Astronomers have detected what they believe to be a planet at least the size of Jupiter that came from another galaxy. If true, the world is the first planetary immigrant ever detected in the Milky Way. The find would also violate the leading hypothesis of how and where planets form.
The planet lives 2200 light-years away inside the Helmi stream, a ring of ancient stars that cuts through the plane of the Milky Way. Astronomers believe the stream formed 6 billion to 9 billion years ago, when the Milky Way ripped another galaxy to shreds, swallowing some of its stars in the process. Astronomer Johny Setiawan of the Max Planck Institute for Astronomy in Heidelberg, Germany, likes looking at these stars because they tend to have unusual properties. But even by these standards, one star in particular caught his eye: HIP 13044.
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