Terrestrial planets, gas giants, comets, moons, asteroids: earth’s solar system has numerous types of heavenly bodies. Planetesimals are unusual rocky celestial objects that can be measured in a few meters or many kilometers. They are located in many parts of the solar system, and some astronomers believe they are key to the history of planets and moons. Planetesimal matter such as rock and dust may have combined with gravity to form a number of the masses orbiting the sun.

Russian astronomer Viktor Safronov theorized that, when the solar system was forming, the attractive force of gravity pulled bits from nebulae --clouds of dust, gasses and plasma-- together, creating rocky planetesimals of various sizes. If the planetesimals nearest the sun were composed of matter that had high melting points, they may have formed the four terrestrial planets. The outer planets could have come from planetesimals made from different materials that formed dense cores, attracting light gases such as hydrogen and helium. This may have resulted in the four planets known as gas giants.

Pluto was once considered one of the nine planets in earth’s solar system. However, in the latter part of the 20th century, many astronomers believed that Pluto was simply not large enough to be considered a major planet. Some of these scientists began referring to Pluto as a planetesimal. By 2006, most astronomers in the International Astronomical Union generally agreed that Pluto was not a planet, although this was a controversial decision for some scientists and non-scientists. Dropping Pluto from the planetary list was intended as a reclassification rather than a demotion.

In 1943, Irish astronomer Kenneth Edgeworth suggested that undiscovered objects lay near the outer boundary of the solar system. In 1951, Gerard Kuiper offered further evidence to support this idea. In fact, a ring of icy bodies, now commonly known as the Kuiper belt, orbits the sun beyond Neptune. Some of the larger objects in the belt are considered planetesimals or "super comets." Since 1992, many have been identified. Pluto is the largest body within this grouping. Smaller members in the belt are labeled “comets.”

Many of the moons orbiting planets are considered planetesimals. The largest of Neptune’s 13 moons, Triton, falls into this category. One of Saturn’s 53 moons, Phoebe, is a planetesimal, as well as both of Mars’ moons, Phobos and Deimos. In addition, Jupiter has 50 moons, and several of these match the criteria for planetesimals.