Resurrection of a venerable SF trope - Planet 4.5

Ever since Bode's Law was formulated in the 18th century, and
especially after the discovery of Ceres in 1801, there has been
speculation about an unknown planet between Mars and Jupiter.
When the extent of the asteroid belt was found, this extended to
the idea that the asteroids were the remnants of an earlier planet
that was somehow destroyed.
A number of SF stories used this trope - up to and including, iirc,
'2001'.
Later science would settle on the notion that the asteroids were
never a planet, they were left over unconsolidated material from
the formation of the solar system.
In the last year, we finally got back some pristine sample of
asteroid material, from Bennu.
Very recently some early research results were published. To
everyone's surprise, the  minerals found included serpentine.
a mineral which forms underwater.
https://www.nasa.gov/missions/osiris-rex/surprising-phosphate-finding-in-nasas-osiris-rex-asteroid-sample/
This suggests that Bennu was once part of a planet, big
enough to support liquid water. Not neccesarily on the surface,
but perhaps as a ice-over ocean, similar the Jovian moons.
So, there really *was* a planet 4.5, between Mars and Jupiter.
An old tool in the SF authors toolbox can be used again.
pt