<p>If it exists, then the orbit has a perihelion of maybe 300 au and an aphelion of perhaps 2700 au.</p>

<p>You can then just scale from the brightness $m=-26.7$ of the Sun at the Earth. It would be between 12.4 and 17.1 magnitudes fainter. So still much brighter than the next brightest star in the sky.</p>

CC BY-SA 3.0
2016-01-21T12:49:09.657Z
+1 because I think it's more useful to compare the brightness to distant stars as seen from Earth. I see that this would make the Sun still much brighter than Venus appears on Earth ($M\approx-4.6$, according to Wikipedia).
Warrick
CC BY-SA 3.0
2016-01-21T07:59:24.237Z
2016-01-21T07:59:24.237Z
ProfRob
<p>Between $1/40,000$ and $1/8,000,000$ of the brightness as seen from Earth, depending on what the actual orbit would turn out to be, and where the planet is in its $15,000$ year orbit period.</p>

<p>Brightness drops as $\dfrac{1}{r^2}$ with distance from the light source. Earth is at $1~\textrm{AU}.$ The theoretical planet is at $200~\textrm{AU}$ when it's closest to the sun, and up to $2800~\textrm{AU}$ at the point furthest from the sun at the upper end of the estimated orbit.</p>

<p><br>So e.g. $1/200^2 = 1/40,000$ of the brightness (luminance) as seen from Earth.</p>

<p>For how a human would experience it, we can convert to <a href="https://en.wikipedia.org/wiki/Exposure_value#Tabulated_exposure_values">exposure value</a> as used in photography: The difference in exposure value (photographic 'stops') is $\log 2$ of the luminance ratio, so we would have $15$ to $23$ stops less light than on Earth. 
Sunny noon on Earth is $15~\textrm{EV}.$ </p>

<p>So the brightness at noon on the planet surface would be:</p>

<ul>
<li><p>At $200~\textrm{AU},$ planet orbit is closest to the sun: About $0~\textrm{EV},$ roughly the same as a dimly lit interior</p></li>
<li><p>$400~\textrm{AU},$ lower bound on semi-major axis: $-2 ~\textrm{EV},$ similar to a landscape lit by the full moon</p></li>
<li><p>$1500~\textrm{AU},$ upper bound on semi-major axis: $-6~\textrm{EV},$ similar to landscape lit by a quarter moon</p></li>
<li><p>$2800~\textrm{AU},$ upper bound on aphelion (the point on orbit most distant from the sun): $-8~\textrm{EV}.$ This would be dark, but you would probably still see enough to avoid running into things.</p></li>
</ul>

CC BY-SA 3.0
2016-01-21T13:50:27.370Z
One other important brightness milestone is 700AU.  That's when the sun would appear as faint as the full moon.  With dark adapted eyes it's still barely possible to see colors under the full moon, although they appear very washed out.
Dan Is Fiddling By Firelight
CC BY-SA 3.0
2016-01-21T13:56:40.497Z
Thinking a little more, At its brightest Venus is 1 billion times dimmer than the Sun.  In really dark locations it's apparently bright enough to cast a shadow; so you'd definitely be able to see something at 2800 AU.
Dan Is Fiddling By Firelight
CC BY-SA 3.0
2016-01-21T14:25:20.410Z
@DanNeely Yes, at 2800 AU with dark adapted eyes, you should be able to see just fine for navigation purposes. But no colors, and no reading. Already at 200 AU we're into [mesopic vision](https://en.wikipedia.org/wiki/Purkinje_effect) with reduced color perception. A bit beyond 2000 AU  we should be purely monochromatic. (Barring mistakes in my calculations.)
j-g-faustus
CC BY-SA 3.0
2017-04-27T14:58:22.597Z
@DanNeely ~ 700 AU is also the milestone where a significant number of the brightest stars would become visible to the naked eye during "day light" if you were to stand on the surface of Planet 9 or another body with a clear atmosphere.
RobertF
CC BY-SA 3.0
2016-01-21T08:02:58.303Z
2016-01-21T13:58:58.890Z
2016-01-21T13:58:58.890Z
j-g-faustus
<p>Theoretical <a href="http://www.findplanetnine.com/">Planet Nine</a>, <a href="http://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523">proposed</a> by Caltech's <a href="https://twitter.com/plutokiller">Mike Brown</a> and <a href="http://web.gps.caltech.edu/~kbatygin/Home.html">Konstantin Batygin</a>, is said to have a 15-20 thousand year orbit. Approximately how bright would the sun appear from Planet Nine aphelion and perihelion? </p>

<the-sun><planet><9th-planet>
How bright would the sun appear from the hypothetical Planet Nine proposed by Caltech?
joseph.hainline
# How bright would the sun appear from the hypothetical Planet Nine proposed by Caltech?

(Asked by: username_0 on 2016-01-21)

Theoretical [Planet Nine](http://www.findplanetnine.com/), [proposed](http://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523) by Caltech's [Mike Brown](https://twitter.com/plutokiller) and [Konstantin Batygin](http://web.gps.caltech.edu/~kbatygin/Home.html), is said to have a 15-20 thousand year orbit. Approximately how bright would the sun appear from Planet Nine aphelion and perihelion?

**Tags:** the-sun, planet, 9th-planet

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## 2 answers

### Answer by username_3 on 2016-01-21. Score: 28 | Accepted answer

Between $1/40,000$ and $1/8,000,000$ of the brightness as seen from Earth, depending on what the actual orbit would turn out to be, and where the planet is in its $15,000$ year orbit period.

Brightness drops as $\dfrac{1}{r^2}$ with distance from the light source. Earth is at $1~\textrm{AU}.$ The theoretical planet is at $200~\textrm{AU}$ when it's closest to the sun, and up to $2800~\textrm{AU}$ at the point furthest from the sun at the upper end of the estimated orbit.

So e.g. $1/200^2 = 1/40,000$ of the brightness (luminance) as seen from Earth.

For how a human would experience it, we can convert to [exposure value](https://en.wikipedia.org/wiki/Exposure_value#Tabulated_exposure_values) as used in photography: The difference in exposure value (photographic 'stops') is $\log 2$ of the luminance ratio, so we would have $15$ to $23$ stops less light than on Earth.
Sunny noon on Earth is $15~\textrm{EV}.$

So the brightness at noon on the planet surface would be:

* At $200~\textrm{AU},$ planet orbit is closest to the sun: About $0~\textrm{EV},$ roughly the same as a dimly lit interior
* $400~\textrm{AU},$ lower bound on semi-major axis: $-2 ~\textrm{EV},$ similar to a landscape lit by the full moon
* $1500~\textrm{AU},$ upper bound on semi-major axis: $-6~\textrm{EV},$ similar to landscape lit by a quarter moon
* $2800~\textrm{AU},$ upper bound on aphelion (the point on orbit most distant from the sun): $-8~\textrm{EV}.$ This would be dark, but you would probably still see enough to avoid running into things.

> **Comments:**
> **username_4**: One other important brightness milestone is 700AU.  That's when the sun would appear as faint as the full moon.  With dark adapted eyes it's still barely possible to see colors under the full moon, although they appear very washed out.  
> **username_4**: Thinking a little more, At its brightest Venus is 1 billion times dimmer than the Sun.  In really dark locations it's apparently bright enough to cast a shadow; so you'd definitely be able to see something at 2800 AU.  
> **username_3**: @DanNeely Yes, at 2800 AU with dark adapted eyes, you should be able to see just fine for navigation purposes. But no colors, and no reading. Already at 200 AU we're into [mesopic vision](https://en.wikipedia.org/wiki/Purkinje_effect) with reduced color perception. A bit beyond 2000 AU  we should be purely monochromatic. (Barring mistakes in my calculations.)  
> **username_5**: @DanNeely ~ 700 AU is also the milestone where a significant number of the brightest stars would become visible to the naked eye during "day light" if you were to stand on the surface of Planet 9 or another body with a clear atmosphere.  

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### Answer by username_1 on 2016-01-21. Score: 11

If it exists, then the orbit has a perihelion of maybe 300 au and an aphelion of perhaps 2700 au.

You can then just scale from the brightness $m=-26.7$ of the Sun at the Earth. It would be between 12.4 and 17.1 magnitudes fainter. So still much brighter than the next brightest star in the sky.

> **Comments:**
> **username_2**: +1 because I think it's more useful to compare the brightness to distant stars as seen from Earth. I see that this would make the Sun still much brighter than Venus appears on Earth ($M\approx-4.6$, according to Wikipedia).  

---

