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Reading the atmosphere of another world means catching starlight as it filters through that planet's air during a transit. The signal is about one part in a million and a true Earth twin transits only once a year, so you cannot simply repeat the measurement until the noise averages away. Each attempt has to work on its own. A team led from Shanghai has proposed the alternative to building an impossibly large mirror in space - nine hundred, one metre telescopes, flying together, watching the same transit and analysing the light together.

Blazars are galaxies with a jet aimed almost directly at us, and they flicker across every part of the spectrum at once. Almost everything we think we know about them comes from short observing campaigns just a handful of days, every year or two, in one narrow band of energy. A Polish and German team has now done the opposite, following a single blazar for nearly twenty years with two orbiting observatories and in two observations from 2012 they found something in the spectrum that nobody had predicted.

The eROSITA telescope has released its second great catalogue of the X-ray sky with close to two million sources across half the sky, roughly double what it published before. Buried in it is a result from Bonn that matters more than the headline number. A team there went after the faintest part of a massive galaxy cluster, the outer region where fresh material is still arriving along threads of gas stretching between clusters. Nobody had measured that emission before. The gas turns out to be hotter, denser and poorer in heavy elements than the models say it should be.

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