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Looking at all discovered exoplanets (4393 exoplanets), I found than only 17 of them (less than one percent!) have masses less or equal to Earth's mass. Why so?

  • Is it because it is very difficult to discover an exoplanet of a low mass?
  • Is it because of the mass distribution, so that Earth-mass planets are very rare?
  • Is it because of the some other physical limitations?

According to Wikipedia:

The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.

From another article:

A dwarf planet, by definition, is not massive enough to have gravitationally cleared its neighbouring region of planetesimals: it is not known quite how large a planet must be before it can effectively clear its neighbourhood, but one tenth of the Earth's mass is certainly sufficient.

So, where are all these planets that are lighter than Earth? Personally, I suspect that it's very difficult to detect these (relatively) low-mass planets. If so, are there any theoretical limitations that prevent formation of low-mass planets?

Note 1: most of the planets (around 70%) from the mentioned catalog do not have masses (i.e. there's no estimate for the mass of a planet). Most of the rest have $\sin i$ mass estimates. That might be one of the reasons.

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    $\begingroup$ Personally, I suspect that it's very difficult to detect these (relatively) low-mass planets. What (non-personal) reasons do you have to suspect this? Elaborate in your post please. $\endgroup$
    – Daddy Kropotkin
    Commented Dec 28, 2020 at 16:23
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    $\begingroup$ @N.Steinle - it's just a feeling, I don't have any rationale behind that. That's why I am asking :) $\endgroup$
    – Peter
    Commented Dec 28, 2020 at 16:24
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    $\begingroup$ @B--rian About 1325 (out of 4393), IIRC. $\endgroup$
    – Peter
    Commented Dec 28, 2020 at 17:34
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    $\begingroup$ The answer is extremely simple: it's easier to find bigger planets. $\endgroup$
    – Fattie
    Commented Dec 29, 2020 at 3:06
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    $\begingroup$ Note also that exoplanet discovery is very biased in favour of planets near to their suns. There’s no way we could yet have discovered Jupiter around another star, for example, because it would need at least 25 years of data to establish the periodicity in the transits or motions of the star.. $\endgroup$
    – Mike Scott
    Commented Dec 31, 2020 at 17:59

1 Answer 1

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There are a number of methods of detecting exoplanets, but all of them favour detection of larger planets over smaller ones, albeit for slightly different definitions of large:

  1. Radial velocity measurement — this detects the small movement of the star towards and away from us as the planet and the star orbit their mutual barycenter. This movement is fastest when the planet is massive (so the barycenter is further from the center of the star) and close to the star (so the orbital velocity is highest). I also needs the planet's orbit not to be "face-on" to the Earth. This method produces measurements for the $mass\times \sin(i)$ since a more massive planet in a less inclined orbit produces the same motion as a less massive planet in a more inclined orbit

  2. Transverse displacement — this detects the small movement of the star from side to side (against the background of distant stars) as the planet and the star orbit their mutual barycenter. The displacement is largest when the planet is massive and far from the star (although distant planet require observation over a long period of time). It works best on stars close to us.

  3. Transit — this detects the tiny reduction in the brightness of the star when the planet moves between us. It is more likely to detect large planets, and more likely to notice if the orbital period of the planet is fairly small.

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  • $\begingroup$ Perhaps you could comment on selection effects in observing exoplanets via transit (which the vast majority of exoplanets are discovered by), e.g. how transit method generally selects larger radii planets which tend to have mass larger than the Earth. Of course there are outliers to this, like the smallest ever discovered exoplanets, but these also tend to have correspondingly small host stars. $\endgroup$
    – Daddy Kropotkin
    Commented Dec 30, 2020 at 14:26
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    $\begingroup$ (2) says a massive planet far from the star which seems backwards to me. Shouldn't it be close to the star? Gravitational forces are of course inversely proportional to distance squared. Thus for a given exoplanet mass, a closer, faster orbit would increase both the displacement of the star and the frequency of displacement. A larger, higher frequency signal should be easier to detect. $\endgroup$
    – Technophile
    Commented Dec 30, 2020 at 16:47
  • $\begingroup$ @DaddyKropotkin I did say that transit is more likely to detect large planets. Feel free to edit and elaborate on this. $\endgroup$
    – Steve Linton
    Commented Dec 30, 2020 at 17:21
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    $\begingroup$ @Technophile The magnitude of the displacement increases with the distance, although the frequency and velocity drop (which is why it may take a long time to notice the effect of a large but distant planet. If the mass ration of the star and the planet is $M$ and the separation is $R$ the star moves with radius $R/(1+M)$. $\endgroup$
    – Steve Linton
    Commented Dec 30, 2020 at 17:24

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