Article History
Received: 6 April 2018
Accepted: 14 February 2019
First Online: 25 March 2019
Change Date: 24 April 2019
Change Type: Update
Change Details: In the version of this Article originally published, the following ‘Journal peer review information’ was missing: “<i>Nature Astronomy</i> thanks Don Ellison, Giovanni Morlino and the other anonymous reviewer(s) for their contribution to the peer review of this work.” This statement has now been added.
Competing interests
: The authors declare no competing interests.