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I have been learning about the solar system from popular science shows. In these shows they suggest that, after having seeing around 2500 other solar systems, astronomers have concluded that our solar system is not the normal one. They see sun hugging hot jupiters and super earths close to their star. They find most systems have most of their matter closer to their star. They draw conclusions about our system, namely that our system is a freak.

We know that the first exoplanets found were hot jupiters. And they found super earths close to their star. We also know that telescope technology is always improving. With increasing telescope quality, we can find dimmer objects. These would naturally be smaller planets orbiting distant stars at further distances from their stars.

My question is this: how do we know that the apparent rare quality of our system is not an artifact of limited observing power that selects for larger objects close to their star?

        

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    $\begingroup$ Relevant question on Astronomy: astronomy.stackexchange.com/q/13301/1559. In short, if we'd been looking at the solar system from the outside, we'd have only found Jupiter, Saturn, and if everything lined up just right, maybe Earth or Venus. $\endgroup$
    – Mark
    May 3 at 22:34
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    $\begingroup$ Note that using the Kepler mission’s criterion of three transits to “discover” an exoplanet, the best case for discovering something in a Saturn-like orbit would be transits observed in years 2009, 2039, and 2069. Many other techniques are also biased towards short-period orbits. $\endgroup$
    – rob
    May 4 at 20:29
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    $\begingroup$ @Mark Though it should very much be noted that even those would be very obviously much farther from the Sun than the exoplanets we've observed so far. But there's definitely observation bias at play anyway - closer to the star means shorter orbital periods, which helps most of the ways we try to find exoplanets, among other things that heavily favour such systems. $\endgroup$
    – Luaan
    May 5 at 8:56
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The solar system cannot be said (yet) to be "rare" because we lack the ability to examine the planetary systems around other stars in detail. In particular, the census of low-mass planets and planets that are more than an astronomical unit from their star is very incomplete.

Nevertheless, enough is known to say that the solar system is unusual in some respects. The main oddity about the solar system is that it doesn't contain any "super-Earths" or "sub-Neptunes" at all (i.e. planets intermediate in size between Earth and Neptune), despite them being common in other systems, and the close-in planets are all small and rocky. Most ($\sim 70$%) solar-type stars have at least one exoplanet larger than the Earth orbiting with a period of 100 days or less (e.g. Kunimoto & Matthews 2020). Most of these close-in planets are 2-5 times the size of the Earth.

The presence of a Jupiter-sized planet at 3-7 au is also somewhat unusual - occurring in $<10$% of solar-type stars Wittenmyer 2016).

The lack of a hot Jupiter in the solar system is not unusual, since the occurrence rate of these is only of order 1%.

These frequencies are corrected for the known and well-understood biases in detection sensitivity associated with system geometry, signal-to-noise ratio and observing cadence. These factors can easily be accounted for in a forward modelling approach. This is where you simulate your exoplanet population, then "observe" it, in software, including all the observation biases and detection thresholds, and then adjust the characteristics of the simulated population until the simulated observations match the real observations.

Note for those wanting to discuss the anthropic principle. It seems to me that whether the planetary system around our Sun has an unusual architecture has nothing to do with our presence. What the anthropic principle may have a bearing on is explaining why we live in an unusual solar system or why we might expect the solar system to be unusual.

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    $\begingroup$ If we assume, for the sake of argument, that our solar system is the only one within range of our detectors which is capable of supporting life, then we should reasonably expect to find that our solar system has some unusual features. You have catalogued various ways in which the solar system differs from all other observed planetary systems, yet you seem unwilling to make the logical inference that the observed anomolies account for the uniqueness of our situation. As data continues to be collected, the differences seem to multiply: nowhere do we observe a situation paralleling our own. $\endgroup$
    – Ed999
    May 3 at 15:12
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    $\begingroup$ The question wasn't to "account for the uniqueness" of the solar system (and there is no evidence that the solar system is unique or "differs from all other planetary systems"); it was to address the question of whether the solar system was unusual and how that can be established given the observational selection effects. @Ed999 $\endgroup$
    – ProfRob
    May 3 at 15:50
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    $\begingroup$ @jamesqf Of course. That is why one uses a forward modelling approach, as I say in my answer. The numbers I've quoted above account for these biases. $\endgroup$
    – ProfRob
    May 3 at 17:14
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    $\begingroup$ @ChristopherSchultz that's overly simplistic. A sufficiently massive planet will be able to hold on to its gases even in close proximity to the star, just as well as a smaller one can only do further away. $\endgroup$ May 4 at 8:41
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    $\begingroup$ @ChristopherSchultz That was much of the thinking that has been essentially disproved by exoplanet observation so far. And plenty of hypotheses even before that lay serious doubt on that logic - including the fact that it seems that our gas giants were much closer to the Sun in the past, when the Sun's activity was much stronger. Mercury is tiny; even without the proximity to the Sun, we wouldn't expect it to hold on to much more than a tenuous atmosphere. In contrast, Venus is very close to the Sun, and has a massive atmosphere (though granted, made out of heavy gases, not hydrogen). $\endgroup$
    – Luaan
    May 5 at 9:00
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I know this is something that scientists take into account, but perhaps their words have been twisted a bit by the popular science shows: there is a measurement bias.

We know what we know about exoplanets because of several ways in which we are able to measure them. Namely, by observing the planets' occultation of their stars (the light dims when the planet crosses in front of it) and by observing the gravitational wobble of the stars due to the planets.

Both of these are going to be easier to see for large planets that are closer to their star.

So I don't know if we are really at a point yet where we can make definitive statements about things such as "our solar system is rare".

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  • $\begingroup$ There's also a selection bias going on: The scientists performing the experiment could only do it from the planet on which life appeared. Whether life evolves or not is impacted by the type of planets in your system. $\endgroup$
    – Jeffrey
    May 3 at 13:07
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    $\begingroup$ @Jeffrey that has no bearing on whether the solar system is unusual or not, only on what conclusions you can draw about why we live in an unusual solar system. $\endgroup$
    – ProfRob
    May 3 at 14:07
  • $\begingroup$ @ProfRob I'm pretty sure that if OP lived in a different system, he would not care about whether this one is unusual, but I see your point :-) </offtopic> $\endgroup$
    – Jeffrey
    May 3 at 14:08
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    $\begingroup$ This answer seems to be arguing that these biases can't be accounted for. Which is partially untrue. As long as there is any sensitivity to a particular parameter space then that sensitivity can be used to correct the measured frequencies. $\endgroup$
    – ProfRob
    May 3 at 17:17
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    $\begingroup$ @Luaan I haven't said what you claim. Biases in detection sensitivity can (and are) corrected for in all respected work on the topic of exoplanet frequency. Results are only quoted where there is some sensitivity to detection. $\endgroup$
    – ProfRob
    May 5 at 9:30

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