#### Imparcticle

www.badastronomy.com is the best site for anyone who cares to find the astronomy related errors in movies, news, and other misconceptions.

The site goes over the astronomical discrepencies in Star Wars, Men In Black 1 and 2, The Mummy Returns etc.

ENJOY.

Related Astronomy and Astrophysics News on Phys.org

#### Imparcticle

This excerpt is from badastronomy.com concerning the beginning of Men In Black 2. My question is, do the calculations depend on the planet's size?

In the beginning, we see a montage of aliens blowing up planets. A beam weapon is fired, and the planets explode in giant pieces.

Good:
It's really hard to blow up a planet! It's possible that the alien technology did something like cause a chain reaction which totally disintegrated the atoms, or something like that.

But that's no fun. Well, it might be fun, but really, how much energy does it take to blow up a planet? I did this calculation once before, to show that a comet impact could not destroy a planet. To blow up a planet, you have to overcome the gravity of the planet for each piece. Let's put it another way: Imagine taking a rock and throwing it up so hard it escapes from the planet entirely. For the Earth, that means you have to give it a pretty big velocity, about 11 kilometers per second. It takes a lot of energy to do that, the amount of which depends on the mass of the rock and the amount of gravity you are fighting. Now take another rock, and do it again. And again. And againandagainandagain. Repeat for a gazillion times. If you do it enough times, the planet is gone. You've destroyed it. Congratulations!

It's possible to calculate that total energy you expended. It's called the gravitational binding energy, and is basically the amount of energy locked up in the planet's gravity. I'll spare you the math (but you can find it here if you'd like), but for the Earth that energy is roughly 1039 ergs. An erg is a tiny unit of energy, but 1,000,000,000,000,000,000,000,000,000,000,000,000,000 is a lot of them! For comparison, the Sun puts out about 4 x 1033 ergs every second, so the amount of energy needed to vaporize the Earth is about the total amount of energy the Sun emits in 3 days! That's a whole lot of energy. I doubt a single ship could generate that much. But who knows? They're aliens!

#### Nereid

Staff Emeritus
Gold Member
Originally posted by SquareItSalamander
This excerpt is from badastronomy.com concerning the beginning of Men In Black 2. My question is, do the calculations depend on the planet's size?
Yes, if by 'size' you mean mass. There's also a 'radius' dependency.
"I'll spare you the math (but you can find it here if you'd like) ..."
Did you find?

#### Imparcticle

Originally posted by Nereid
Did you find? [/B]
the calculations? my computer wouldn't load to the site for some reason. thanks though.

#### Phobos

Staff Emeritus
Gold Member
The Bad Astronomer, Phil Plait, is a definite favorite among astronomy forums. He also publishes many articles, a book, and is invited to many radio interviews. Always good fun. If he's not being mentioned here in the PF A&C forum, then he's mentioned in the PF debunking forum. You'll find me & some other PF members in his discussion forums too.

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