<p>Let's take a look at the star and the planet's characteristics first. We have the star TRAPPIST-1:</p>

<ul>
<li>$M = 0.082 M_{\odot}$</li>
<li>$R = 0.117 R_{\odot}$</li>
</ul>

<p>The planets are all roughly in the scale of $0.7 R_{\oplus}$ to $1.2 R_{\oplus}$ with their semi-major axes of:</p>

<ul>
<li>b: $1.66 \times 10^6 \; \mathrm{km}$</li>
<li>c: $2.88 \times 10^6 \; \mathrm{km}$</li>
<li>d: $3.14 \times 10^6 \; \mathrm{km}$</li>
<li>e: $4.19 \times 10^6 \; \mathrm{km}$</li>
<li>f: $5.57 \times 10^6 \; \mathrm{km}$</li>
<li>g: $6.73 \times 10^6 \; \mathrm{km}$</li>
</ul>

<p>So if we would stand on the surface of TRAPPIST-1g and look towards the star, we would be at $1/25$ the distance to the star compared to the sun. The star however is roughly $1/8 R_{\odot}$. That would make the star appear about at about $3$ times the diameter compared to our Sun from the Earth.</p>

<p>TRAPPIST-1f would every few days pass TRAPPIST-1g with a closest approach of $1.16 \times 10^6 \; \mathrm{km} \approx 3 \times r_{\mathrm{Earth-Moon}}$ which is roughly three times the distance to the Moon. </p>

<p>Since $R_{\mathrm{TRAPPIST-1f}} \approx R_{\oplus}$ and $R_{\oplus}/R_{\mathrm{Moon}} = 3.67$ it would seem that TRAPPIST-1f would appear roughly the size of the Moon on closest approach.</p>

<p>The other planets are similar in radius but their distances are $2$ to $5$ times further away and would thus appear smaller by that factor.</p>

CC BY-SA 3.0
2017-03-02T14:18:12.507Z
2017-03-03T06:19:15.800Z
2017-03-03T06:19:15.800Z
Adwaenyth
<p>There's a lot of interest with the recent discovery of seven earth sized planets around <a href="http://www.trappist.one/" rel="nofollow noreferrer">TRAPPIST-1</a>. I saw <a href="https://exoplanets.nasa.gov/resources/2159/" rel="nofollow noreferrer">this poster</a>, and the <a href="https://www.nasa.gov/press-release/nasa-telescope-reveals-largest-batch-of-earth-size-habitable-zone-planets-around" rel="nofollow noreferrer">NASA video</a> says that in the system you could see a lot of other planets whipping around in their orbits. </p>

<p>It got me wondering:</p>

<ul>
<li>Obviously that poster is exaggerated, but how exaggerated? </li>
<li>What would you actually see with the naked eye, standing on TRAPPIST-1g, and looking back toward the other planets? </li>
<li>How big would they appear?</li>
<li>What might TRAPPIST-1 look like?</li>
</ul>

CC BY-SA 3.0
2017-03-01T09:37:03.713Z
Is this really a different question to http://astronomy.stackexchange.com/questions/20200/how-many-things-are-wrong-in-this-artist-view-of-the-trappist-1-system ?
ProfRob
CC BY-SA 3.0
2017-03-02T15:02:48.740Z
@RobJeffries This question is better written than the other though. I'd be inclined to merge the earlier question with this one.
called2voyage
<trappist-1><dwarf-star>
What would the view be list from TRAPPIST-1g, looking back towards TRAPPIST-1?
joseph.hainline
# What would the view be list from TRAPPIST-1g, looking back towards TRAPPIST-1?

(Asked by: username_0 on 2017-03-01)

There's a lot of interest with the recent discovery of seven earth sized planets around [TRAPPIST-1](http://www.trappist.one/). I saw [this poster](https://exoplanets.nasa.gov/resources/2159/), and the [NASA video](https://www.nasa.gov/press-release/nasa-telescope-reveals-largest-batch-of-earth-size-habitable-zone-planets-around) says that in the system you could see a lot of other planets whipping around in their orbits.

It got me wondering:

* Obviously that poster is exaggerated, but how exaggerated?
* What would you actually see with the naked eye, standing on TRAPPIST-1g, and looking back toward the other planets?
* How big would they appear?
* What might TRAPPIST-1 look like?

**Tags:** trappist-1, dwarf-star

> **Comments:**
> **username_1**: Is this really a different question to http://astronomy.stackexchange.com/questions/20200/how-many-things-are-wrong-in-this-artist-view-of-the-trappist-1-system ?  
> **username_2**: @RobJeffries This question is better written than the other though. I'd be inclined to merge the earlier question with this one.  

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## 1 answers

### Answer by username_3 on 2017-03-02. Score: 1 | Accepted answer

Let's take a look at the star and the planet's characteristics first. We have the star TRAPPIST-1:

* $M = 0.082 M\_{\odot}$
* $R = 0.117 R\_{\odot}$

The planets are all roughly in the scale of $0.7 R\_{\oplus}$ to $1.2 R\_{\oplus}$ with their semi-major axes of:

* b: $1.66 \times 10^6 \; \mathrm{km}$
* c: $2.88 \times 10^6 \; \mathrm{km}$
* d: $3.14 \times 10^6 \; \mathrm{km}$
* e: $4.19 \times 10^6 \; \mathrm{km}$
* f: $5.57 \times 10^6 \; \mathrm{km}$
* g: $6.73 \times 10^6 \; \mathrm{km}$

So if we would stand on the surface of TRAPPIST-1g and look towards the star, we would be at $1/25$ the distance to the star compared to the sun. The star however is roughly $1/8 R\_{\odot}$. That would make the star appear about at about $3$ times the diameter compared to our Sun from the Earth.

TRAPPIST-1f would every few days pass TRAPPIST-1g with a closest approach of $1.16 \times 10^6 \; \mathrm{km} \approx 3 \times r\_{\mathrm{Earth-Moon}}$ which is roughly three times the distance to the Moon.

Since $R\_{\mathrm{TRAPPIST-1f}} \approx R\_{\oplus}$ and $R\_{\oplus}/R\_{\mathrm{Moon}} = 3.67$ it would seem that TRAPPIST-1f would appear roughly the size of the Moon on closest approach.

The other planets are similar in radius but their distances are $2$ to $5$ times further away and would thus appear smaller by that factor.


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