Some of the first exoplanets identified as candidates for habitable worlds turn out to be mirages conjured up by magnetism on their host star.
Earlier studies looked at tiny changes in the motion of the star Gliese 581 and concluded that at least five planets must circle it. Of these, two planets, dubbed GJ 581d and GJ 581g, were thought to be at a distance that would allow liquid water to exist on their surface. But a team led by Paul Robertson of Pennsylvania State University in University Park studied emissions from hydrogen in the star's spectrum and discovered magnetic disturbances within Gliese 581.
These magnetic changes, as they rotate around the star, mimic the signal an exoplanet would produce, and probably misled the earlier researchers.
Science http://doi.org/th6 (2014)
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The exoplanets that were not. Nature 511, 128 (2014). https://doi.org/10.1038/511128b