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2019-04-04. Astronomers spy an iron planet stripped of its crust around a burned-out star.

posted Apr 6, 2019, 9:20 AM by Alan Gould

By Daniel Clery, Science Magazine. [https://www.sciencemag.org/news/2019/04/astronomers-spy-iron-planet-stripped-its-crust-around-burned-out-star] For GSS A Changing Cosmos chapter 8. Excerpt: In a glimpse of what may be in store for our own solar system, astronomers have discovered what appear to be the shattered remains of a planet orbiting a white dwarf, the burned-out ember of a star like our sun. If the team’s calculations are correct, the orbiting object may be the iron core of a small planet that had its outer layers ripped off by the white dwarf’s intense gravity. Although astronomers know of thousands of exoplanets in the Milky Way, they struggle to see anything much smaller than Earth. The new object is by far the smallest, more of an asteroid than a planet. ...Finding the planetesimal, 400 light-years from Earth, wasn’t easy.  ...Most exoplanets can’t be seen directly, but are found when they cast a shadow crossing the face of their star or when they tug their star back and forth with the force of their gravity. Manser’s team...picked apart the light coming from the disk to see its spectrum of frequencies and zoomed in on three bright spectral lines produced by calcium ions, which act as a flag for the gas circulating in the disk. ...his team...saw that the two peaks in each calcium line rose and fell in opposition to each other every 2 hours like clockwork. “It was a really exciting discovery,” Manser says. ...this is the signature of a planetesimal orbiting the star. ...the calcium lines are not from the planetesimal itself, but from a cloud of gas that surrounds it, either because it is being battered by disk debris or because radiation from the star causes it to emit gas. As that gas cloud follows the planetesimal in its orbit, it boosts one emission peak while moving toward Earth and, an hour later, the other peak while moving away....