<p>First, Pluto isn't a planet, and hasn't been for about a decade. It's a <a href="https://en.wikipedia.org/wiki/Dwarf_planet" rel="nofollow noreferrer">dwarf planet</a>, and is better grouped with other small, rocky objects beyond Neptune's orbit, such as <a href="https://en.wikipedia.org/wiki/Haumea" rel="nofollow noreferrer">Haumea</a>. These objects, and other minor planets, have more in common with objects in the Kuiper Belt and Oort Cloud than the eight planets of the Solar System.<sup>1</sup></p>

<p>Therefore, we see a different pattern: Four relatively low-mass planets (Mercury, Venus, Earth and Mars) in the inner Solar System (the four <a href="https://en.wikipedia.org/wiki/Terrestrial_planet" rel="nofollow noreferrer">terrestrial planets</a>) and four relatively high-mass planets (Jupiter, Saturn, Uranus, and Neptune) farther out (the four <a href="https://en.wikipedia.org/wiki/Giant_planet" rel="nofollow noreferrer">giant planets</a>). The question now is why the two groups are arranged as they are, why the terrestrial planets are close to the Sun while the giant planets are further away.</p>

<p>I wrote about this in <a href="https://astronomy.stackexchange.com/a/14608/2153">my answer to Why do the gas giants in the Solar System have comparatively large orbits compared to the inner planets?</a>. It boils down to something called <a href="https://en.wikipedia.org/wiki/Frost_line_(astrophysics)" rel="nofollow noreferrer">the frost line</a>, a rough boundary before which ices of water, ammonia, methane, and other compounds cannot form. Beyond it, they can form ice crystals (not just water ice, of course), which made it easier to accrete large amounts of material from the protoplanetary disk. This leads to the more massive gas giants.</p>

<p>In general, this explains the mass distribution of the planets, although there are exceptions (e.g. <a href="https://en.wikipedia.org/wiki/Hot_Jupiter" rel="nofollow noreferrer">Hot Jupiters</a>), which often result form planet-planet interactions, rather than forming at distances inconsistent with the frost line.</p>

<hr>

<p><sup>1 I exclude <a href="https://en.wikipedia.org/wiki/Planet_Nine" rel="nofollow noreferrer">Planet Nine</a> from this list, as its existence has yet to be verified.</sup></p>

CC BY-SA 3.0
2016-07-17T14:34:18.227Z
2016-07-17T14:34:18.227Z
2017-04-13T12:59:06.910Z
HDE 226868
<p>Is it coïncidence that the mass of a planet has about its maximum in the middle planets (Jupiter), while the mass of the closest and furthest planets has a minimum (Mercury and Pluto)?</p>

<planet><solar-system><mass>
A question about the distribution of mass among the planets in the Solar System
Deschele Schilder
# A question about the distribution of mass among the planets in the Solar System

(Asked by: username_0 on 2016-07-17)

Is it coïncidence that the mass of a planet has about its maximum in the middle planets (Jupiter), while the mass of the closest and furthest planets has a minimum (Mercury and Pluto)?

**Tags:** planet, solar-system, mass

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## 1 answers

### Answer by username_1 on 2016-07-17. Score: 5

First, Pluto isn't a planet, and hasn't been for about a decade. It's a [dwarf planet](https://en.wikipedia.org/wiki/Dwarf_planet), and is better grouped with other small, rocky objects beyond Neptune's orbit, such as [Haumea](https://en.wikipedia.org/wiki/Haumea). These objects, and other minor planets, have more in common with objects in the Kuiper Belt and Oort Cloud than the eight planets of the Solar System.1

Therefore, we see a different pattern: Four relatively low-mass planets (Mercury, Venus, Earth and Mars) in the inner Solar System (the four [terrestrial planets](https://en.wikipedia.org/wiki/Terrestrial_planet)) and four relatively high-mass planets (Jupiter, Saturn, Uranus, and Neptune) farther out (the four [giant planets](https://en.wikipedia.org/wiki/Giant_planet)). The question now is why the two groups are arranged as they are, why the terrestrial planets are close to the Sun while the giant planets are further away.

I wrote about this in [my answer to Why do the gas giants in the Solar System have comparatively large orbits compared to the inner planets?](https://astronomy.stackexchange.com/a/14608/2153). It boils down to something called [the frost line](https://en.wikipedia.org/wiki/Frost_line_(astrophysics)), a rough boundary before which ices of water, ammonia, methane, and other compounds cannot form. Beyond it, they can form ice crystals (not just water ice, of course), which made it easier to accrete large amounts of material from the protoplanetary disk. This leads to the more massive gas giants.

In general, this explains the mass distribution of the planets, although there are exceptions (e.g. [Hot Jupiters](https://en.wikipedia.org/wiki/Hot_Jupiter)), which often result form planet-planet interactions, rather than forming at distances inconsistent with the frost line.

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1 I exclude [Planet Nine](https://en.wikipedia.org/wiki/Planet_Nine) from this list, as its existence has yet to be verified.


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