I have been writing a scifi book in which I wanted to include real stars and planets as locations. It was simple enough to find a chart on Wikipedia that lists possibly habitable exoplanets, but I quickly noticed that every single one has a greater mass than Earth, with the exception of the planets around TRAPPIST-1. The main reason this caught my attention was that I wanted to calculate the gravity on each planet as obviously that should have a pretty significant affect on whether or not we can colonize it. It would be terribly disappointing if we manage to master interstellar travel only to have every other habitable planet crush us with its gravity.

Is it just a weird correlation, or is there actually some aspect of how we estimate the mass of exoplanets that is likely to cause higher estimations?

Link to the Wikipedia page that includes the chart HERE.

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    $\begingroup$ I think it is an artefact of how we detect planets. The bigger they are the easier to detect them. Small ones are harder. $\endgroup$
    – geoffc
    Jun 11, 2020 at 17:17
  • $\begingroup$ Not a complete answer because I'm not an astronomer, but this image provides some insight on why we can't go too much smaller than Earth. This one expands on it a little $\endgroup$ Jun 11, 2020 at 17:17
  • $\begingroup$ As a fellow scifi writer I wouldn't worry about naming things after real stars. It's cool if you can, but don't let it stop you from telling your story. $\endgroup$
    – GdD
    Jun 11, 2020 at 17:18
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    $\begingroup$ @geoffc's point can't be stressed enough. If you go back another 10 years, all we found were Hot Jupiters, and 10 years before that we hadn't found much at all. The types and sizes of planets we can find has always been biased by the available detection methods and technology. $\endgroup$ Jun 11, 2020 at 17:36
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    $\begingroup$ We don't know about all possibly habitable exoplanets, we only know about those we found yet. We will know about exoplanets of similar mass than Earth if we develop methods to find these. $\endgroup$
    – Uwe
    Jun 11, 2020 at 20:41

1 Answer 1


There is a nice Wikipedia article about the transit method for finding exoplanets by detection of the light intensity drop caused by the planet when passing before its central star. Unfortunately this article is available in german but not in english language.

If we take the perspective of an alien astronomer searching for explanets around a star of the Milky Way, our Sun, we can do some calculations using the diameters of the planets and the Sun.

The probability for viewing from the proper direction to measure a passage of a planet before the star is:

enter image description here

The probability is higher for planets close to the sun.

The light intensity drop is:

enter image description here

The light intensity drop is higher for big planets.

To detect Earth instead of Jupiter you need more than 100 times more resolution for light intensity drop measurement. But light intensity of our Sun is not constant, it is influenced by Sunspots, stars do have starspots too. You have to distinguish starspots from planetary transits by different timing. Starspot activity may hide a transit, especially in periods of high spot activity.

All tables from the Wikipedia article.

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    $\begingroup$ @Speedphoenix many thanks to Speedphoenix for translating the tables from german to english. $\endgroup$
    – Uwe
    Jun 13, 2020 at 10:05

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