"While the sun was still forming, the building material was formed into the innermost planets," says Tim Lichtenberg, a planetary physicist at the University of Oxford and lead author of a study published in this week's Science.

Meteorites are time capsules

He and his research group have analyzed meteorites, which are a kind of time capsule from that time.

They reveal how the solar system was built then.

- They have looked at the chemical composition of meteorites and discovered that those that come from the innermost parts of the solar system contain a larger proportion of residues from radioactive substances, says Alexis Brandeker, astrophysicist at Stockholm University.

Made the planets dry

Radioactive substances decompose and then heat is developed.

That heat caused the planetary system's first boulders to melt.

Heavier substances sank and water came to the surface and evaporated.

- It may have contributed to the innermost planets being much drier than those at the far end of the solar system, says Alexis Brandeker.

The earth is certainly a wet planet on the surface.

But in terms of the earth's mass, the proportion of water is less than one per mille.

The celestial bodies at the far end of the solar system can consist of up to 50 percent water.

Is fresh produce

But how can there be radioactive substances in the cloud from which the sun and the planets were formed?

Radioactive substances are fresh produce.

They decompose into lighter elements and cease to be radioactive.

One of the substances that has traces of it is aluminum-26 and it has a half-life of 700,000 years.

It is a relatively short time in cosmic terms and shows that the radioactive substance may not have existed for very long.

So where did the aluminum-26 come from that managed to heat the inner parts of the solar system?

Supernova

A probable theory is that a supernova explosion took place near the sun's birth clouds and that explosion contributed, among other things, newly manufactured radioactive substances.

Thanks to the supernova, the earth was able to develop into the reasonably dry planet it is today.

Play the video above to see how a supernova helped create our solar system.