Have you ever seen a moon being born?

It starts with a cloud of gas and dust. The cloud contracts into a disk and a star ignites at its center. Planetoids form around the star in the protoplanetary disk and grow into planets. Around the planets, in turn, are dust disks that eventually form moons. Up to that last step, this theory of solar system formation had long been confirmed by observations. But no telescope had yet discovered a dust disk around a planet.

Now one has. ALMA, the Atacama Large Millimeter/submillimeter Array made the first such observation in the young solar system of PDS 70, which is about 370 light years from Earth. The European Southern Observatory’s Very Large Telescope (VLT) had already confirmed the existence of two gas giants with Jupiter-size dimensions in that system. Now ALMA can also show that a dust disk has formed around the outer planet and might form multiple moons. With some patience, we might even be able to observe the birth of an exo-moon there.

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