That sounds like you're describing Ceres, the largest object in the asteroid belt.

Here's why:

* Irregular Shape: Ceres, while roundish, isn't perfectly spherical. It has a slightly flattened shape.

* Orbits the Sun: Ceres, like all asteroids, orbits the Sun.

* Just Outside the Orbit of Mars: The asteroid belt, where Ceres resides, lies between the orbits of Mars and Jupiter.

While there are other objects in the asteroid belt, Ceres is the largest and most massive, making it a standout candidate for your description.