“It may be that a planetary system like our own, where all the planets are in WorldMags.net stable orbits, is a rare occurrence” SPACE
Planet formation
Most formation theories start with a disc of dust and gas that eventually coalesces into planets
Why new discoveries have challenged our thinking of how different worlds develop across the universe The long-standing theory of how our Solar System was created is that the planets and the Sun were born out of a nebula that flattened into a rotating disc. At the heart of this disc formed our Sun, while the dust and gas surrounding it slowly coalesced into the planets and other objects such as asteroids and comets. However, more recent observations suggest that our current theories may not be wholly applicable to other planetary systems. Over the past decade we have found a treasure trove of exoplanets in the Milky Way, and a lot of these do not conform to our current understanding of planetary formation. Unlike our Solar System, where all the major planets orbit within the same plane, we’ve found systems where planets are flung in highly eccentric orbits away from the plane of the other planets. Similarly, we thought no gas giant planets would be able to form near stars, but ‘hot Jupiters’ have been found orbiting their host stars in tight orbits.
It is likely that planets found incredibly close to their host star migrated there after being influenced by other planets within their own systems (see the diagram below). Indeed, it may be that a planetary system like our own, where all the planets are in stable orbits, is a rare occurrence. However, while our theories of how planetary systems form may need to be rewritten, our understanding of how planets themselves form is thought to be fairly sound. It is believed that as the dust and gas within a protoplanetary disc begin to clump together, they become the dominating force within their orbit and begin to attract more and more material. Over a period of hundreds of millions or billions of years, these ‘building blocks’ of planets, known as planetesimals, gradually grow into the fully-fledged planets that we can see in our own Solar System today. Those planetesimals that do not become a planet remain in a system as asteroids and, in some instances, comets.
Tidal parades vs jumping Jupiters A closer look at the main stages in two formation theories
4 2
3
7. Thrown out The result is that one or more planets are thrown out of the system, while the rest remain in highly eccentric orbits.
6. No stability Here, the orbits and masses of the planets influence each other and make them unstable.
7 1
5. Jumping Jupiters 5
However, an unbalanced system can send planets into disarray.
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1. Formation
2. Tidal parade
3. Destruction
4. Close orbit
Systems are thought to begin with a rotating disc of dust and gas that forms a star and planets.
In a tidal parade system, multiple planets, like the three here, form in orbit around their host star.
The farthest and largest planet slows the orbits of the inner planets so they fall into the star.
The larger planet is in turn pulled towards the centre, but its orbit then stabilises around the star.
070 | How It Works
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