Page 94 - How It Works - Book Of Amazing Answers To Curious Questions, Volume 05-15

What if

two

planets

collided?

The Solar System may seem
calm now, but long ago it was
a chaotic and violent place…

he planets in our Solar System currently
orbit the Sun in stable orbits, always far
Tenough away from the other planets to
avoid a collision. This isn’t always the case,
though. Planets can and do collide, usually
either when they are very young or very old.
Planets are made through collisions: young
stars are surrounded by discs of gas and dust
particles that collide and stick together, going
on to form progressively larger chunks. A young
planetary system can have dozens of

‘protoplanets’ flying around on unstable orbits.
These crash and smash into each other, the
debris from the collisions coalescing into larger
and larger bodies.
Earth is probably the result of many violent
collisions, the last of which formed the Moon.
Scientists using NASA’s Spitzer Space Telescope
have witnessed the dusty debris clouds that are
the aftermath of such a collision around the star

HD 172555, where two planets crashed at 36,000 flung in all directions: some will collide with collide. We see evidence for this in the form of
kilometres (22,400 miles) per hour. each other, or with their star, or be thrown out the debris from these collisions contaminating
Some of the planets grow so large that they of their planetary system altogether. the surface of the white dwarf.
begin to siphon hydrogen away from the Now, fast-forward billions of years to the Beyond the scale of solar systems, some truly
gaseous dust disc around their young parent death of these stars. Most will end their lives by cosmic collisions take place between entire
star. Their accelerated growth soon sees them becoming red giants, before casting off their galaxies. The Andromeda galaxy is currently
become gas giant planets, like Jupiter. However, outer layers in a planetary nebula, leaving heading straight for our Milky Way and is due to
as they steal gas from the disc around them, behind a white dwarf. As the star swells into a collide in about four billion years. It might
these planets lose angular momentum and red giant, it swallows the innermost planets, sound like the plot of a science fi ction
begin to migrate inwards towards their star, while those planets outside its grasp see their blockbuster, but mergers such as these are
steamrolling anything in their way. Smaller orbits widen due to the giant’s lower mass. This common in the universe and are key to
planets that are in the gas giant’s path can be can cause planets, comets and asteroids to galaxy evolution.

94 How It Works