Is the Solar System special?

There are at least three aspects of the Solar System that are weird or at least unusual:

  1. No hot super-Earths.  About half of stars contain a planet larger than Earth interior to Venus’ orbit.  We don’t have one.  That puts us in the minority.
  2. Jupiter.  Only about 15% of Sun-lilke stars appear to have gas giants like Jupiter, and most of these are on very stretched out, “eccentric” orbits.  This makes us unusual at the ~5% level.
  3. Life.  Earth is the only planet in the whole freaking Universe that we are sure harbors life.  That is pretty awesome, and it makes us extra-special.

Is there a connection between these things?  I think so!  I just wrote a blog post about it here.  Let me summarize the main ideas.

The key question is: how do “hot super-Earths” form?  I think that these planets — or maybe just their building blocks — formed farther away from their stars and then were driven inward by the gaseous protoplanetary disk.  That process is called orbital migration.  It looks something like this:


If one of those migrating planets gets big enough, it can gravitationally attract gas from the disk and become a gas giant planet like Jupiter.  Gas giant planets are so massive (Jupiter is more than 300 times more massive than Earth) that they carve ring-shaped gaps in the disk.  Any more distant super-Earths that try to migrate inward are blocked by the giant planet:


What this means is that we owe a lot to Jupiter!  With no Jupiter, Uranus, Neptune and Saturn’s core may have migrated inward and invaded the inner Solar System!  Jupiter is our protector!

A nice feature of this model is that it is testable.  We expect an anti-correlation between the presence of Jupiters and hot super-Earths around other stars.  This anti-correlation shouldn’t be perfect but it should be there.  We’ll find out in a few years!

FYI, here is the more eloquent blog post I wrote for  And here is the technical scientific paper.


2 thoughts on “Is the Solar System special?

  1. Mr. Raymond:
    In your article “Terrestrial Planet Formation in the Presence of Migrating Super-Earths”, you indicated that computer simulations indicated that terrestrial planet formation in the habitable zone was still likely if migrating Super-Earth passed through such zone relatively quickly (less than 1,000,000 years).
    a. Do you still agree that this is still likely the case?
    b. Would you agree that, if the migrating body is Neptune-size (or larger), then the migrating body will most likely disrupt the formation of any terrestrial planets in the habitable zone.
    c. Speculatively, do you think it is reasonable to assume that many or most of the Earth-size bodies found by Kepler with orbital periods of less than 100 days were shepherded to such tight orbits by migrating Super-Earths or Neptunes?
    d. There seem to be many articles which argue for in-situ formation of many of the bodies found by Kepler in tight orbits. Do you still think that migration is still the likely cause of the presence of planets in very tight orbits? Alternatively, is it reasonable to argue for a formation scenario in which the material from which planets form migrates from the snow-line and then assembles in tight orbits?

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