Will there ever be new planets?

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Our solar system is not making new planets anymore. All of our planets were formed very early in the lifetime of our star, when there was still a lot of heat and energy in the disk of material that surrounded the young Sun. It was this heat and energy that allowed the small grains of dust to stick together and form larger and larger objects, until the rocky and gaseous planets we know today had largely formed. The solar system gradually cooled down. As it did, fewer and fewer small pieces of material were able to stick together, until eventually, everything stopped - stuck the size and shape it had been at the moment it could no longer continue growing. It’s been several billions of years since the formation of the solar system, and most of the material around our Sun has stabilized; it’s either part of a planet already, or it’s something along the lines of an asteroid or comet, whose shapes are quite literally frozen into place.

We may yet discover more dwarf planets in our solar system - the dwarf planets are small, and require a lot of hunting. But as far as rocky or gas giant planets, our solar system is finished.

We are, of course, also searching for planets outside our solar system. Kepler, the planet-hunting satellite, has found 135 confirmed planets, and more than 3500 planet candidates, at last count. Kepler hunts in a very small area of the sky, so there are certainly many many more planets to be discovered outside of our solar system.

But in terms of entirely new planets being created - this is also happening! Planets tend to form around new stars, and our galaxy is forming a few new stars every year. (Usually our Milky Way is quoted as making an average of about 3 new stars every year, which is not too bad for a galaxy of its size.) So every year, we have three new stars around which we can form a few planets. Some estimates state that, on average, each star has at least one planet - so our galaxy is, on average, forming a few new planets every year!

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How do asteroids form?

Asteroids are the left over bits and pieces from the solar system’s early days that didn’t quite make it to being a planet, for one reason or another.

Most planets formed relatively early in the solar system when a given lump of material (called a planetestimal) started crashing into nearby lumps of dust and rock with just enough force to get the other pieces of rock to stick to it, without having the smaller object simply bouncing off or shattering. This method of accretion works until the planetesimal gains enough mass to start attracting objects through gravitational forces, which can help it gain mass even more quickly. Once a particular lump reaches a certain mass, its own gravitational weight will start making it rounder and rounder. For some objects, this process will continue, until they form a fully-fledged planet.

This ideal case of catching smaller lumps of material is rarely a smooth process. In the earliest stages of building up a planetesimal, the rate at which it can gain mass is largely dependent on how much nearby matter there is to be wafted in the right direction to clump on to our object. Ultimately, this means that some planetesimals will grow more rapidly than others, if they find themselves in a region that has a lot of material with which to grow. In turn, this means that the early solar system was a very hectic place, with swarms of little planetesimals of various sizes. Some of these planetesimals will unavoidably be absorbed into one of its larger neighbors, if it doesn’t suffer a more catastrophic fate.

We mentioned earlier that to get a smaller object to stick to you and gain its mass, you have to run into it with the right amount of force. If you run into it with too much force, you can shatter the smaller object into small fragments. There are two main ways to have too much force in your collision- high velocity and large mass. If you’re a much more massive object, you’re likely to just shatter the small object. But if you’re both small objects, and you hit at high speeds, both of you will shatter. (This is what we think happened to create that X-shaped asteroid Hubble spotted a couple years ago.) What’s left over - these shards of former planetesimals - can then continue to wander the solar system in places like the main asteroid belt between Mars and Jupiter, or out beyond Neptune.

There are a few objects that seem to have done okay in building up mass to the point of making themselves round, and also managed to avoid being destroyed in one of these collisions, but then failed to continue gaining mass to the point where there weren’t any other objects in their orbit - these are what we now call dwarf planets. Like brown dwarf stars, they made it partway to being a planet, but didn’t quite make it all the way there.

Something here unclear, or have your own question? Feel free to ask!