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New exoplanet endured a rough start

16 September 2020

As stars enlarge to red giants, they engulf any planets orbiting nearby. But astronomers recently discovered a survivor, a Jupiter-sized exoplanet traveling tightly around a white dwarf.

Artist's impression of a white dwarf orbited by a giant planet
In this artist’s impression, a giant exoplanet orbits a white dwarf star. Credit: NASA Goddard Space Flight Center

To date, astronomers have found thousands of exoplanets, but none closely orbit a white dwarf star. That’s because during the preceding red-giant phase of a star’s evolution, nuclear fusion in its core causes the star to expand and swallow closely orbiting objects. But astronomers may now need to reconsider their search parameters: Andrew Vanderburg of the University of Wisconsin–Madison and his colleagues have identified a potential planet orbiting a white dwarf every 1.4 days.

The researchers detected the exoplanet using the transit method. As the planet moved between the star and the line of sight of NASA’s Transiting Exoplanet Survey Satellite, the star’s light periodically dimmed (see the article by Jonathan Lunine, Bruce Macintosh, and Stanton Peale, Physics Today, May 2009, page 46). Optical observations collected by the Gran Telescopio Canarias and summarized in the figure below show the characteristic brightness drop.

Chart showing drop in brightness due to planetary orbit
Credit: A. Vanderburg et al., Nature 585, 363 (2020)

The researchers used NASA’s Spitzer Space Telescope to measure the IR light from the white dwarf. Those data showed that the thermal flux from the transiting object is at most only 6.1% that of the flux from the white dwarf. That number puts the object in planetary-mass or perhaps low-mass brown dwarf territory. Based on thermal measurements and the white dwarf’s age of six billion years, Vanderburg and his colleagues estimate that the exoplanet’s mass is no more than 14 times Jupiter’s.

To avoid being annihilated by the star during its red-giant phase, the exoplanet would require a distant orbital trajectory earlier in its history. But the transition to the white-dwarf phase could have instigated severe dynamical interactions that, according to the researchers’ calculations, would have dragged the exoplanet into the circular, short-period orbit observed today. The process is slow, but it is consistent with that for a six-billion-year-old white dwarf. (A. Vanderburg et al., Nature 585, 363, 2020.)

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