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Planets with masses that fall between Earth’s and Neptune’s, as shown in the illustration above, might be common around distant stars and are targets for scientists searching for Earthlike worlds. But just how common is up for debate, and the results of two exoplanet surveys disagree. Now, astronomers suggest the super-Earths and sub-Neptunes might fall into two different density classes — and that some searches more easily detect one class than the other, explaining the differing results. Credit: Aldaron/Wikimedia Commons