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Kepler data suggests that the vast majority of planets are smaller than 4 Earth radii, with larger planets quickly becoming vanishingly scarce. Yet somehow, our own solar system harbours not just one, but two giant planets. This seems suspicious.

Are giant planets expected to be as rare as the Kepler data suggests? Or is there some bias inherent in the transit method that makes detecting giant planets more difficult? Is it perhaps the long orbital period of a planet that formed outside the frost line that has caused this apparent dearth of giant planets?

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  • $\begingroup$ Probably worth considering mass here. Large gas giants consume far more mass than puny little rocky planets so, in terms of mass, they are likely the majority of planets material orbiting stars. $\endgroup$ – StephenG Sep 2 at 17:00
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There is an observational bias and it is taken into account when you see inferences about planet frequency.

The methods to find planets are inherently biased towards finding large, close-in planets. Both the transit and doppler-based methods suffer from this bias.

The paper you reference takes into account this bias to arrive at the statistics you highlight in your question. Note however that the figure for Earth-sized planets is that 16.5% of stars have at least one such planet in an orbit with periods less than 85 days. There are no planets of any kind at all in our solar system that have periods less than 85 days. The figure quoted for Neptune-sized planets is 31% with orbital periods less than 245 days and there are no such objects in the solar system. Finally, the figure quoted for giant planets is that 5.2% of stars have a giant planet in an orbit with a period less than 418 days. Again, no such planet exists in the solar system.

You are correct that it should be unlikely for giant planets to form close to their parent star and this is one of the reasons that they are rarely found there. It is widely believed that the close-in gas giant planets that are seen, have migrated inwards from a formation site further out.

That said, it is perhaps true that our solar system is a little unusual. Whilst we do not yet know what fraction of stars have gas giants in Saturn-like orbits (because observations have not been going on long enough), some work (e.g. Rowan et al. 2015) suggests that only $\sim 3$% of Sun-like stars have an analogue of Jupiter (periods of 5-15 years and masses of 0.3-3 times that of Jupiter).

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  • $\begingroup$ Thanks very much for your answer, I will be sure to check out the paper you've provided. I am curious, however, about very small stars such as an M-class red dwarf. Around such a star, the frost line would be very much closer to the star, and therefore even outside that line a Jovian mass planet could have a short period. I assume this is taken into consideration? $\endgroup$ – Arkenstein XII Sep 2 at 21:55
  • $\begingroup$ @ArkensteinXII M-dwarfs aren't sun-like stars. The data for M-dwarfs are more sparse. Data can be considered divided into many subsets and semi-major axis rather than orbital period can and sometimes is used. Sounds like you have a more specific question? $\endgroup$ – Rob Jeffries Sep 3 at 7:27
  • $\begingroup$ Mostly, I'm seeking an understanding of planet formation and the apparent scarcity of gas giants surprised me. I had wondered whether small stars might have offset bias in transit data since even those planets outside the frost line would have short periods. Is planet formation around a red dwarf expected to differ significantly from that of a sun-like star? $\endgroup$ – Arkenstein XII Sep 3 at 20:10
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    $\begingroup$ @ArkensteinXII Yes. It is. $\endgroup$ – Rob Jeffries Sep 3 at 22:05

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