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Kepler shows small exoplanets are either super-Earths or mini-Neptunes

Tally from the prolific space telescope adds 10 more potentially habitable rocky planets to count

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6:39pm, June 19, 2017
Kepler-452b and Kepler-22b

PLANETARY COUSINS Planets may be lumped into two groups: smaller and rocky like Kepler-452b (left), or bigger and gassy like Kepler-22b (right).

Small worlds come in two flavors. The complete dataset from the original mission of the planet-hunting Kepler space telescope reveals a split in the exoplanet family tree, setting super-Earths apart from mini-Neptunes.

Kepler's final exoplanet catalog, released in a news conference June 19, now consists of 4,034 exoplanet candidates. Of those, 49  are rocky worlds in their stars' habitable zones, including 10 newly discovered ones. So far, 2,335 candidates have been confirmed as planets and they include about 30 temperate, terrestrial worlds.

Careful measurements of the candidates’ stars revealed a surprising gap between planets about 1.5 and two times the size of Earth, Benjamin Fulton of the University of Hawaii at Manoa and Caltech and his colleagues found. There are a few planets in the gap, but most straddle it.

That splits the population of small planets into those that are rocky like Earth — 1.5 Earth radii or less — and those

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