Question: For the Universe I am creating humanity has devolved into groups of scattered tribes. This is due to the fact that we ran afoul of a *really* nasty alien race, who decided to leave us homeless. A coalition of races had banded together to fight this menace, but mankind ended up with the short end of the stick. It is the Great *Diaspora*, and we are only hanging on by our fingernails.

The problem I am having is how to thoroughly destroy or at the very least render uninhabitable the places Man once called home. If it were only Earth it would be a simple enough matter, but by the time the attack takes place he *also* has colonies on Luna, Mars, Ceres, Callisto, and Titan. How can I muck them up badly enough that we have to now seek a Promised Land elsewhere?

Ceres I figure is small enough it could just be reduced to rubble, but what about Luna? If you destroy the colony there, what would prevent them from setting up new ones a few hundred miles away? 

At the moment all I can think of is a "Gray Goo" weapon, short of blasting multiple planets and moons to smithereens. I am hoping there is another option out there I haven't considered.

EDIT: I'd like to hold onto the Sun. You know, sentimental reasons. :)

Answer: 
The answers all suggest multiple plausible ways to render the Solar System uninhabitable, but suffer from some flaws, such as requiring a long time frame to work, or require a great deal of resources.

I will refine the process and suggest the aliens simply have to fire one RKKV (Relativistic Kinetic Kill Vehicle) into the Sun. Striking the sun with that amount of energy and using the extreme speed of the RKKV to deliver the energy deep into the Sun, many processes will be disrupted and the Sun will likely emit massive flares and other radiation events which will scour life from all exposed surfaces. Since the Sun will be unstable for years, there is really no hunkering down and waiting to fix the blasted surface installations, especially since we are talking about the sorts of flare events which would strip large portions of the atmosphere from the inner planets. Larry Niven's short story "Inconstant Moon" describes what such a scenario would look like from Earth.

[![enter image description here](https://i.stack.imgur.com/H83uC.jpg)](https://i.stack.imgur.com/H83uC.jpg)

*Use SPF 10,000,000*

<http://news.larryniven.net/biblio/display.asp?key=52>

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> This story raises the question: How would you spend your last night on Earth? When the Moon suddenly starts shining brighter, Stan and Leslie realize the sun must have gone nova and they only have a few hours until the Earth rotates into the deadly sunshine. This story won a Hugo Award and was made into an episode of The Outer Limits.
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As a possible bonus, after the flares settle down, the Sun may also go into a Maunder minimum, producing less energy and making any sort of biological or industrial recovery even more difficult.

Even K2 civilizations have limits, so being able to deal with the opposition using only one RKKV is much more efficient than trying to strike every planet, moon and free flying object in the Solar System.
#### Spheres of cobalt 59 orbiting the sun

The alien placed a lot of spheres of cobalt 59 in orbit around the Sun. The orbits are so eccentric that at the perihelion they graze the Sun surface and the heat ablates a layer of the surface, then the solar radiation turns it into cobalt 60. Now you have a very radioactive solar wind and, since those atoms were not in the plasma, many of them are not ionised therefore they can pass through the barrier of the Earth magnetic field.