TCD Scientist Re-examines Formation History of the Solar System

Posted on: 04 August 2009

An alternative to the conventional view of events immediately following the birth of the solar system was proposed by TCD geologist, Dr Ian Sanders, of the School of Natural Sciences, at the Meteoritical Society meeting in France last month.  This new theory, the subject of an article in the New Scientist magazine, challenges existing ideas on how the present day planets began to evolve in a rotating disk of gas and dust that surrounded the infant Sun 4567 million years ago.

Meteorites are rocks from space – fragments of small early-formed planets, called planetesimals, a few of which still exist today in the asteroid belt beyond the orbit of Mars. Most meteorites are sedimentary, consisting of myriads of compacted frozen droplets of lava, called chondrules, whose origin is enigmatic.  Others are igneous – solid lumps of basalt or iron that was once totally molten.

Recent dating of meteorites has led to a major paradox. Conventional thinking holds that sedimentary meteorites are older than their igneous cousins, as their overall chemical composition is ‘primitive’, similar to that of the Sun.   Igneous meteorites are seen as having formed later, when sedimentary planetesimals became overheated and melted.  Now igneous meteorites turn out to be the older ones, by about 1.5 million years.

Dr ian sanders with a meteorite in tcd.

Dr Ian Sanders from the School of Natural Sciences displays a meteorite from the Geological Museum in TCD.

This paradox was explained by Dr Sanders who suggested that heating due to a high level of the intensely radioactive isotope, aluminium-26, led to rapid meltdown of the first crop of planetesimals in the disk. Since the sedimentary planetesimals did not melt, they must have formed later, after about 1.5 million years, when the aluminium-26 had declined to a much lower level. Speaking at the meeting in France, Dr Sanders commented: “This hypothesis puts a completely different slant on what happened in the early solar system.”

Dr Sanders’ theory also offers a solution to the chondrule mystery. The molten planetesimals probably collided with each other in space and their liquid contents would have been launched as huge cascades of droplets (i.e. chondrules) that were re-cycled back into the dusty disk. Thus the original primitive chemistry of the disk was reconstituted before sedimentary planetesimals were formed. To view examples of meteorites, the Geological Museum situated in the Museum Building in Trinity College, is open Monday to Friday between 10am and 5pm.