<p>According to the <a href="https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html" rel="noreferrer">NASA Pluto fact sheet</a>, the brightest that Pluto gets is an apparent visual magnitude of 13.65 when it is 28.6 au from the Earth and (presumably) about  29.6 au from the Sun.</p>
<p>To work out how bright that would be from Neptune we could  work out how close Neptune can be to Pluto when Pluto is at perihelion. This is complicated by the fact that the semi-major axes of the orbits are not aligned, that the inclination of Pluto to the ecliptic is quite high and that the orbital periods are in a 3:2 resonance.
The closest they can actually get is about 16 au and this will not occur when Pluto is at perihelion. And, when Pluto is at perihelion it will be further than 16 au from Neptune and will not be fully illuminated (or even as illuminated as we can see it from Earth) as seen from Neptune.</p>
<p>Therefore an <em>upper limit</em> to the brightness would be to scale how bright it can appear from Earth to a distance of 16 au, which gives an apparent magnitude <span class="math-container">$&gt;12.4$</span>.</p>
<p>For the reasons stated above it <em>must</em> be fainter than this because it is not fully illuminated as seen from Neptune at 16 au and it is not at perihelion at that time, and so it cannot be anywhere near visible to the naked eye.</p>
<p>EDIT: A paper by <a href="http://articles.adsabs.harvard.edu/pdf/1971AJ.....76..167W" rel="noreferrer">Williams &amp; Benson (1971)</a> puts the closest approach of Pluto and Neptune at 16.8 au, but crucially, this occurs when Pluto is near aphelion. They also show that this is true for a timespan exceeding 1 million years.</p>

CC BY-SA 4.0
2021-01-19T13:11:41.093Z
Do you happen to know how reliable the planets positions according to Stellarium in ~100000 years are? Because in 99861, the distance between Neptune and Pluto would be only 3 AU, for a brightness of 10 mag.
Eric Duminil
CC BY-SA 4.0
2021-01-19T13:27:53.637Z
@EricDuminil that would make another good question. I think the 16 au is based on the current orbits. The closest the lines of the orbits ever come to each other is 2.4 au. AFAIK they are in a *stable* configuration, so I can only assume that there is some numerical accuracy problem in Stellarium (which contains no physics remember), but I'm surprised by that.
ProfRob
CC BY-SA 4.0
2021-01-19T18:30:43.847Z
Stellarium uses VSOP87 unless the user has downloaded DE430 or DE431.
Mike G
CC BY-SA 4.0
2021-01-19T20:41:32.850Z
@MikeG: VSOP87, then. According to https://en.wikipedia.org/wiki/VSOP_(planets)#VSOP87, it should still be accurate for Neptune in -2527 ("over 6000 years before and after J2000"). So is it unreliable for Pluto?
Eric Duminil
CC BY-SA 4.0
2021-01-19T21:25:02.463Z
@EricDuminil Dunno. Certainly no more reliable than for Neptune.
Mike G
CC BY-SA 4.0
2021-01-19T13:08:17.583Z
2021-01-19T13:37:31.567Z
2021-01-19T13:37:31.567Z
ProfRob
<p>That's great question! ... I'm just thinking that just for comparison, Pluto is roughly 70% the diameter of our Moon and Pluto would be far away from Neptune than the Moon is from Earth. Moon is also sort of &quot;fixed&quot; to Earth, but Pluto is in a different orbit than Neptune. I think that Pluto will not be visible with  the naked eye given its size and distance from the observer.</p>

CC BY-SA 4.0
2021-01-18T19:50:28.300Z
Welcome to the site, Daniel! Yes, as mentioned in the comments on planetmaker's answer, even the closest approach distance is rather large. By way of comparison, the mean orbital radius of Uranus is 19.19 au. Also, it's rather dark way out there. Neptune's mean orbital radius is ~30.1 au, so by the inverse square law, the intensity of sunlight is ~1/900 of its intensity near the Earth & Moon.
PM 2Ring
CC BY-SA 4.0
2021-01-19T12:47:33.010Z
This isn't a correct way to think about the problem. The surface of Pluto is much less "luminous" than that of the Moon because it is much further away from the illuminating Sun.
ProfRob
CC BY-SA 4.0
2021-01-18T19:17:06.643Z
2021-01-18T19:17:06.643Z
Daniel Morales
<p>No, it cannot. Far from it.</p>
<p>The closest approach between both planets is <a href="https://astronomy.com/magazine/ask-astro/2005/08/how-close-does-plutos-orbit-come-to-neptune" rel="noreferrer">roughly 16 AU</a> due to the 3:2 orbit resonance. Pluto will even then be a tiny dot among many with a brightness around 14 mag.</p>
<p>You can try that with Stellarium yourself, placing the observer on Neptune and looking for Pluto. You just have to find the right time. One such time is approx. in the year 2877.</p>

CC BY-SA 4.0
2021-01-18T13:23:27.673Z
Thank you for the year. I just studied Stellarium, looking at Pluto from Proteus, but the closest I got Pluto was in end of 2003 / beginning of 2004 at 27.29 au from me.
Plutos Loyer
CC BY-SA 4.0
2021-01-18T13:35:43.510Z
Alright, in spring 2881 I got Pluto as close as 20.05 au from me on Proteus. It wasn't visible even in a good telescope.
Plutos Loyer
CC BY-SA 4.0
2021-01-18T18:42:41.237Z
I found an 18.9 au approach in 1896.
Mike G
CC BY-SA 4.0
2021-01-19T07:24:57.587Z
@MikeG Indeed, 18.88 au from me in fall 1896. Still not visible at 70x magnification.
Plutos Loyer
CC BY-SA 4.0
2021-01-19T10:25:36.900Z
14 mag? Isn't it the same brightness as what we see from Earth?
Eric Duminil
CC BY-SA 4.0
2021-01-19T13:08:30.323Z
In Stellarium, I found a 9.8 AU distance between Pluto and Neptune, in -2527. Not sure if it's really reliable, though. With a 12.9 mag, as seen from Neptune. Oooooh. And 10 mag at 3 AU in October 99861. XD
Eric Duminil
CC BY-SA 4.0
2021-01-19T18:41:19.913Z
@EricDuminil Pretty sure this is fake news.
ProfRob
CC BY-SA 4.0
2021-01-20T19:48:09.033Z
So, does this mean people on Neptune are still perfectly unaware of Pluto's existence?
mdomino
CC BY-SA 4.0
2021-01-20T23:47:15.633Z
People on earth also know about pluto without being able to see it with the unaided eye
planetmaker
CC BY-SA 4.0
2021-01-18T13:08:17.000Z
2021-01-19T11:44:50.873Z
2021-01-19T11:44:50.873Z
planetmaker
<p>When Neptune and Pluto are closest, about 100 million mi (160 million km) from each other, would an observer on Neptune (or rather on one of its moons, since Neptune is gaseous) be able to see Pluto, and maybe even Charon, with the naked eye? If not, could Pluto be seen in average binoculars? I think Pluto would appear a bit smaller than Mercury from Earth (but at much lower apparent brightness because of the distance from the Sun).</p>

CC BY-SA 4.0
2021-01-18T12:42:10.993Z
Hi, this is not an answer to your question, just an idea. If you have [Stellarium](https://stellarium.org/), you could change the planet in location window to Neptune and watch Pluto from there. It gives you an endless opportunity to tweak and try things. All the best!
lordparthurnaax
CC BY-SA 4.0
2021-01-18T12:51:10.557Z
@lordparthurnaax Thank you. I do have Stellarium, but I dunno when Neptune and Pluto came/come closest. I think it was in the 1970s/80s when Pluto was closer than Neptune. I'll research their intersection dates.
Plutos Loyer
CC BY-SA 4.0
2021-01-18T20:50:09.907Z
As a rule of thumb, halving the distance to an object decreases its apparent magnitude by about $-2.5 \log_{10} 4 ≈ -1.5$.  To get Pluto into the visible range, you'd have to decrease the mangitude by about 7.5 relative to its magnitude on Earth, so you'd need to be about 32 times closer to Pluto than the Earth is to (barely) see it with the naked eye.  If I've worked my numbers right, this very conveniently works out to be about 1 AU away (maybe a bit less.)
Michael Seifert
CC BY-SA 4.0
2021-01-19T20:11:33.967Z
Pluto and Neptune never get within 160 million km of each other.  More like 2.5 billion km at their closest. See: https://astronomy.stackexchange.com/questions/22304/when-and-where-are-pluto-and-neptune-closest-to-each-other
Connor Garcia
<observational-astronomy><pluto><naked-eye><neptune>
Can Pluto be seen with the naked eye from Neptune when Pluto and Neptune are closest?
Plutos Loyer
# Can Pluto be seen with the naked eye from Neptune when Pluto and Neptune are closest?

(Asked by: username_0 on 2021-01-18)

When Neptune and Pluto are closest, about 100 million mi (160 million km) from each other, would an observer on Neptune (or rather on one of its moons, since Neptune is gaseous) be able to see Pluto, and maybe even Charon, with the naked eye? If not, could Pluto be seen in average binoculars? I think Pluto would appear a bit smaller than Mercury from Earth (but at much lower apparent brightness because of the distance from the Sun).

**Tags:** observational-astronomy, pluto, naked-eye, neptune

> **Comments:**
> **username_1**: Hi, this is not an answer to your question, just an idea. If you have [Stellarium](https://stellarium.org/), you could change the planet in location window to Neptune and watch Pluto from there. It gives you an endless opportunity to tweak and try things. All the best!  
> **username_0**: @username_1 Thank you. I do have Stellarium, but I dunno when Neptune and Pluto came/come closest. I think it was in the 1970s/80s when Pluto was closer than Neptune. I'll research their intersection dates.  
> **username_2**: As a rule of thumb, halving the distance to an object decreases its apparent magnitude by about $-2.5 \log_{10} 4 ≈ -1.5$.  To get Pluto into the visible range, you'd have to decrease the mangitude by about 7.5 relative to its magnitude on Earth, so you'd need to be about 32 times closer to Pluto than the Earth is to (barely) see it with the naked eye.  If I've worked my numbers right, this very conveniently works out to be about 1 AU away (maybe a bit less.)  
> **username_3**: Pluto and Neptune never get within 160 million km of each other.  More like 2.5 billion km at their closest. See: https://astronomy.stackexchange.com/questions/22304/when-and-where-are-pluto-and-neptune-closest-to-each-other  

---

## 3 answers

### Answer by username_9 on 2021-01-18. Score: 31 | Accepted answer

No, it cannot. Far from it.

The closest approach between both planets is [roughly 16 AU](https://astronomy.com/magazine/ask-astro/2005/08/how-close-does-username_0-orbit-come-to-neptune) due to the 3:2 orbit resonance. Pluto will even then be a tiny dot among many with a brightness around 14 mag.

You can try that with Stellarium yourself, placing the observer on Neptune and looking for Pluto. You just have to find the right time. One such time is approx. in the year 2877.

> **Comments:**
> **username_0**: Thank you for the year. I just studied Stellarium, looking at Pluto from Proteus, but the closest I got Pluto was in end of 2003 / beginning of 2004 at 27.29 au from me.  
> **username_0**: Alright, in spring 2881 I got Pluto as close as 20.05 au from me on Proteus. It wasn't visible even in a good telescope.  
> **username_6**: I found an 18.9 au approach in 1896.  
> **username_0**: @username_6 Indeed, 18.88 au from me in fall 1896. Still not visible at 70x magnification.  
> **username_5**: 14 mag? Isn't it the same brightness as what we see from Earth?  
> **username_5**: In Stellarium, I found a 9.8 AU distance between Pluto and Neptune, in -2527. Not sure if it's really reliable, though. With a 12.9 mag, as seen from Neptune. Oooooh. And 10 mag at 3 AU in October 99861. XD  
> **username_4**: @username_5 Pretty sure this is fake news.  
> **username_10**: So, does this mean people on Neptune are still perfectly unaware of Pluto's existence?  
> **username_9**: People on earth also know about pluto without being able to see it with the unaided eye  

---

### Answer by username_4 on 2021-01-19. Score: 16

According to the [NASA Pluto fact sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html), the brightest that Pluto gets is an apparent visual magnitude of 13.65 when it is 28.6 au from the Earth and (presumably) about 29.6 au from the Sun.

To work out how bright that would be from Neptune we could work out how close Neptune can be to Pluto when Pluto is at perihelion. This is complicated by the fact that the semi-major axes of the orbits are not aligned, that the inclination of Pluto to the ecliptic is quite high and that the orbital periods are in a 3:2 resonance.
The closest they can actually get is about 16 au and this will not occur when Pluto is at perihelion. And, when Pluto is at perihelion it will be further than 16 au from Neptune and will not be fully illuminated (or even as illuminated as we can see it from Earth) as seen from Neptune.

Therefore an *upper limit* to the brightness would be to scale how bright it can appear from Earth to a distance of 16 au, which gives an apparent magnitude $>12.4$.

For the reasons stated above it *must* be fainter than this because it is not fully illuminated as seen from Neptune at 16 au and it is not at perihelion at that time, and so it cannot be anywhere near visible to the naked eye.

EDIT: A paper by [Williams & Benson (1971)](http://articles.adsabs.harvard.edu/pdf/1971AJ.....76..167W) puts the closest approach of Pluto and Neptune at 16.8 au, but crucially, this occurs when Pluto is near aphelion. They also show that this is true for a timespan exceeding 1 million years.

> **Comments:**
> **username_5**: Do you happen to know how reliable the planets positions according to Stellarium in ~100000 years are? Because in 99861, the distance between Neptune and Pluto would be only 3 AU, for a brightness of 10 mag.  
> **username_4**: @username_5 that would make another good question. I think the 16 au is based on the current orbits. The closest the lines of the orbits ever come to each other is 2.4 au. AFAIK they are in a *stable* configuration, so I can only assume that there is some numerical accuracy problem in Stellarium (which contains no physics remember), but I'm surprised by that.  
> **username_6**: Stellarium uses VSOP87 unless the user has downloaded DE430 or DE431.  
> **username_5**: @username_6: VSOP87, then. According to https://en.wikipedia.org/wiki/VSOP_(planets)#VSOP87, it should still be accurate for Neptune in -2527 ("over 6000 years before and after J2000"). So is it unreliable for Pluto?  
> **username_6**: @username_5 Dunno. Certainly no more reliable than for Neptune.  

---

### Answer by username_7 on 2021-01-18. Score: 5

That's great question! ... I'm just thinking that just for comparison, Pluto is roughly 70% the diameter of our Moon and Pluto would be far away from Neptune than the Moon is from Earth. Moon is also sort of "fixed" to Earth, but Pluto is in a different orbit than Neptune. I think that Pluto will not be visible with the naked eye given its size and distance from the observer.

> **Comments:**
> **username_8**: Welcome to the site, username_7! Yes, as mentioned in the comments on username_9's answer, even the closest approach distance is rather large. By way of comparison, the mean orbital radius of Uranus is 19.19 au. Also, it's rather dark way out there. Neptune's mean orbital radius is ~30.1 au, so by the inverse square law, the intensity of sunlight is ~1/900 of its intensity near the Earth & Moon.  
> **username_4**: This isn't a correct way to think about the problem. The surface of Pluto is much less "luminous" than that of the Moon because it is much further away from the illuminating Sun.  

---

