<blockquote>
<p>How might an alien astronomer calculate Uranus's density without realizing it has rings?</p>
</blockquote>
<p>Hold my beer...</p>
<p>He would be able to determine the density of Uranus in the same way we determine the density of other exoplanets.</p>
<ol>
<li><p>You calculate the diameter from the light curve</p>
</li>
<li><p>Then infer the volume from the radius and the fact that it's spherical.</p>
</li>
<li><p>Then we assume, by induction on observations of the sol system, the density as a function of volume. An established relation exists between outer-planet volume and density. It is explicitly used to estimate exoplanet densities (<a href="https://ethz.ch/content/dam/ethz/special-interest/phys/particle-physics/quanz-group-dam/documents-old-s-and-p/Courses/ExtrasolarPlanetsFS2016/exop2016_chapter3_part1_UPDATED.pdf" rel="nofollow noreferrer">Table 3.1</a>).</p>
</li>
</ol>
<p>Rings, cotton candy, and planetary plumage don't enter into it.</p>

CC BY-SA 4.0
2024-06-25T14:09:50.937Z
I''m afraid this is wrong. To find a super-puff planet we need the mass estimate, from radial velocity measurement.
Heopps
CC BY-SA 4.0
2024-08-31T02:13:01.720Z
How do you infer  mass from radial velocity?
Miss Understands
CC BY-SA 4.0
2024-09-02T13:33:22.710Z
Radial velocity method (called also Doppler method) allows to obtain mass and period of a planet revolving around a star, by measuring Doppler shift in star's spectral line in time. Transit method can give us period and radius of a planet, but not mass. Without mass estimate we can't say is the planet a super-puff (low mass with big radius) or a normal Jupiter-like heavy planet. The third point in your answer "3.Then we assume .." is not correct. Also with this assumption we'll newer tell the exoplanet is super-puff, because we have no super-puff  analog in Solar syslen.
Heopps
CC BY-SA 4.0
2024-09-02T13:42:08.723Z
Aa far as I know there is no consensus currently (in 2024) what are the interiors of super-puff planets. Also super-puffs are rare, less than dosen candidates have been found so far if I remember correctly. I would not even be surprised if some or all of them will be found to have errors in density edtimates after futher observations. Maybe in next decade we'll see more clear picture do super-puff planets really exist, and if yes how often they occur and in which conditions. Maybe, maybe )
Heopps
CC BY-SA 4.0
2024-06-25T04:45:22.843Z
2024-06-25T05:58:45.100Z
2024-06-25T05:58:45.100Z
Miss Understands
<p>It has been suggested that at least some of the &quot;super-puff&quot; planets may actually be ordinary planets with rings. Saturn, for instance, may appear as a &quot;super-puff&quot; to an alien astronomer light years away. Uranus also has rings, albeit less extensive than Saturn's. How might an alien astronomer calculate Uranus's density without realizing it has rings?</p>
<p><a href="https://earthsky.org/space/super-puff-cotton-candy-exoplanets-with-rings" rel="nofollow noreferrer">https://earthsky.org/space/super-puff-cotton-candy-exoplanets-with-rings</a>
<a href="https://sciencealert.com/the-puffiness-of-super-puff-planets-could-be-huge-rings" rel="nofollow noreferrer">https://sciencealert.com/the-puffiness-of-super-puff-planets-could-be-huge-rings</a></p>

CC BY-SA 4.0
2024-06-25T03:27:27.017Z
I'm not sure why the first link didn't work. That's weird.
Joe Peters
CC BY-SA 4.0
2024-06-25T16:21:31.043Z
Now it works. I'm not sure why it didn't last night.
Joe Peters
CC BY-SA 4.0
2024-06-26T07:43:21.017Z
You did some self-research, it's good! But we should discern between scientific papers with certain discoveries and papers that suggest that "such-and-such objects may exist". The probability of the assertion being true can be low. Also, astromomers don't "see" most of exoplanets, including super-puff ones. They see a star dimming than exoplanet transits. Uranus rings are too thin to be seen during transit. For planet with dense rings - transit photometry pattern will differ from a planet without rings.
Heopps
CC BY-SA 4.0
2024-06-26T07:45:50.533Z
You can do further self-research with requests like "exoplanet with rings transit pattern"
Heopps
CC BY-SA 4.0
2024-06-27T03:16:33.213Z
Here are my thoughts, if a planet have rings, during transit, there will be a small decrease of the star's brightness before the planet transit through the star, just like how planets with atmosphere will cause a little decrease of star's brightness when they transit, but the decrease is just too small for us to detect with our current equipment. Resulting in a seemingly smooth light curve.
Polaris5744
<planet><gas-giants>
What would Uranus look like from light years away?
Joe Peters
# What would Uranus look like from light years away?

(Asked by: username_0 on 2024-06-25)

It has been suggested that at least some of the "super-puff" planets may actually be ordinary planets with rings. Saturn, for instance, may appear as a "super-puff" to an alien astronomer light years away. Uranus also has rings, albeit less extensive than Saturn's. How might an alien astronomer calculate Uranus's density without realizing it has rings?

<https://earthsky.org/space/super-puff-cotton-candy-exoplanets-with-rings>
<https://sciencealert.com/the-puffiness-of-super-puff-planets-could-be-huge-rings>

**Tags:** planet, gas-giants
> **Comments:**
> **username_0**: I'm not sure why the first link didn't work. That's weird.  
> **username_0**: Now it works. I'm not sure why it didn't last night.  
> **username_1**: You did some self-research, it's good! But we should discern between scientific papers with certain discoveries and papers that suggest that "such-and-such objects may exist". The probability of the assertion being true can be low. Also, astromomers don't "see" most of exoplanets, including super-puff ones. They see a star dimming than exoplanet transits. Uranus rings are too thin to be seen during transit. For planet with dense rings - transit photometry pattern will differ from a planet without rings.  
> **username_1**: You can do further self-research with requests like "exoplanet with rings transit pattern"  
> **username_2**: Here are my thoughts, if a planet have rings, during transit, there will be a small decrease of the star's brightness before the planet transit through the star, just like how planets with atmosphere will cause a little decrease of star's brightness when they transit, but the decrease is just too small for us to detect with our current equipment. Resulting in a seemingly smooth light curve.  

---

## 1 answers

### Answer by username_3 on 2024-06-25. Score: -6

> How might an alien astronomer calculate Uranus's density without realizing it has rings?

Hold my beer...

He would be able to determine the density of Uranus in the same way we determine the density of other exoplanets.

1. You calculate the diameter from the light curve
2. Then infer the volume from the radius and the fact that it's spherical.
3. Then we assume, by induction on observations of the sol system, the density as a function of volume. An established relation exists between outer-planet volume and density. It is explicitly used to estimate exoplanet densities ([Table 3.1](https://ethz.ch/content/dam/ethz/special-interest/phys/particle-physics/quanz-group-dam/documents-old-s-and-p/Courses/ExtrasolarPlanetsFS2016/exop2016_chapter3_part1_UPDATED.pdf)).

Rings, cotton candy, and planetary plumage don't enter into it.

> **Comments:**
> **username_1**: I''m afraid this is wrong. To find a super-puff planet we need the mass estimate, from radial velocity measurement.  
> **username_3**: How do you infer  mass from radial velocity?  
> **username_1**: Radial velocity method (called also Doppler method) allows to obtain mass and period of a planet revolving around a star, by measuring Doppler shift in star's spectral line in time. Transit method can give us period and radius of a planet, but not mass. Without mass estimate we can't say is the planet a super-puff (low mass with big radius) or a normal Jupiter-like heavy planet. The third point in your answer "3.Then we assume .." is not correct. Also with this assumption we'll newer tell the exoplanet is super-puff, because we have no super-puff  analog in Solar syslen.  
> **username_1**: Aa far as I know there is no consensus currently (in 2024) what are the interiors of super-puff planets. Also super-puffs are rare, less than dosen candidates have been found so far if I remember correctly. I would not even be surprised if some or all of them will be found to have errors in density edtimates after futher observations. Maybe in next decade we'll see more clear picture do super-puff planets really exist, and if yes how often they occur and in which conditions. Maybe, maybe )  

---

