The Fate of Former Planetary Systems Revealed by the Helix Nebula

Category Space

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In a paper recently published in The Astronomical Journal, researchers studied the Helix Nebula to understand its peculiar infrared radiation. Observations from SOFIA and ALMA, as well as the Herschel and Spitzer Space Telescope, suggest that dust grains from the destroyed planetary systems are returning towards the Helix Nebula's core to cause the infrared excess, with around 500 million grains present over the lifetime of the planetary nebula.

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Once a star evolves beyond the main sequence – the longest stage of stellar evolution, during which the radiation generated by nuclear fusion in a star’s core is balanced by gravitation – the fate of any planetary system it may have had is an enigma. Astronomers generally don’t know what happens to planets beyond this point, or whether they can even survive.

In a paper published recently 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 one potential explanation for the fate of these planetary remains.

The Helix Nebula is the closest planetary nebula to Earth and is located about 700 light-years away from Earth.

The Helix Nebula is an old planetary nebula – expanding, glowing gas ejected from its host star after its main-sequence life ended. 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, the astronomers first determined where it could not have come from.

The Helix Nebula was discovered by Friedrich von Hahn in 1811.

Collisions between planetesimals – small, solid objects formed out of cosmic dust left over from the creation of a planetary system around a star – can produce this type of excess emission, but SOFIA and ALMA failed to see the large dust grains required for such objects to exist, ruling out one option. The astronomers also didn’t find any of the carbon monoxide or silicon monoxide molecules characteristic of the gas disks that can surround evolving post-main-sequence stellar systems that precede objects like the Helix Nebula, excluding another potential explanation.

It is estimated that the even spread of dust found around the Helix Nebula might come from the destruction of two objects larger than Earth-sized.

Different strands of evidence place strict constraints on the size, structure, and orbit of the source of the emission, and eventually come together to identify the same culprit: dust – from full-fledged planets destroyed during the nebula’s formation – returning toward its inner regions.

"In piecing together the size and shape of the excess emission, and what those properties infer regarding the dust grains in the white dwarf environment, we conclude 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 on the paper and a researcher at Academia Sinica in Taiwan.

The mass of the white dwarf at the centre of the Helix Nebula is estimated to be about 50% of the sun's mass.

Once they realized the remnants of a former planetary system are at the origin of the infrared emission, they calculated how many grains need to be returning to the Helix Nebula’s center to account for the emission: about 500 million over the 100,000-year lifetime of the planetary nebula, conservatively.

SOFIA’s capabilities fell right into a gap between the previous Spitzer and Herschel observations, allowing the group to understand the shape and brightness of the dust, and improving the resolution of how far it spreads out.

The Helix Nebula spans 15 light years across and its inner part is expanding at 20 kilometers per second.

"This gap lay around where we expected the dust emission to peak," Marshall said. "Pinning down the shape of the dust emission is vital to constraining the proper size of the grains." .

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