Maybe it was because our sun was smaller, but there were hot gases present from the first suprenovae explosion.

We are a by-product of a second generation star after all.

If our sun was smaller in mass as is predicted our heliosphere was small as well and may have been unstable, therefore those rocky and icy things we've come to know as comets may reached our rocky planets when our solar system was young.

Yes, these things may have suffered the consequences when 'things' were settling down and as time passed:

Mercury's volcanism ceased.

Venus started it s greenhouse effect.

The Moon got heavily bombarded.

Life started on earth.

Mars lost its atmosphere

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And here we are.