Expert: Courtney Seligman - 1/16/2009

Question
QUESTION: Hi if the universe is approximately 13.73 billion years old, Mars and earth about 4.5 billion years old, would it be fare to say that these two planets were created 9 billion years after the big bang? And which planet was created first Mars or Earth?
Thanks
thanks

ANSWER: As far as the 9 billion years, yes, that would be correct, presuming the 13.73by is correct (I know that's the currently accepted number, but I'm not convinced that it's as accurate as claimed). In other words, about 2/3 of the current age of the Universe had already passed when our Solar System formed.

As far as the timing of the formation of the various planets, the giant planets must have formed in 3 to 6 million years, and the smaller ones in 12 to 25 million years; but I don't think we have any information that would allow us to say exactly when Mars or the Earth was "finished". They should have both grown at similar rates, in comparison to their final size, during the 12 to 25 million years it took them to form, and reached similar "finished" fractions of their final structures at approximately the same time. Certainly, in comparison to the 4475+ million years since then, any difference in their "ages" would be very small.

Courtney Seligman

---------- FOLLOW-UP ----------

QUESTION: Thanks for your previous answer. If earth is protected from asteroids and comets by Jupiter's gravitational pull, how is it that the asteroid or comet that destroyed the dinasours made it to earth?
Thanks

Answer
There are two reasons for that:

(1) Jupiter does not significantly protect the Earth from comets or asteroids. In fact, it is more likely to alter their orbits to cause them to run into us, than the other way around.

However, the chance that Jupiter can do either is very, very small. Think of what it looks like in the night-time sky -- just a dot, too small to see as anything other than a dot, even with binoculars. Now imagine objects heading in our direction, or at least somewhere toward the Sun, from all directions in the sky (which is the way it works, for comets). How many of those are going to happen to pass anywhere near the tiny dot that is Jupiter? Not very many, compared to the ones that don't. Because of that, even if Jupiter were some sort of magical shield, it couldn't possibly prevent more than a billionth of a percent of the collisions that we're likely to have with objects coming from beyond its orbit.

(2) Many of the impacts that we suffer don't have to do with anything far enough out in the solar system to be affected by Jupiter's gravity. In fact, there is substantial circumstantial evidence that in the last 150 to 250 million years, at least half of the collisions suffered by inner-solar-system objects (like the Earth and Moon) were caused by a collision between two asteroids that were located fairly close to the orbit of Mars, and therefore quite far from Jupiter's influence.

So, to summarize, Jupiter doesn't really do much to protect us, if anything at all; and even if it could protect us from some things, there are a heck of a lot of things out there that it can't protect us from, anyway.

Still, the chances of our being hit by anything big enough to be concerned with during a given human lifetime is pretty small, so it isn't something I'd stay up nights, worrying about.

Best wishes to you and to yours,

Courtney Seligman