I understand that formation theories for gas giants suggest they should be born further away where there is more gas for them to monopolize vs the sun, and then to form Hot Jupiters they need to migrate inwards, which would disrupt the orbits of rocky planets.

Are there any quibbles with this? Out of all the extrasolar systems is there even one where you have hot or warm gas giants and then cold Earth or Super-Earth sized terrestrials? Bodies like we have in the Kuiper belt lie beyond our gas giants but they are mostly made of ices and represent a small leftover fraction of the system mass, so they don't really count here.

  • $\begingroup$ For the purposes of the question, how do you want to define gas giants? Do you want to consider Neptune-like planets to be gas giants? $\endgroup$
    – user24157
    Commented Jun 15, 2020 at 22:00
  • 2
    $\begingroup$ Neptune-like planets are usually considered ice giants. $\endgroup$
    – Axion
    Commented Jun 17, 2020 at 16:15

1 Answer 1


The Kepler-20 system has planets with masses in the following order, going outwards from the star:

  • Kepler 20b: $\approx 10 M_\oplus$
  • Kepler 20e: $\approx M_\oplus$
  • Kepler 20c: $\approx 16 M_\oplus$
  • Kepler 20f: $\approx 1.5 M_\oplus$
  • Kepler 20g: $\approx 20 M_\oplus$
  • Kepler 20d: $< 20 M_\oplus$

If Wikipedia is to be believed, Kepler 20b may be a rocky world; but at the very least, Kepler 20c is a hot Neptune and has a rocky planet (Kepler 20f) outside its orbit.

  • 1
    $\begingroup$ 20c has f outside of its orbit, but I can understand how that could sporadically happen. Here the system is still weighted on the outside. I'm wondering whether there's an alternative formation pathway that produces a consistent relationship for all of the planets. $\endgroup$
    – Axion
    Commented Jun 17, 2020 at 16:18

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