NASA Reveals a Destroyed Planetary System

 

The spectral energy distribution of WD 2226-210 superimposed on an image of the Helix Nebula from the Hubble Space Telescope. The graph combines optical, infrared, and millimeter photometry, the mid-infrared spectrum from Spitzer, and upper limits from WISE, Spitzer, SOFIA, Herschel, and ALMA. Models of the white dwarf photosphere (solid) and IR excess show good fits to data detections (circles) and upper limits (triangles). Helix Nebula. Credit: NOIRLab; SED Credit: J. P. Marshall.

Once a star evolves beyond the main sequence, the longest stage of stellar evolution, during which radiation generated by nuclear fusion in the star’s core balances with gravity, the fate of any planetary system it might have had remains an enigma. Astronomers generally do not know what happens to the planets beyond this point, or if they can survive.

In a recently published article in the Astronomical Journal, researchers used new data from the Stratospheric Observatory for Infrared Astronomy (SOFIA) and the Atacama Large Millimeter/submillimeter Array (ALMA), as well as archival data from the Spitzer Space Telescope and the Herschel Space Observatory, to study the Helix Nebula. These observations provide a potential explanation for the fate of these planetary remnants.

A process of elimination and a disruptive origin
The Helix Nebula is an old planetary nebula: bright, expanding gas ejected from its host star after it ended its life on the main sequence. The nebula has a very young white dwarf at its center, but this central white dwarf is peculiar. It emits more infrared radiation than expected. To answer the question of where this excess emission comes from, astronomers first determined where it could not come from.

Collisions between planetesimals (small, solid objects formed from leftover cosmic dust from the creation of a planetary system around a star) can produce this type of excess emission, but SOFIA and ALMA could not see the large dust grains needed for such objects to exist, ruling out one option. Astronomers also did not find any of the carbon monoxide or silicon monoxide molecules characteristic of gas disks that can surround evolving post-main-sequence star systems preceding objects like the Helix Nebula, excluding another possible explanation.

Different lines of evidence impose strict constraints on the size, structure, and orbit of the emission source, ultimately coming together to identify the same culprit: dust from whole planets destroyed during the nebula’s formation, returning to its inner regions.

“By piecing together the size and shape of the excess emission, and what those properties infer regarding the dust grains in the white dwarf’s environment, we concluded that a disrupted planetary system is the best solution to the question of how the Helix Nebula’s infrared excess was created and maintained,” said Jonathan Marshall, the lead author of the paper and a researcher at Academia Sinica in Taiwan.

Once they realized that remnants of an ancient planetary system are the origin of the infrared emission, they calculated how many grains must return to the center of the Helix Nebula to account for the emission: around 500 million over the 100,000-year lifespan of the planetary nebula, conservatively.

The role of SOFIA
SOFIA’s capabilities filled a gap between previous observations by Spitzer and Herschel, allowing the group to understand the dust’s shape and brightness and improve the resolution of how far it spreads.

“This gap was around where we expected the dust emission to peak,” Marshall said. “Nailing down the shape of the dust emission is vital to constraining the properties of the dust grains producing that emission, so the SOFIA observation helped refine our understanding.”

Although researchers do not plan any follow-up observations of the Helix Nebula in particular, this study is part of a larger effort to use observations to understand what happens to planetary systems once their star evolves beyond the main sequence. The group hopes to study other late-stage stars using similar techniques.

Reference: “Evidence of the Disruption of a Planetary System During the Formation of the Helix Nebula” by Jonathan P. Marshall, Steve Ertel, Eric Birtcil, Eva Villaver, Francisca Kemper, Henri Boffin, Peter Scicluna, and Devika Kamath, December 19, 2022, The Astronomical Journal. DOI: 10.3847/1538-3881/ac9d90

SOFIA was a joint project of NASA and the German Aerospace Center (DLR). DLR provided the telescope, scheduled aircraft maintenance, and other mission support. NASA’s Ames Research Center in California’s Silicon Valley managed the SOFIA program, science, and mission operations in cooperation with the Universities Space Research Association, headquartered in Columbia, Maryland, and the German SOFIA Institute at the University of Stuttgart. The aircraft was maintained and operated by NASA’s Armstrong Flight Research Center Building 703, in Palmdale, California.

 

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