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A glimpse of a planetary system’s final stages

19 April 2019

Detailed spectroscopy measurements may help astronomers discover bodies orbiting white dwarfs.

Illustration of a planetesimal orbiting a white dwarf
Credit: University of Warwick/Mark Garlick

When the hydrogen fuel that keeps a star like our sun burning brightly is exhausted, the star expands into a red giant before collapsing into a hot, dense white dwarf. Although the stellar swelling engulfs nearby planets, theoretical models suggest that some planets and planetary cores up to hundreds of kilometers in diameter can survive the star’s death and fall into closer orbit. But identifying solid bodies around a dim stellar core is difficult. Now Christopher Manser (University of Warwick) and colleagues have used a new spectroscopic method to identify a planetesimal orbiting a white dwarf 400 light-years from our solar system.

Astronomers have discovered most exoplanets—including an asteroid-like body orbiting a white dwarf—via the transit method, identifying periodic dimming as an object passes in front of its host star. But the method requires a lucky geometry of the planetary system’s orbital plane relative to Earth. Manser and his team instead turned to short-cadence optical spectroscopy using data from the 10.4 m Gran Telescopio Canarias in Spain. They focused on one of just a few white dwarfs that, based on metal emission lines in the stellar and disk spectra, are suspected to be surrounded by disks of gas and dust. Minute-by-minute observations over several nights in 2017 and 2018 let the researchers deconstruct the light emanating from the disk and determine how much variation had occurred over a year.

The researchers zoomed in on calcium-ion emission lines, a signature of metal-rich gases. As expected, they found that each spectral line had two peaks. The peaks came from light that is shifted in frequency as the swirling gas disk moved toward and away from Earth. But in addition, another wave of light shifted from one peak to the other every two hours. That periodic variation led Manser to conclude that the emissions came from a cloud of gas surrounding a potentially iron-rich metallic planetary core (illustrated above). The gas originated from either colliding debris as the planetesimal sped around its host or irradiation by the stellar core. Calculations suggested that a core rich in iron and nickel would be sufficiently dense and strong to resist being torn apart by the white dwarf’s gravity.

Future observations with the next generation of large ground telescopes could help astronomers learn about the chemical composition and structure of planetary cores. (C. J. Manser et al., Science 364, 66, 2019.)

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