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RandomFactor [soylentnews.org] writes:

The Kepler Space Telescope (retired in place last year after running out of hydrazine [soylentnews.org]) has racked up over 2600 planetary discoveries in its nine years of operation. It's successor, the Transiting Exoplanet Survey Satellite (TESS) has already found hundreds itself. In 2017 a mysterious gap was identified in the sizes of exoplanets discovered by Kepler (and which continues in those since discovered by TESS.) There are significantly less planets between 1.5 and 2x the size of Earth vs. planets either larger or smaller than that. This has left astronomers puzzling over this peculiar patch of planetary paucity [quantamagazine.org]

This sparsely populated range of planetary sizes is referred to as the 'Fulton Gap' [wikipedia.org] after Benjamin J. Fulton the lead author of the paper [iop.org] that first described it.

There are three general theories that attempt to explain the gap.

One possibility [...] is a reverse-Goldilocks scenario in which medium-sized rocky planets with atmospheres can’t last. “You are either going to be big enough to hold on to your atmosphere, or if you are intermediate in size, then you are probably not big enough and you are going to lose it all pretty quickly,” [astronomer Diana Dragomir] said. “It’s like a tug of war; it’s really hard to stay in the middle.”

Another theory [according to Sara Seager, an astronomer at MIT] holds that the gap results directly from planetary genesis, maybe because of the location or makeup of the gas and dust left over from the star’s birth.

[...]a third theory proposes, planets’ own cooling processes might cause their atmospheres to evaporate, an effect called “core-powered mass loss.

TESS, which is just getting started, will enable observation of 400 times more sky than Kepler was able to survey and will return data on planets further out from their stars, and also on many nearer stars that can then be observed for followup with Earth based telescopes.

Tiny balls of rock or high gravity super Earths. Time will tell if there really is no place like home.

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