Why Are Planets So Different?

by Carson
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Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. While they form from the same materials and orbit the same star, their properties are diverse. They are made of different materials and come in different sizes, shapes, and colors. Why is this the case? Let’s explore in this article.

The Process of Planet Formation

Before we start exploring the differences in the planet’s compositions, one thing that we should know is that this indicates that they formed differently. Specifically, this indicates that the objects took shape with different materials or processes. Let’s briefly examine the planet formation process in this section.

With an external trigger such as a supernova, an instability begins, in which material unevenly distributes. The regions with slightly higher density attract nearby material. Thus, the matter accumulates and concentrates around a specific point due to the gravitational forces of the materials’ clump. Eventually, that object becomes so massive that nuclear fusion starts, making a star.

When a star forms, there are also the leftovers, which is the protoplanetary disk. Various events in the disk can also trigger instabilities and form clumps, which become the planets we know. If you want to learn more about this process, we have a dedicated article about planet formation. But if you understand the process, we’ll introduce some of the more advanced concepts to explain why planets are so different.

The Frost Line

There are a few theories on why planets have different chemical compositions, but one of the most convincing explanations involves the frost line. It is the distance to a star in which certain types of material can condense if they’re farther away. For example, the water snow line of the Solar System is estimated to be about 2.7 to 3 AU from the Sun. Beyond that distance, the temperature is low enough for water particles to condense there, even in the vacuum of space and unfiltered sunlight.

An illustration explaining the frost line
An illustration on how frost lines work

The frost line of different chemicals meant that material availability varies with different distances from a star. Generally, the farther it is from the star, the more matter is available for accretion. Gas giants form outside the frost lines due to this reason. At these distances, the abundant ices accumulate together to form larger planets. These large planetesimals attract lots of hydrogen and helium, thus creating a massive envelope of gases enshrouding a small rocky core.

On the other hand, the region of terrestrial planets contains less ready-to-accrete material as it’s within the frost line of most materials. This means that planets are smaller and are unable to capture as many gases as gas giants do. They thus end up becoming small solid bodies with thin atmospheres.

The Individual Differences

The frost lines might explain some of the diversity of planets, but it’s far from a complete picture. What is causing the individual differences between the planets? Although they form from the same system, they might not look like they are made of the same materials. For instance, why does Earth have large oceans of water while the other planets don’t? And why does Venus have thick layers of carbon dioxide in its atmosphere, making it hotter than Mercury?

Well, it comes down to the individual evolutions within the planetary system. In fact, the relationships are so complex that researchers are still trying to understand the exact processes that shaped our Solar System. However, it mostly comes down to the factors that occur after the planets start to form.

For example, instability events can occur in a planetary system during or after its formation, causing the material distribution to change. For example, the Grand Tack scenario has been famously proposed, in which Jupiter migrated inward through the asteroid belt and then out beyond the frost line again. This truncates the inner protoplanetary disk to around 1 AU, limiting the amount of material Mars can grow from. Thus, it explained why its actual mass is so tiny (~0.1 Earth masses), much smaller than the estimated masses from simulations.

An illustration explaining how the Grand Tack scenario might have occurred.
Note that sizes are not to scale, and the arrows indicate long-term change in orbital distance

In addition, processes that take place after a planet forms can also influence its appearance. For example, it is well-known that Mars might have once had water on its surface, just like Earth. However, through some mechanism, it was lost from our sight. There are still a few theories as to why this is the case, ranging from the loss of a protective magnetic field to simply water going underground. However, regardless of what this process was, this probably made Mars look different from Earth.

Conclusion

In this article, we briefly explained why planets are different from each other. This could be due to frost lines influencing material availability across various distances, or it could be some other processes that take place after the planets start grabbing material. If you would like us to include more details in this article, please comment below to give your suggestions.

References

  1. Lecar et al. (2006, February 9). “On the Location of the Snow Line in a Protoplanetary Disk”. Retrieved April 8, 2023, from https://arxiv.org/abs/astro-ph/0602217
  2. Mulders et al. (2015, May 13). “The snow line in viscous disks around low-mass stars”. Retrieved April 8, 2023, from https://arxiv.org/abs/1505.03516
  3. (n.d.). “Overview of the Solar System”. Retrieved April 8, 2023, from https://ircamera.as.arizona.edu/Astr2016/lectures/solarsysovervw.htm
  4. Wall, M. (2014, May 1). “Why Is Mars So Much Smaller Than Earth?” Retrieved April 8, 2023, from https://www.space.com/25710-mars-size-planet-formation-theories.html
  5. Beall, A. (2017, April 7). “Mars is so small because Jupiter shook up its formation”. Retrieved April 8, 2023, from https://www.newscientist.com/article/2127273-mars-is-so-small-because-jupiter-shook-up-its-formation/
  6. (2011, August 19). “Jupiter’s “Grand Tack” Reshaped the Solar System”. Retrieved April 8, 2023, from https://astrobiology.nasa.gov/news/jupiters-grand-tack-reshaped-the-solar-system/

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