Protoplanets are small celestial objects that are the size of a moon or a bit bigger. They are small planets, like an even smaller version of a dwarf planet. Astronomers believe that these objects form during the creation of a solar system.
The most popular theory of how a solar system is formed says that a giant cloud of molecular dust collapsed, forming one or more stars. Then a cloud of gas forms around the new star. As a result of gravity and other forces, the dust and other particles in this cloud collide and stick together forming larger masses. While some of these objects break apart on impact, a number of them continue to grow. Once they reach a certain size – around a kilometer – these objects are large enough to attract particles and other small objects with their gravity. They continue to get larger until they form protoplanets. Some protoplanets continue colliding and growing until they form planets while others stay that size.
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As the protoplanets grew to become planets, parts of them melted due to radioactivity, gravitational influences, and collisions. Where the objects had melted, the composition of the planets changed. Heavier elements sank, forming the cores of the planets, and lighter objects rose to the surface. This process is called planetary differentiation and explains why planets have heavy cores. Astronomers have discovered that even some asteroids have differentiated, so their cores are heavier than their surfaces.
Protoplanets used to be highly radioactive due to how they were formed. However, over thousands of years, the radioactivity of these objects has greatly decreased because of radioactive decay. Astronomers are still discovering new protoplanets, and most likely, they will discover many more. With better technology, astronomers are now able to find protoplanets in other star systems. Last year, scientists discovered a protoplanet HL Tau b that will probably turn into an actual planet one day. Astronomers say that will not happen for about a million years though because the protoplanet’s star is also very young. In its final form, HL Tau b will look like Jupiter – a gas giant around the same size as that massive planet. It is hard to believe that thousands of years ago our planets were objects about the size of a moon, which were slowly evolving and growing. Astronomers continue to study protoplanets, the same way they study planetesimals, to find out more about how the Solar System was formed.
Astronomy Cast has an episode on how old the universe is.
When is an Asteroid Not an Asteroid?
From Planetesimals to Terrestrial Planets: Habitable Planet Formation in Binary Star Systems