Here is the full list of all 15 Little Comet achievements.
Reach the Plutoids
Reach Neptune
Reach Uranus
Reach Saturn
Reach Jupiter
Reach Ceres and the Asteroid belt
Reach Mars
Reach Earth
Reach Venus
Reach Mercury
Reach the Sun
Earn 25 stars
Earn 50 stars
Earn 100 stars
Earn all 144 stars