A really good theory but...

How planetary systems form

Until a few years ago we were pretty confident we knew how planetary systems form around stars.

A cloud collapses into a disc. The centre of the disc forms a star, and the rest of the disc forms the planets. What's left over forms a cloud way out in the outer reaches of the system. The belts in the disc, where the would-be planets form, get wider as they get further from the star, so the outer planets tend to be bigger than the inner ones.

All the planets collect the same recipe of rocky and icy stuff, and gas, but the inner planets get a lot of that gas burned away by the young star's heat, so they tend to be rocky, while the outer planets retain their gas, forming gas giants. One planet would do well at the expense of its neighbours.

Jupiter fits the bill in our Solar System. In fact, it became massive enough for its gravity to perturb the material in the next belt inward, between Mars and Jupiter, so that a planet could not form. This left the missing planet's orbit being filled with thousands of asteroids and other debris. It all sounds fairly obvious and logical.

An early hint of problems came from studies of the moon. It came from counting and measuring craters. The moon has no atmosphere, no erosion by water, and no plate tectonics. Its surface is a history book.

When a lump of rock hits the moon, it makes a crater. The size of the crater is an indicator of the size of the impacting body. Over time, the streaks of material ejected by an impact tend to fade as they get covered by dust ejected by later impacts. In addition, when one crater is laid down overlapping another, it is easy to see which crater formed first. By measuring crater sizes and estimating ages, or at least the order of the impacts, we get a record of the early history and formation of the moon.

Our nice tidy theory for the formation of the Solar System suggests there was a lot of impacting at the beginning, with some of the impacting objects being quite big. Then, as the construction of the Solar System continued, the building material was swept up and the size and frequency of the impacts tailed off.

However, that's when scientists found something odd. There was the expected period of impacting on the moon, which gradually tailed off, and then, unexpectedly, there was another period of intense bombardment. This event is referred to as the "Late, Heavy Bombardment".

What caused that? A possible explanation appeared when we became able to detect planets orbiting other stars. Our commonsensical model for the formation of the Solar System suggested that most other stars should have planetary systems like ours. That was not the case. We have now found thousands of planets orbiting other stars, and almost all them live in systems unlike the Solar System. It seems the formation of a planetary system is more like a game of pool than a tidy process. Now we have a new idea as to how the Solar System formed.

Jupiter and Saturn formed first. They gobbled up most of the gas and dust, so later comers, such as Uranus and Neptune got less and wound up smaller.

The interplay of the gravity pulls of the two giants then catapulted Neptune and Uranus outwards, which made Jupiter move inwards towards the sun. Jupiter then swept out the belt forming a planet next out from Mars, leaving a belt of asteroids. Lots of planet-forming lumps of material were thrown out of the Solar System. Other lumps moved inwards, hitting the moon and inner planets, causing a second bombardment, and a period of additional planet building.

This sort of cosmic pool game process turns each planetary system into a unique construction project.


• Venus shines very brightly in the west after sunset.

• Mars, much less bright, and reddish, lies higher in the southwest.• Saturn, golden-coloured and moderately bright, lies very low in the dawn glow.

• The Moon will reach Last Quarter on May 12.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton, B.C

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

More Skywatching articles

About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

Previous Stories