I recently read a bit about "grand tack" hypothesis. It was extremely interesting and it seems to reasonably well explain many features of our solar system. At the same time it supposedly is consistent with our knowledge of other planet systems, which I don't understand. What exactly happened in solar system and at which point , that is so rare out in the other systems? Combination of Jupiter/Saturn showing up at the right places and the right time? Isn't it common that the biggest gas giants appear somewhere over the edge of frost line?


1 Answer 1


We do not yet know whether our solar system is a common or rare outcome of the planetary process.

The statistical properties of exoplanetary systems are severely affected by observational biases. It is far easier to detect close-in giant planets; objects like Saturn, at Saturn-like distances from their host star, have not been found due to the limitations of the observations.

It is therefore far too early to say whether the Grand Tack model may or may not be applicable to a wide variety of exoplanetary systems. What we do know, is that something drastically different happened in the case of those systems that have a Jupiter-sized planet within an astronomical unit or so of their parent star; but these are found in only a few per cent of stars.

As an addendum, it is also the case, that despite taking 10+ years of data to establish them, there are now many cases of Jupiter-like planets in Jupiter-like orbits, though they are probably not that common. The overall conclusion at present is that the solar system does not look that unusual, although the lack of close-in planets larger than the Earth is notable (see Martin & Livio 2015).

  • $\begingroup$ But as far as i understand, we dont need to see earth size exoplanets to suspect that some planet system might be similar to ours. All we need is some jupiter size body(it would be seen, right?) in a respectable distance from its star and a bunch of empty space inside its orbit. Then we could suspect that there are smaller terrestrial planets there. Meanwhile, we mostly find super-earth size bodies pretty close to their stars, which makes those planet systems completely not like ours. $\endgroup$
    – gytis
    Commented Jun 19, 2016 at 2:53
  • $\begingroup$ @gytis Yes, it is true that many (but a lot less than 100%) stars have larger planets than the Earth orbiting closer than the Earth. I am no longer sure what your question is. $\endgroup$
    – ProfRob
    Commented Jun 19, 2016 at 6:51
  • $\begingroup$ I was under impression from http://www.reasons.org/articles/recent-research-strengthens-the-creation-friendly-grand-tack-model that somehow this hypothesys explains what we see in exoplanets. The exact wording was "The fine-tuning inherent in the Grand Tack model is consistent with the observation that no other of the 1,293 planetary systems in the exoplanet catalog (maintained by the exoplanet team) comes close to mimicking the characteristics of ours". $\endgroup$
    – gytis
    Commented Jun 19, 2016 at 7:12
  • $\begingroup$ @gytis Right, now we know where this is coming from. See my answer. The selection effects are far too strong to say that there is anything particularly unusual about the solar system. $\endgroup$
    – ProfRob
    Commented Jun 19, 2016 at 7:27
  • $\begingroup$ @gytis And there are plenty of doppler-discovered, Jupiter-sized planets in Jupiter-sized orbits. $\endgroup$
    – ProfRob
    Commented Jun 19, 2016 at 7:33

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