How vast is our solar system ? We know that the Sun is at the center of the solar system. How far does it extend ? The outer edge of the solar system has been extending since the ancient times to today. There is no unambiguous and precise definition of the outer edge. There are couple of considerations we can look at. Let us start with what the solar system consist of.
Firstly, the solar system has planets orbiting around the Sun. There are eight of them in all. Starting from Mercury to Neptune. Each of the planets have their own moons which circle them. While the smaller ones may have one or two moons, the bigger planets have several of them. The distances in our solar system are not measured in miles or kilometers. Instead we use AU (Astronomical Unit). 1 AU is the distance from the Sun to the Earth (about 93 million miles or 150 million kilometers). Once we go beyond our solar system, we use light years or parsecs to measure distances across other stars and galaxies.
The four inner planets – namely Mercury, Venus, Earth, Mars – are all rocky planets. They are comparatively close to each other and to the Sun as well. They are all solid by and large and have a very thin gaseous atmosphere. Next we have the asteroid belt which contains a large number of small objects, essentially made of minerals and ores. These are perceived to be remnants from the formation of our solar system about five billion years ago. Like the rubble left after constructing a building. The garbage was thrown just a few blocks from the backyard !
Next four planets are the outer gaseous giants – namely Jupiter, Saturn, Uranus, Neptune. They are much larger than the inner rocky planets. And they consist largely of gases with a very small solid core at the center. And the distance increases as we go towards the outer fringes of the solar system. Saturn is at 10 AUs from the Sun. Similarly, Uranus is at 20 and Neptune at 30 AUs. It is like the stations on a railway line coming at increasing distances as we go away from a city.
Planets up to Saturn were known to our ancients as they are visible to the naked eye. Uranus had to wait till the invention of the telescope, to be discovered. Neptune was predicted theoretically based on mathematical calculations of gravitational tug on Uranus and other planets. And later observed through a telescope, thus validating the prediction.
As we find the limit of the solar system is extending, the cold body called Pluto was discovered and accorded the status of a planet. In the last decade, hundreds of such bodies, some of them larger than Pluto were found. Pluto, at a distance of 40 AUs from the Sun, was demoted to be a dwarf planet or a planetesimal. All these cold bodies were found to be part of a region named as the Kuiper belt, named for the astronomer Gerard Kuiper, extending from 40 to 60 AUs. We found a new suburb of our solar system, again presumably the remnants of the formation of our solar system !
The NASA spacecraft Voyager 1, which was launched in 1977 on a mission to visit the outer planets, has gone past the Kuiper belt. Now It is at a distance of 140 AUs from the Sun. This is considered as beyond the edge of the solar system, as per one definition.
The Sun gives out energetic particles called the solar wind. They travel very quickly at the speed of the light and encounter a similar wind which is known as inter-stellar wind, which blows between the stars. This wind is more of a radiation than a wind we normally know on the Earth. With the inter-stellar wind, space is not truly empty. At about 95 AUs, the solar wind slows down and becomes hotter. This region is called the termination shock. At about 125 AUs, the influence of the solar wind wanes and the inter-stellar wind takes over. This is the edge of the solar system, as per the realm of the solar wind.
There is a second consideration of how much gravitational influence the Sun has. There are comets, which are largely icy bodies with tails, which can be observed from time to time with a naked eye. Some of these comets come from the outer parts of the solar system. They are in an elongated orbit and come closer to the Sun once in a hundred years or so. They are predicted to be formed in a region called the Oort cloud, named for a Danish astronomer Jan Oort. This region is a spherical region enveloping our solar system, unlike the planets and the asteroid and the Kuiper belts which are all roughly on a single plane. And the Oort region is at a whopping distance of fifty thousand AUs ! Yet, the Sun has a gravitational influence over the objects in this region. Hence, this can be considered as the outer limit of the solar system.
But the problem is that Oort cloud has not been observed using any telescope. Presumably due to the small size of the objects scattered in a vast region. It is just a prediction shared by the community of astronomers. Someday we might be able to photograph the Oort cloud. As per the distance of the Oort cloud, Voyager at just 140 AUs has long long way to go to be considered as away from home.
Editors’ note: Please check a related article on this topic titled “The Earth as a Peppercorn” by Victor Davis.