This is the outreach projects "Ask an astronomy/particle physics question" page. I'll attempt to answer as many questions as possible, but because of time constraints, I am unfortunately often unable to answer all of them. I do not accept more than one question per email and no more than one question per week from an individual user. I aim to answer a question within two weeks.

If you have a question which is not answered in the list below, please fill the form you will find by following this link: Ask a Scientist.
Answers to popular questions will be added to the "Big Questions!" list.

Here are the answers to some of the most common questions I have been asked.

The Sun formed by the shrinking of a huge cloud of gas as it was pulled together by gravity.
As it shrank it heated up and started to shine but it also spun faster like a skater pulling in their arms.
The spin became so fast it threw the outer layers of the Sun off and they formed a rotating disc of gas and dust, which cooled and formed the planets, comets and meteors.

We don't know!
Until about ten years ago we only knew of our own stellar system 'a system with a star and planets around it' but we thought there would be others.
Recent technology has let us detect the presence of planets around some other stars.
The total detected changes very quickly. In November 2006 we had found about 165 systems, containing at least 195 known planets. Alex Willman's web page gives the most up to date information and some useful links.
The fact that we can only detect big planets of stars close by suggests that there are many of them out there.
Even supposing only one star in every 100 had planets, that would mean about 100 000 000 000 000 000 000 stellar systems in the visible Universe.

There are now more than 100 known systems, beyond our Solar System, where stars are orbited by planets.
Even supposing only one star in every 100 had planets, that would mean about 100 000 000 000 000 000 000 stellar systems in the visible Universe.
There is no reason to think that the Earth is a particularly unusual planet, and life may be common.

Some of the meteorites found on Earth are actually pieces of the planet Mars.
By September 1998, thirteen 'Martian Meteorites' had been found.
Scientists have studied these meteorites for evidence of tiny ancient bacteria to see if life once existed on Mars.
According to some scientists, the tube-like structures shown in the picture in red, found in a meteorite which came from Mars, are fossils of bacteria-like organisms.
Most scientists now think this isn't right!

Yes.
But you would have to squeeze the Earth to the size of a large pea for it to become a black hole!