Gemini South spectra put WASP-189b's magnesium-to-silicon ratio close to its star's

The inference has two links, and the measurement bears directly on only one of them. Stars and their disks form from the same cloud, so astronomers have long assumed the two hold similar proportions of rock-forming elements ^[6]. A giant planet's outer atmosphere is thought to keep clues about the environment where it formed ^[5]. If both ideas hold, a giant planet whose magnesium-to-silicon proportion closely matches its star's ^[8] is what the assumption predicts.

The release describes the agreement only as a close match and does not publish the ratios or their uncertainties ^[12]. Without those numbers there is no way to tell whether the agreement sits within a few percent or within a wide margin.

The sample is also small. Until now the assumption rested mainly on evidence from the Solar System ^[7], and this adds one planet around one other star ^[8].

Jorge Antonio Sanchez, the Arizona State University graduate student who led the team ^[3], framed the result in terms of terrestrial planets. "WASP-189b gives us a much-needed observational anchor in our understanding of terrestrial planet formation since it offers a measurable quantity that validates the presumed resemblance of stellar composition and the proportion of rocky material around host stars used to form planets," Sanchez said ^[10].

The planet he measured is a giant ^[5]. A rocky planet's bulk composition goes untested here. In our view the result is one data point on whether a giant planet's makeup tracks its star's.