No, can't be a former planet, it's too tiny. Total mass is 4% of the mass of our Moon.
It's thought to be what's left over from the early solar system, and disturbed by Jupiter, wasn't possible for a large planet to form.
However there was enough matter for one dwarf planet, Ceres, which is massive enough to be in hydrostatic equilibrium.
It's next largest asteroid 4 Vesta is in between an asteroid and a dwarf planet.
Both are thought to have a differentiated interior like a planet. Ceres may even have a "muddy ocean" beneath its surface and is a possible candidate for present day life. Enough so, that the Dawn spacecraft will be left in a long term orbit around Ceres instead of crashed onto its surface as originally planned, to make sure it doesn't contaminate it with Earth life.
The origin of some of the asteroids is somewhat mysterious. Ceres particularly may have too much ice in its composition to have originated there; if so, it may have came in from the outer solar system beyond Jupiter. There's a strange event called the Late Heavy Bombardment very early on but after our Moon formed. There are various theories for why it happened, but it caused the big craters on the Moon and other terrestrial planets. Seems to be objects that were in some way brought into the inner solar system from further out. So this is something that happens.
"In the "Nice model,” an enormous quantity of KBOs are injected into the outer asteroid belt, >2.6 AU (Levison et al., submitted). Most are dynamically lost or collisionally destroyed, yet the remnant of this embedded population may be the source of the D-type asteroids. This raises the issue of whether larger KBOs were also embedded, and what they would look like today (and can we tell?). It is interesting to note that Ceres, the largest asteroid, is not that different from what we imagine dwarf planet KBOs to be like: differentiated, ice-rich (0.72-0.77 anhydrous rock by mass), and possessing unusual surface chemistry"
On The Possibility Of Large KBOs Being Injected Into The Outer Asteroid Belt
And in this paper the author speculates that Ceres was a moon of a planet which he calls Yurus which came into the inner solar system from the asteroid belt in the very early solar system - and Yurus itself was ejected, leaving it's moon Ceres behind.
http://arxiv.org/ftp/arxiv/paper...
Or perhaps it formed where it is but got more ice than expected? After all the Earth got a lot more water than you'd expect from its location close to the sun. We still aren't sure how Earth managed to get quite so much water - the observations of comet 67p showed that our water didn't come from comets similar to comet 67p because the hydrogen to deuterium ratio is wrong - but that's just one comet. It's an open field for investigation to try to figure out how Earth got its water.
In short, it's possible that some of the asteroids could have come from beyond Jupiter. Or else that the ice in some of the asteroids did if not the entire asteroid.
Not likely that an entire large asteroid like Ceres could be captured into the asteroid belt in the current solar system but in the very early solar system with lots of debris, and many collisions, it would be easier to capture a large asteroid inside of Jupiter, and though most would be destroyed or ejected, some could survive just because there was so much stuff passing through the inner solar system in those days.
About the Author
