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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    $\begingroup$ 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. $\endgroup$
    – user10106
    Commented Mar 15, 2018 at 16:50
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    $\begingroup$ 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. $\endgroup$ Commented Mar 15, 2018 at 17:12
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    $\begingroup$ Thanks for setting me straight. Good thing I didn't go into the real world bragging yet. $\endgroup$ Commented Mar 15, 2018 at 17:47
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    $\begingroup$ Pluto is so small that it fails to fully occult stars. That's small enough to twinkle. But I still think you fooled yourself. $\endgroup$
    – Joshua
    Commented Mar 15, 2018 at 21:12
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    $\begingroup$ 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. $\endgroup$
    – swbarnes2
    Commented Mar 16, 2018 at 17:00

2 Answers 2


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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    $\begingroup$ 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. $\endgroup$
    – peterh
    Commented Mar 15, 2018 at 19:30
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    $\begingroup$ It surely wasn't Pluto indeed. You cannot be sure it was a star though (e.g. satellite, plane, ...) $\endgroup$ Commented Mar 16, 2018 at 20:37
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    $\begingroup$ @mathreadler: Planes that are far away but flying in your exact direction can be surprisingly bright and stable in the sky. $\endgroup$ Commented Mar 18, 2018 at 10:42
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    $\begingroup$ Obviously stars are fixed, or they'd all come crashing down on us. $\endgroup$ Commented Mar 19, 2018 at 8:27
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    $\begingroup$ @uhoh "Fixed star" is a commonly used term in astronomy, although nowadays it's more of a historic interest, since we now understand that most star-like objects visible with the naked eye, which appear to be unmoving relative to each other, are indeed stars. Even so, it's still in use, so don't be surprised when you hear it. en.wikipedia.org/wiki/Fixed_stars $\endgroup$ Commented Mar 20, 2018 at 20:21

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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    $\begingroup$ 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. $\endgroup$ Commented Mar 16, 2018 at 8:29
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    $\begingroup$ @MichaelBorgwardt they tested a whole 3 people... hardly good methodology $\endgroup$ Commented Mar 16, 2018 at 10:51
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    $\begingroup$ "other, more intense signals" - such as good old thermal noise in the amplifier stack. $\endgroup$ Commented Mar 16, 2018 at 12:01
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    $\begingroup$ @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). $\endgroup$
    – sgf
    Commented Mar 16, 2018 at 16:46

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