Is our solar system unique?
 
History and astronomy are interconnected. After all, as we look into the night sky, we are looking back in time at light that travelled for thousands of years from its star. As I started to think about the structure of our solar system I was curious about some of the historical perspectives of our solar system and came across a short paper by Robert Aiktken, director emeritus of the Lick Observatory, published in 1938. He asked a question I’ve been wondering about too, is the solar system unique? His perspective was that other solar systems probably existed, but it was unlikely that scientists would never be able to observe them, or even gather data that infers the presence of planets around other stars. This statement needs a little historical context about the astronomy of the era; in 1923 Edwin Hubble provided evidence that Andromeda was in fact another galaxy; in 1929 Hubble published his observations that the universe was expanding; in 1930 Pluto was discovered by Clyde Tombaugh.
 
Leap forward over 80 years and astronomers research into solar systems is beyond what Aitken could have imagined. We know that our solar system isn’t the only one in the galaxy; there are currently (as of January 3, 2021) around 3200 planetary systems identified beyond our own. To try and answer Aitken’s question of whether it is unique, we need to look at its structure.
 
Our solar system consists of one G-type main sequence star (Sun) and everything gravitationally bound to it (8 planets, dwarf planets, 200+ moons and millions of asteroids, comets and meteoroids). The four inner planets are smaller and rocky and the four outer planets gas and ice giants. Six of the eight planets have moons orbiting them, even dwarf planet Pluto and some of the asteroids have moons.
 
Thanks to open data available through the Open Exoplanet Catalogue it is relatively easy to start comparing our solar system to some of the other planetary systems discovered. To start, I focused on some of the basic characteristics of our solar system and selected the relevant criteria on the Open Exoplanet Catalogue.

Number of stars in the system: Our system has only one star. Some of the systems discovered so far have up to three stars. I filtered the results to exclude any systems with multiple stars.

Number of planets in the system: The Exoplanet Catalogue includes Pluto as a planet in its database so it lists nine planets in our solar system. It is easy to sort the results based on the number of planets. There is one other exosystem discovered so far that also has nine planets. This number will change as more planetary systems are discovered and more planets withing existing systems are discovered.

Spectral type: This refers to the type of star and is a system used to classify stars based on the light coming from the star. This helps astronomers to determine important information such as the star’s age and surface temperature.

Radius and mass: These measurements can be used to calculate a planet’s density, which will help astronomers determine if the planet is rocky, gas or ice. Mass and density are also important to determine whether the planet can support an atmosphere.

Age of solar system: The age of the solar system will help astronomers hypothesize what might be happening within the solar system.
 
This screen shot of the Exoplanet Catalogue results reveals the eight planets in our solar system followed by the planetary systems with multiple planets. The entire list is very long and would certainly suggest that our solar system is not that unique.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

There is, however, one more criterion we can use—the habitable zone. The habitable zone is the area within a solar system where water could exist in a liquid state. Liquid water is necessary for life as we know it to survive. Adding this to our search in the Open Exoplanet Catalogue results in a much shorter list, starting with the three planets in our solar system’s habitable zone (Venus, Earth, Mars) and a shorter list of solar systems that may be more like ours.

 

 

 

 

 

 

 

 

 

 

 

 

The next step would be to examine the details of each solar system more closely. The Open Exoplanet catalogue makes this easy for you – each system has its own page. Have a look at the information for a very interesting planetary system around the star TRAPPIST-1.
 
There is still one criterion we can add to help address the question whether our solar system is unique,  and that is the presence of life. So far, we have only identified one solar system, ours, that supports life as we know it. So, for now the answer is yes – it is unique because of us but some day that answer might change.

There is so much more to discover about the solar system. Below are a few resources to get you started.