How typical is the Solar System?

Venus comes as close as it ever does to Earth next weekend, and (from the UK and Ireland at least) has been as bright as ever just above the setting Sun at nightfall over the past few weeks.

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In terms of size, Venus is remarkably similar to Earth, but is of course closer to the Sun (71% to 72% as far on average). It is one of four inner, terrestrial (rocky) planets, the smallest and closest to the Sun being Mercury and the most habitable other than Earth being the outermost Mars (roughly between Mercury’s and Earth’s size).

As a child I was gripped by the Solar System from the moment I observed Jupiter’s Red Spot (a storm then three times the size of Earth on the surface of the planet) through a telescope given to me for my ninth birthday. Jupiter is of course the largest planet (all the others in the Solar System would fit into it), the nearest of the outer planets and a “gas giant” a little over five times the distance of the Earth from the Sun; Saturn, also a “gas giant” and famous for its rings, is next, followed by the smaller “ice giants” Uranus and Neptune (similar in size and mass to each other, neither of which were known to the ancients and the latter of which is not even close to visible with the naked eye even in absolutely ideal conditions).

As well as moons around the planets (except Venus and Mercury) and various other objects with share orbits with the planets, the Solar System also possesses an Asteroid belt (essentially of small rocks) between Mars and Jupiter (much closer, in fact, to the former) and series of outer clouds and belts which are not all well understood. Some of these contain dwarf planets such as Pluto; outer ones have comets with weird elongated orbits. A full light year from the Sun (for scale, Earth is eight light minutes away and Neptune four light hours), the Solar System absolutely ends as the solar wind meets what is essentially outer space (known as the “interstellar medium”).

What has happened in astronomy in the 30 years since I first picked up that telescope is astounding. Then, it was not known whether there even were other planetary systems. Now, not only have 4000 or so “exoplanets” (planets orbiting other stars) been found, but entire systems are beginning to be understood. We are still at the beginning of this voyage of scientific discovery, but there is now some indication of how “normal” we are.

The Sun itself is often deemed a “typical” star but in fact it is abnormal in the sense it is among only 15% or so which are visible from other systems (the Sun would be visible with the naked eye in a night sky equivalent to our own up to about 80 light years away). Also, the Sun is among a minority (albeit a large minority) of stars which are alone – just over half are part of binary or multi-star systems (some are confirmed to have as many as four, and up to six may be possible). Of the lone visible stars like the Sun, that is to say 7-8% of the total star systems (at least in the nearby part of our galaxy), the Sun is fairly average, although some stars are vastly bigger (often thousands of times more massive and luminous) and life ranges can vary hugely (from a few million years to potentially a trillion; the Sun is halfway through its fairly average ten billion year life cycle). Stars are different colours too – the Sun, for the record, is white (it appears yellow-orange to us due to our atmosphere).

So of these single visible star systems, how typical is ours?

Well, the range of planets is fairly typical (although by no means universal). It is quite common to have small terrestrial inner planets and large gaseous outer planets. It is hard to say for certain, because by definition the exoplanets found so far tend to be large and close to their star, but the range of sizes would also appear to be quite normal, although many planets have been found to be between the size of Earth and Neptune, and quite a high number between Neptune and Jupiter. There is no reason to doubt that moons are fairly typical also. It may be that most systems have rather more than eight identifiable planets on average (some are already known to have as many as six closer to their star than Mercury is to the Sun).

Two things do stand out as definite and slightly surprising, however.

Firstly, as expected, planets tend to orbit their stars along the star’s plane in the same direction as the star rotates (that direction, in the case of the Solar System, is anti-clockwise looking down from the north). However, it was also expected that planets would generally rotate the same way as they orbit (six of the eight in the Solar System do, and the remaining two were thought to be peculiar – Uranus effectively spins upright on a horizontal axis, and Venus spins theoretically clockwise but incredibly slowly, with its rotational day in fact lasting longer than its orbital year). Remarkably, it turns out that the rotational direction of stars and planets is random (roughly 50% clockwise and 50% anti-clockwise), and there is no connection between them in any given system – in other words, planets are as likely to rotate against their orbital direction as with it, and indeed seemingly entire planetary systems are as likely to rotate against their direction of travel through the galaxy as with it. (There are even systems where some planets do orbit the opposite way to their star’s rotation, particularly large planets close to their star, but this is atypical.)

Secondly, planets in the Solar System (particularly Earth) have relatively circular orbits, with little eccentricity (i.e. little difference between their closest point to and further point from the Sun). It turns out this is unusual. In most planetary systems, most or indeed all planets have more eccentric orbits than even the most eccentric in our Solar System; indeed, none has yet been discovered where even the average eccentricity is lower than that of Mercury, the Solar System’s most eccentric planet. There are also systems with large planets close to the star which are misaligned – that is to say the plane of their orbit does not match the plane of their star’s rotation. (This is all potentially a problem for the development of complex life forms, which is thought to be easier on planets with less eccentric orbits.)

Another thing which has become evident is that many planetary systems have planets which are locked to their star (i.e. they take precisely as long to rotate as they do to orbit and thus always show the same face to their star, the way the Moon does to Earth), or which are still orbiting in an interlinked manner (known as “orbital resonance”; for example planets which complete precisely three orbits for every two the next planet out completes). None of this occurs in our Solar System (at least as regards planets and the Sun). As noted above there is a bias here, however, towards systems with large inner planets, so we cannot yet determine whether such locking and interlinking is really common.

Thus our own Solar System is in some ways typical but in notable ways abnormal. The search for other planetary systems will no doubt make us able to determine just how typical and abnormal in the coming years.

The result matters profoundly. It could be that the Solar System is particularly or even uniquely capable of supporting complex life.

Or not…

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One thought on “How typical is the Solar System?

  1. interesting ian I remember growing up i was interested in these things Patrick Moore was resident at Armagh Observatory but i remember being teased in school with the old double entree “what about uranus” i replied it was discovered in 1781 and the approximate diameter -this bemused people who said they did not think it was that old and so big!

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