<p>Of the gas giants, the number of "hot Jupiters" is about the same as the number of "hot Neptunes." A little under 9% of known gas giants are in the habitable zones.</p>

<p>An <a href="https://www.popularmechanics.com/space/deep-space/a13733860/all-the-exoplanets-weve-discovered-in-one-small-chart/" rel="nofollow noreferrer">article</a> by John Wenz has the discoveries thus far collated into a chart:</p>

<p><a href="https://i.sstatic.net/AMFTl.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/AMFTl.jpg" alt="All the Exoplanets We’ve Discovered in One Small Chart"></a></p>

<p>The claim, "most of them are "Hot Jupiters", orbiting so close to their stars that the heat causes them to swell up like a hot air balloon," is at best a bit of a stretch - the only reference that I found for this was a <em>Gizmodo</em> article about Kelt-9b. The <a href="https://arxiv.org/abs/1706.06723" rel="nofollow noreferrer"><em>Nature</em> paper</a>, <em>A giant planet undergoing extreme ultraviolet irradiation by its hot massive-star host</em>, that discusses Kelt-9b says, </p>

<blockquote>
  <p>The planet is also extremely inflated relative to theoretical models, with a radius of $\sim 1.9\ R_J$. Poor redistribution of
  heat and radius inflation (both of which are also observed in WASP-33b), have been linked to high stellar insolation, although the exact physical mechanisms remain uncertain.</p>
</blockquote>

<p>So they did observe that Kelt-9b had a larger radius than is normally expected from a planet of that mass, and it receives massive amounts of radiation from its star, but it cannot be concluded from that that it is the same as putting a balloon filled with air near a heat source.</p>

CC BY-SA 4.0
2018-07-27T10:42:43.807Z
2018-07-27T10:42:43.807Z
Mick
<p>I was curious about the figure given in <a href="https://astronomy.stackexchange.com/a/27114/2153">Mick's answer</a> (~9%), so I did some data-crunching of my own. I looked at the <a href="http://exoplanetarchive.ipac.caltech.edu" rel="nofollow noreferrer">NASA Exoplanet Archive</a>, in particular the table of confirmed planets. I was interested in four parameters:</p>
<ul>
<li><span class="math-container">$a$</span>, the planet's semi-major axis</li>
<li><span class="math-container">$M\sin i$</span>, the planet's minimum mass</li>
<li><span class="math-container">$T$</span>, the temperature of the star</li>
<li><span class="math-container">$R$</span>, the radius of the star</li>
</ul>
<p>From the final two, I could approximate the luminosity of the star by the <a href="https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law" rel="nofollow noreferrer">Stefan-Boltzmann law</a>:
<span class="math-container">$$L=4\pi\sigma R^2T^4$$</span>
I downloaded the dataset (3774 exoplanets) including only those parameters, then filtered out all the planets that lacked all four (leaving 932). I then created lists of all four parameters. Going back through this list, I removed planets of less than <span class="math-container">$\sim0.05$</span> Jupiter masses (i.e. Neptune-mass or less) and objects of more than <span class="math-container">$\sim20$</span> Jupiter masses (which are really essentially brown dwarfs - the cutoff's fuzzy). This left me with 751 probable giant planets, including ice giants.</p>
<p>I estimated the habitable zone by <a href="https://en.wikipedia.org/wiki/Effective_temperature#Blackbody_Temperature" rel="nofollow noreferrer">using a black body approximation</a> for the surface temperature of a planet:
<span class="math-container">$$T_p=\left(\frac{L(1-a)}{16\pi\sigma d^2}\right)^{1/4}$$</span>
where <span class="math-container">$a$</span> is albedo and <span class="math-container">$d$</span> is the mean distance to the star. I set <span class="math-container">$a\approx0$</span>, for simplicity. Then, for each planet, I calculated the orbital radii at which water would freeze and boil, and finally compared the measured semi-major axis to see if it fit inside the two. I ended up with 72 giant planets in the habitable zone - a total of 9.6% of the giant planets I considered, similar to the result given in Mick's answer.</p>
<p>In summary, according to the NASA data I looked at, around 10% of giant planets lie in the habitable zone, although uncertainties in planetary and stellar parameters could change this.</p>
<h3>Notes:</h3>
<ul>
<li>Increasing the minimum mass even up to <span class="math-container">$1M_J$</span> changed my result by no more than 1% of the population; lowering it down to <span class="math-container">$0.01M_J$</span> also changed it by only about 1% of the population.</li>
<li>Varying the albedo from <span class="math-container">$0.1$</span> to <span class="math-container">$0.9$</span> changed the results by about 1.5% of the population, going in both directions.</li>
<li>I assumed no greenhouse effect when calculating the effective surface temperature.</li>
<li>Removing the lower mass restriction entirely (i.e. including terrestrial planets) brought the result down by about 1% of the population.</li>
<li>I used Python after downloading the <code>.csv</code> file from the Archive (<a href="https://github.com/HDE226868/Gas-giant-distribution" rel="nofollow noreferrer">code available on GitHub</a>).</li>
</ul>

CC BY-SA 4.0
2018-07-27T18:15:10.820Z
2018-07-27T19:59:01.097Z
2020-06-17T09:47:40.653Z
HDE 226868
<p>Approximately what percent of exoplanet gas giants discovered so far are in the habitable zones of their stars? I hear that most of them are "Hot Jupiters", orbiting so close to their stars that the heat causes them to swell up like a hot air balloon. </p>

<exoplanet><gas-giants><habitable-zone>
What percentage of gas giant exoplanets are in the habitable zone?
The Literary Lord
# What percentage of gas giant exoplanets are in the habitable zone?

(Asked by: username_0 on 2018-07-27)

Approximately what percent of exoplanet gas giants discovered so far are in the habitable zones of their stars? I hear that most of them are "Hot Jupiters", orbiting so close to their stars that the heat causes them to swell up like a hot air balloon.

**Tags:** exoplanet, gas-giants, habitable-zone


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## 2 answers

### Answer by username_1 on 2018-07-27. Score: 3

Of the gas giants, the number of "hot Jupiters" is about the same as the number of "hot Neptunes." A little under 9% of known gas giants are in the habitable zones.

An [article](https://www.popularmechanics.com/space/deep-space/a13733860/all-the-exoplanets-weve-discovered-in-one-small-chart/) by John Wenz has the discoveries thus far collated into a chart:

[![All the Exoplanets We’ve Discovered in One Small Chart](https://i.sstatic.net/AMFTl.jpg)](https://i.sstatic.net/AMFTl.jpg)

The claim, "most of them are "Hot Jupiters", orbiting so close to their stars that the heat causes them to swell up like a hot air balloon," is at best a bit of a stretch - the only reference that I found for this was a *Gizmodo* article about Kelt-9b. The [*Nature* paper](https://arxiv.org/abs/1706.06723), *A giant planet undergoing extreme ultraviolet irradiation by its hot massive-star host*, that discusses Kelt-9b says,

> The planet is also extremely inflated relative to theoretical models, with a radius of $\sim 1.9\ R\_J$. Poor redistribution of
> heat and radius inflation (both of which are also observed in WASP-33b), have been linked to high stellar insolation, although the exact physical mechanisms remain uncertain.

So they did observe that Kelt-9b had a larger radius than is normally expected from a planet of that mass, and it receives massive amounts of radiation from its star, but it cannot be concluded from that that it is the same as putting a balloon filled with air near a heat source.


---

### Answer by username_2 on 2018-07-27. Score: 3

I was curious about the figure given in [username_1's answer](https://astronomy.stackexchange.com/a/27114/2153) (~9%), so I did some data-crunching of my own. I looked at the [NASA Exoplanet Archive](http://exoplanetarchive.ipac.caltech.edu), in particular the table of confirmed planets. I was interested in four parameters:

* $a$, the planet's semi-major axis
* $M\sin i$, the planet's minimum mass
* $T$, the temperature of the star
* $R$, the radius of the star

From the final two, I could approximate the luminosity of the star by the [Stefan-Boltzmann law](https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law):
$$L=4\pi\sigma R^2T^4$$
I downloaded the dataset (3774 exoplanets) including only those parameters, then filtered out all the planets that lacked all four (leaving 932). I then created lists of all four parameters. Going back through this list, I removed planets of less than $\sim0.05$ Jupiter masses (i.e. Neptune-mass or less) and objects of more than $\sim20$ Jupiter masses (which are really essentially brown dwarfs - the cutoff's fuzzy). This left me with 751 probable giant planets, including ice giants.

I estimated the habitable zone by [using a black body approximation](https://en.wikipedia.org/wiki/Effective_temperature#Blackbody_Temperature) for the surface temperature of a planet:
$$T\_p=\left(\frac{L(1-a)}{16\pi\sigma d^2}\right)^{1/4}$$
where $a$ is albedo and $d$ is the mean distance to the star. I set $a\approx0$, for simplicity. Then, for each planet, I calculated the orbital radii at which water would freeze and boil, and finally compared the measured semi-major axis to see if it fit inside the two. I ended up with 72 giant planets in the habitable zone - a total of 9.6% of the giant planets I considered, similar to the result given in username_1's answer.

In summary, according to the NASA data I looked at, around 10% of giant planets lie in the habitable zone, although uncertainties in planetary and stellar parameters could change this.

### Notes:

* Increasing the minimum mass even up to $1M\_J$ changed my result by no more than 1% of the population; lowering it down to $0.01M\_J$ also changed it by only about 1% of the population.
* Varying the albedo from $0.1$ to $0.9$ changed the results by about 1.5% of the population, going in both directions.
* I assumed no greenhouse effect when calculating the effective surface temperature.
* Removing the lower mass restriction entirely (i.e. including terrestrial planets) brought the result down by about 1% of the population.
* I used Python after downloading the `.csv` file from the Archive ([code available on GitHub](https://github.com/username_2/Gas-giant-distribution)).


---

