Planetesimal
Topic: Origin and Evolution of the Earth and the Solar System · NCERT: Class 11, Ch 2 "The Origin and Evolution of the Earth"
Meaning
A planetesimal is a small solid body that forms in the disc of gas and dust around a young star. Matter in the disc condenses into small, rounded objects, which stick together to form planetesimals. Collisions and gravity then join planetesimals into larger bodies. In the last stage, many planetesimals come together by accretion (crashing and sticking under gravity) to form a few large planets.
Example
The asteroid belt between Mars and Jupiter is made of leftover rocky bodies that never joined into a full planet. Ceres, a dwarf planet, is the largest body in it.
Don't confuse with
- Planetesimal hypothesis (Chamberlin and Moulton, early 1900s): this was a failed theory in which a passing star pulled material out of the Sun. A planetesimal, on the other hand, is simply a small building block of the planets and is still part of today's accepted model.