Feb 12 2021

How Big is the Solar System?

Published by under Astronomy
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Like many things in the universe, the complexity of reality defies our attempts at simple categorization or clean demarcation lines. One humorous example sometimes offered – is a taco a sandwich? But there are many serious challenges in categorization: What is a planet? There are reptiles that give birth to live young and two mammals that lay eggs. Disease classification in medicine is often a mess of blurry lines and statistical probabilities.

Along these lines – where is the edge of our solar system? I do think there is a reasonable answer here, but the solar system actually has several meaningful boundaries. The main part of the solar system, the part we all think of and which is represented in most models, contains the eight planets and everything within their orbits, including asteroids, comets, and dwarf planets. Let’s talk distance – Neptune is 4,609,592,833 km from the sun, or 30.8 au (astronomical units). An au is the distance from the sun to the Earth, which is about 150 million km. These are average distances. Neptune’s orbit is somewhat elliptical, 30.8 is its current and near maximal distance, but it gets as close as 28.8 au.

But clearly we would not end the solar system at Neptune. We have to include Pluto, which is the transition to the Kuiper belt and all the Kuiper belt objects (KBOs). The Kuiper belt is a ring in the plane of the solar system beginning at the orbit of Neptune, around 30 au, and extending out to about 1,000 au. So right there we have extended the size of the solar system in terms of distance from the sun by a factor of 33 times. The Kuiper belt is comprised of dwarf planets like Pluto, but also including Haumea, Makemake, and Eris. There are lots of bits of rick and ice, and likely many more dwarf planets and smaller planetoids to be discovered. Astronomers recently confirmed the orbit of the farthest confirmed individual object in the Kuiper belt, called Farfarout, with a highly elliptical orbit that takes it out to 135 au.

We can also consider the sun’s influence on surrounding space. The sun has a powerful magnetic field, which is called the heliosphere. The edge of its influence is called the heliopause, beyond this point interstellar conditions prevail and there is no longer any influence from the heliosphere. The heliopause is about 123 au from the sun.

Gravity, however, is a much longer distance force than electromagnetism. How far does the influence of the sun’s gravity reach into space? There is no limit or demarcation line where the sun’s gravity stops, it just fades away with distance according to the inverse square law. So how can we use gravity to determine the limits of our solar system? We can determine the outer edge of where our sun’s gravity is strong enough to keep objects in its orbit, and to keep them from drifting away or from the gravitational influence of other suns. This depends, then, on the distance to the nearest other stars. Here we are talking about the Oort cloud – a sphere (not a ring) of icy objects like a halo around our sun.

We cannot see individual objects in the Oort cloud. We can only infer their existence by extrapolating from observed long period comets. When their distant orbit is perturbed by passing stars they might rain into the inner solar system as long period comets, and may be bumped again into closer orbits to “briefly” exist as short period comets until they burn up in the sun’s glare. We cannot be sure exactly how far away the Oort cloud is, but the estimate are that its inner edge begins 1000-5000 au from the sun, and its outer edge ends 10,000 to 100,000 au from the sun. That is a pretty big range, an order of magnitude, but even at the small end of the range this is not a thin shell but a massive region of space. Most of the solar system, therefore, is the Oort cloud, with a tiny seed in the middle comprising everything else through the Kuiper belt.

We can also think of the size of the solar system in terms of the speed of light. It takes light 8 minutes to reach the Earth, 4.5 hours to reach Neptune, 17 hours to reach the heliopause, and 10-28 days to reach the inner edge of the Oort cloud. It will then take light 1 to 1.5 years to reach the outer edge of the Oort cloud. That may help put these distances into a little bit of mental focus.

The closest star to our sun is Proxima Centauri, which is 4.23 light years away. This means that the Oort cloud of the Centauri system is not far from our own. In parts of the galaxy where stars are a bit closer together, less than 3 light years, what are their Oort clouds like? Do they mesh together, or do they mess them up? Do they constantly exchange objects at the edges as they dip over from from star’s gravitational influence to the other?¬†We cannot see extrasolar Oort clouds or even infer their existence. We can only assume they likely exist because there is no reason for our solar system to be unique. But we have no direct data about them. So these are questions that will likely go unanswered for a long time.

To answer my initial question – the solar system is big. I tend to think of the solar system as being everything out to the edge of the Kuiper belt, which includes the heliosphere. Beyond that is interstellar space. The Oort cloud is mostly empty space, but there are ice chunks there drifting in interstellar space that are trapped temporarily in the gravity of our sun. It is a diffuse halo around our solar system. But it is also perfectly reasonable to consider the Oort cloud part of our solar system. There is no objective answer.

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