# Why are most discovered exoplanets heavier than Earth?

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.

• 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. – Daddy Kropotkin Dec 28 '20 at 16:23
• @N.Steinle - it's just a feeling, I don't have any rationale behind that. That's why I am asking :) – Peter Dec 28 '20 at 16:24
• @B--rian About 1325 (out of 4393), IIRC. – Peter Dec 28 '20 at 17:34
• The answer is extremely simple: it's easier to find bigger planets. – Fattie Dec 29 '20 at 3:06
• 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.. – Mike Scott Dec 31 '20 at 17:59

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
• @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 $R/(1+M)$. – Steve Linton Dec 30 '20 at 17:24