## What is the size of the Solar System? (Intermediate)

What is the diameter of our solar system and how many times would our solar system fit between us and the nearest star?

Defining the size of the Solar System is a hard thing to do because it does not have a clear boundary. I will answer the question by calculating it two ways. First, using the orbit of Pluto as the boundary, and second using the orbit of the farthest comets we know.

1 - Orbit of Pluto To calculate this we will use the orbit of Pluto as the limit of the solar system. A problem is that the orbit of Pluto is not circular, it is rather an ellipse. All the planets orbit the Sun on ellipses. For most of the planets the ellipses are almost circles, but not for Pluto. This means that the distance from Pluto to the Sun varies quite a bit. In fact, at some time,s it is closer to the Sun than Neptune! So we will take the average distance between the Sun and Pluto as the radius of the Solar System (which is 5,913,520,000 km, or 39.5 AU, where AU stands for Astronomical Unit)

2 - Orbit of comets Beyond the orbit of Pluto, there are object that orbit the Sun. These are the comets. Two populations of comets have been identified: the Kuiper Belt and the Oort Cloud. The Oort Cloud has a larger radius, estimated at about 50,000 AU (or 7.5x1012 km). As you can see, comets are found much farther from the Sun than any of the planets!

Now the nearest star to the Sun is Proxima Centauri which is located at a distance of 4.3 light years (one light year is the distance traveled by light in one year). Now, 1 light year is 63,270 AU, which means that the distance to the nearest star is 272,061 AU.

We took the radius of the solar system to be 39.5 AU, which means it has a diameter of 79 AU. This means you could put the Solar System about 3440 times between the Sun and the nearest star taking this definition.

If you include all the comets like we did in the second part, then the Solar System has a diameter of about 100,000 AU, which means it would fit 2.7 times between the Sun and the nearest star.

Update: Another way of defining the size of the solar system is through the location of the heliopause (link). This is the layer where the solar wind and the interstellar medium push on each other with equal pressure. Close to the Sun, the solar wind is dense. This enables to exert a large pressure and force out the low density interstellar medium. As we move farther away from the Sun, the solar wind density decreases and consequently, its pressure as well. Ultimately, there will be a location where the pressure exerted by the solar wind becomes small enough to match that exerted by the interstellar medium.

This page was last updated Jan 28, 2019.

### About the Author

#### Amelie Saintonge

Amelie is working on ways to detect the signals of galaxies from radio maps.

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