt they tested a whole 3 people... hardly good methodology – Cursed Mar 16 at 10:51
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    "other, more intense signals" - such as good old thermal noise in the amplifier stack. – John Dvorak Mar 16 at 12:01
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    @Cursed Why? If I want to show that something is possible, showing that it works on 3/3 participants seems good enough to me. (And it's not like that were 3 trials, those were thousands of trials with 3 participants). – sgf Mar 16 at 16:46

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