I can not understand the part of the paragraph because I think they are contradiction or irony. Could you explain the process of it? Thank you!

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    $\begingroup$ It's certainly not irony. Hot Jupiters cannot form where we find them right now, so they must have migrated there from larger distances. That's all the paragraph says. $\endgroup$ Commented Nov 22, 2016 at 13:13
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    $\begingroup$ Purple Rain Kim -- The title of the question makes me think that you think the article is writing about planets orbiting the Sun. It's not. It's about planets (technically, exoplanets) that have been discovered orbiting other stars. $\endgroup$ Commented Nov 22, 2016 at 17:59

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I think you're misinterpreting the passage.

Gas giants form via a process know as core accretion. First, a massive core forms from solid material, initially with a small amount of gas. The envelope then grows, producing an object we would recognize as a gas giant - a large gaseous atmosphere, surrounding a comparatively small "rocky" core.

The problem with Hot Jupiters is that gas giants, according to current theories of planet formation, cannot form extremely close to a star. Inside a boundary called the frost line (which changes over time), there is not enough solid material for such a massive core to form and accrete enough gas to form a gas giant before the protoplanetary disk dissipates. Therefore, any gas giants that exist inside the snow line should have formed outside it, and then traveled inward via a process known as migration, which can happen through several different mechanisms.

The phrase "spiraled inwards" doesn't mean that the planets will spiral into the star, merely that their semi-major axes will shrink and their orbits will move closer and closer to their parent star, possibly stopping at some finite distance.

Like David Hammen, I'm a bit confused as to whether you think this happened in the Solar System, or just other exoplanetary systems. As far as I know, this sort of formation and migration could not have happened in the Solar System, as per the leading models of its early evolution, because it would have led to a drastically different layout than the one we know today.


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