<p>The situation is more complex than a simple correlation. A better plot would show the full mass distribution of discovered exoplanets versus their semi-major axis, but for that you need to plot the axes logarithmically. Here is one taken from data held at exoplanets.org. Note that the mass axis is actually &quot;<span class="math-container">$M\sin i$</span>&quot; because the mass usually has a <span class="math-container">$\sin i$</span> ambiguity when measured by the radial velocity technique (as most are).</p>
<p><a href="https://i.sstatic.net/OXOzu.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/OXOzu.png" alt="Mass distribution of known exoplanets" /></a></p>
<p>You can see in this plot that the correlation that you saw is actually a gap between the &quot;hot Jupiters&quot; and the &quot;hot super-Earths&quot; - this is known as the <a href="https://en.wikipedia.org/wiki/Neptunian_Desert" rel="nofollow noreferrer">hot Neptune desert</a>. The gap seems to close up at around <span class="math-container">$a=0.06$</span> au, but is wider at smaller separations.</p>
<p>The predominant explanation for this gap (it is by no means a settled issue) is that a low-mass hot-Jupiter will be unable to hang on to its envelope in the face of photoevaporative radiation from the star it is orbiting. They will lose that envelope and become smaller &quot;super-Earths&quot;. The closer to the star the hot-Jupiter is then the more massive it needs to be to hang onto its envelope and this may account for the sloping lower boundary of the hot-Jupiter distribution in this plot.</p>
<p>The reason that the density behaves in a similar way is that, for giant exoplanets at least, there is a strong correlation between mass and density in the sense that more massive giant exoplanets are more dense.</p>

CC BY-SA 4.0
2022-02-03T17:33:56.807Z
Thanks, nice explanation. Never knew about Neptune desert. So it seems a system in that region is unstable,- there's a bifurcation point around $M \sin(i) \approx 0.1$. If cosmic body will reach this mass in Neptune desert,- you'll never know if it's own gravity will win and it will become next hot Jupiter or in reverse - star will blow already accumulated atmosphere away and it will degenerate into hot super-Earth. Concurrent processes and chaotic behavior in planet formation, so to say. Interesting.
Agnius Vasiliauskas
CC BY-SA 4.0
2022-01-18T09:46:42.707Z
2022-01-18T10:18:47.543Z
2022-01-18T10:18:47.543Z
ProfRob
<p>From the NASA <a href="https://exoplanetarchive.ipac.caltech.edu/cgi-bin/TblView/nph-tblView?app=ExoTbls&amp;config=PS&amp;constraint=default_flag=1&amp;constraint=disc_facility+like+%27%25TESS%25%27" rel="nofollow noreferrer">exoplanet archive system</a> it can be seen that in the range of <span class="math-container">$0.02-0.06~\text{AU}$</span> distance an exoplanet is to its star, as distance drops down, the planet density increases linearly:</p>
<p><a href="https://i.sstatic.net/mZyic.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/mZyic.png" alt="enter image description here" /></a></p>
<p>A similar inverse correlation between exoplanet mass VS distance exists too:</p>
<p><a href="https://i.sstatic.net/GIA3T.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/GIA3T.png" alt="enter image description here" /></a></p>
<p>So in the mentioned distance range, as we get closer and closer to the star, exoplanets gets more heavier and more dense. Why it is so ? My intuition says that it should be in reverse. The closer to a star, an planet receives more heat, which should evaporate more mass from the planet, making things in an orbital body more gas/plasma like. But instead of that, planet mass accumulates and density increases too. This was unexpected to me. What are reasons behind that? Maybe closer to the star there are more events or collisions of orbital bodies happening which in the end accumulates mass/density or something completely different?</p>

CC BY-SA 4.0
2022-01-17T16:53:44.627Z
Have you made sure that both the mass and radius of the exoplanets in your list are measured?
ProfRob
CC BY-SA 4.0
2022-01-17T16:57:15.617Z
In this question i'm just interested of reasons behind mass-distance correlation.
Agnius Vasiliauskas
CC BY-SA 4.0
2022-01-17T17:08:27.540Z
Well, more massive giant planets are more dense, so your first graph would seem to follow from the second (modulo, some close-in giant planets may be inflated by insolation).
ProfRob
CC BY-SA 4.0
2022-01-17T18:04:24.200Z
_more massive giant planets are more dense_ In general it does not hold, Jupiter is a lot more heavier that Earth, but Jupiter is $4.2\times$ less dense than Earth. So it may depend what types of planets are compared (giants maybe?) and still I'm not sure if that also does not depends on exact distance range to star also.
Agnius Vasiliauskas
CC BY-SA 4.0
2022-01-17T18:08:17.727Z
But why closer to the star more massive giants should exist ? It's because of accretion disk formation rules or so ?
Agnius Vasiliauskas
CC BY-SA 4.0
2022-01-17T19:30:31.117Z
The Earth is not a giant planet. The smallest objects on your plot are more massive than Saturn.
ProfRob
CC BY-SA 4.0
2022-01-18T07:47:34.897Z
@ProfRob Yes, you are right. Still, I need explanation of these both charts roots, i.e. why proximity to a star gives birth to more massive giant planets.
Agnius Vasiliauskas
CC BY-SA 4.0
2022-01-18T10:44:59.760Z
Remember that there are also observational biases, e.g. more massive planets closer to their star are easier detected than far-out low-mass planets.
AtmosphericPrisonEscape
<exoplanet><star-formation><planetary-formation>
The reason for an exoplanet density increase of planets closer to a star
Agnius Vasiliauskas
# The reason for an exoplanet density increase of planets closer to a star

(Asked by: username_0 on 2022-01-17)

From the NASA [exoplanet archive system](https://exoplanetarchive.ipac.caltech.edu/cgi-bin/TblView/nph-tblView?app=ExoTbls&config=PS&constraint=default_flag=1&constraint=disc_facility+like+%27%25TESS%25%27) it can be seen that in the range of $0.02-0.06~\text{AU}$ distance an exoplanet is to its star, as distance drops down, the planet density increases linearly:

[![enter image description here](https://i.sstatic.net/mZyic.png)](https://i.sstatic.net/mZyic.png)

A similar inverse correlation between exoplanet mass VS distance exists too:

[![enter image description here](https://i.sstatic.net/GIA3T.png)](https://i.sstatic.net/GIA3T.png)

So in the mentioned distance range, as we get closer and closer to the star, exoplanets gets more heavier and more dense. Why it is so ? My intuition says that it should be in reverse. The closer to a star, an planet receives more heat, which should evaporate more mass from the planet, making things in an orbital body more gas/plasma like. But instead of that, planet mass accumulates and density increases too. This was unexpected to me. What are reasons behind that? Maybe closer to the star there are more events or collisions of orbital bodies happening which in the end accumulates mass/density or something completely different?

**Tags:** exoplanet, star-formation, planetary-formation
> **Comments:**
> **username_1**: Have you made sure that both the mass and radius of the exoplanets in your list are measured?  
> **username_0**: In this question i'm just interested of reasons behind mass-distance correlation.  
> **username_1**: Well, more massive giant planets are more dense, so your first graph would seem to follow from the second (modulo, some close-in giant planets may be inflated by insolation).  
> **username_0**: _more massive giant planets are more dense_ In general it does not hold, Jupiter is a lot more heavier that Earth, but Jupiter is $4.2\times$ less dense than Earth. So it may depend what types of planets are compared (giants maybe?) and still I'm not sure if that also does not depends on exact distance range to star also.  
> **username_0**: But why closer to the star more massive giants should exist ? It's because of accretion disk formation rules or so ?  
> **username_1**: The Earth is not a giant planet. The smallest objects on your plot are more massive than Saturn.  
> **username_0**: @ProfRob Yes, you are right. Still, I need explanation of these both charts roots, i.e. why proximity to a star gives birth to more massive giant planets.  
> **username_2**: Remember that there are also observational biases, e.g. more massive planets closer to their star are easier detected than far-out low-mass planets.  

---

## 1 answers

### Answer by username_1 on 2022-01-18. Score: 4 | Accepted answer

The situation is more complex than a simple correlation. A better plot would show the full mass distribution of discovered exoplanets versus their semi-major axis, but for that you need to plot the axes logarithmically. Here is one taken from data held at exoplanets.org. Note that the mass axis is actually "$M\sin i$" because the mass usually has a $\sin i$ ambiguity when measured by the radial velocity technique (as most are).

[![Mass distribution of known exoplanets](https://i.sstatic.net/OXOzu.png)](https://i.sstatic.net/OXOzu.png)

You can see in this plot that the correlation that you saw is actually a gap between the "hot Jupiters" and the "hot super-Earths" - this is known as the [hot Neptune desert](https://en.wikipedia.org/wiki/Neptunian_Desert). The gap seems to close up at around $a=0.06$ au, but is wider at smaller separations.

The predominant explanation for this gap (it is by no means a settled issue) is that a low-mass hot-Jupiter will be unable to hang on to its envelope in the face of photoevaporative radiation from the star it is orbiting. They will lose that envelope and become smaller "super-Earths". The closer to the star the hot-Jupiter is then the more massive it needs to be to hang onto its envelope and this may account for the sloping lower boundary of the hot-Jupiter distribution in this plot.

The reason that the density behaves in a similar way is that, for giant exoplanets at least, there is a strong correlation between mass and density in the sense that more massive giant exoplanets are more dense.

> **Comments:**
> **username_0**: Thanks, nice explanation. Never knew about Neptune desert. So it seems a system in that region is unstable,- there's a bifurcation point around $M \sin(i) \approx 0.1$. If cosmic body will reach this mass in Neptune desert,- you'll never know if it's own gravity will win and it will become next hot Jupiter or in reverse - star will blow already accumulated atmosphere away and it will degenerate into hot super-Earth. Concurrent processes and chaotic behavior in planet formation, so to say. Interesting.  

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