Butterfly Nebula (NGC 6302) Reveals Clues To Earth’s Creation

Clues about how worlds like Earth may have formed have been found buried at the heart of a spectacular ‘cosmic butterfly’.

With the help of the James Webb Space Telescope, researchers say they have made a big leap forward in our understanding of how the raw material of rocky planets comes together.

This cosmic dust – tiny particles of minerals and organic material which include ingredients linked to the origins of life – was studied at the core of the Butterfly Nebula, NGC 6302, which is located about 3,400 light-years away in the constellation Scorpius.

From the dense, dusty torus that surrounds the star hidden at the centre of the nebula to its outflowing jets, the Webb observations reveal many new discoveries that paint a never-before-seen portrait of a dynamic and structured planetary nebula.

They have been published today in Monthly Notices of the Royal Astronomical Society.

Most cosmic dust has an amorphous, or randomly oriented-atomic structure, like soot. But some of it forms beautiful, crystalline shapes, more like tiny gemstones.

“For years, scientists have debated how cosmic dust forms in space. But now, with the help of the powerful James Webb Space Telescope, we may finally have a clearer picture,” said lead researcher Dr Mikako Matsuura, of Cardiff University.

“We were able to see both cool gemstones formed in calm, long-lasting zones and fiery grime created in violent, fast-moving parts of space, all within a single object.

“This discovery is a big step forward in understanding how the basic materials of planets, come together.”

The Butterfly Nebula’s central star is one of the hottest known central stars in a planetary nebula in our galaxy, with a temperature of 220,000 Kelvin.

This blazing stellar engine is responsible for the nebula’s gorgeous glow, but its full power may be channelled by the dense band of dusty gas that surrounds it: the torus.

The new Webb data show that the torus is composed of crystalline silicates like quartz as well as irregularly shaped dust grains. The dust grains have sizes on the order of a millionth of a metre — large, as far as cosmic dust is considered — indicating that they have been growing for a long time.

Outside the torus, the emission from different atoms and molecules takes on a multilayered structure. The ions that require the largest amount of energy to form are concentrated close to the centre, while those that require less energy are found farther from the central star.

Iron and nickel are particularly interesting, tracing a pair of jets that blast outward from the star in opposite directions.

Intriguingly, the team also spotted light emitted by carbon-based molecules known as polycyclic aromatic hydrocarbons, or PAHs. They form flat, ring-like structures, much like the honeycomb shapes found in beehives.

On Earth, we often find PAHs in smoke from campfires, car exhaust, or burnt toast.

Given the location of the PAHs, the research team suspects that these molecules form when a ‘bubble’ of wind from the central star bursts into the gas that surrounds it.

This may be the first-ever evidence of PAHs forming in a oxygen-rich planetary nebula, providing an important glimpse into the details of how these molecules form.

NGC 6302 is one of the best-studied planetary nebulae in our galaxy and was previously imaged by the Hubble Space Telescope.

Planetary nebulae are among the most beautiful and most elusive creatures in the cosmic zoo. These nebulae form when stars with masses between about 0.8 and 8 times the mass of the Sun shed most of their mass at the end of their lives. The planetary nebula phase is fleeting, lasting only about 20,000 years.

Butterfly Nebula NGC 6302 (Hubble, Webb, ALMA) larger image

Contrary to the name, planetary nebulae have nothing to do with planets: the naming confusion began several hundred years ago, when astronomers reported that these nebulae appeared round, like planets.

The name stuck, even though many planetary nebulae aren’t round at all — and the Butterfly Nebula is a prime example of the fantastic shapes that these nebulae can take.

The Butterfly Nebula is a bipolar nebula, meaning that it has two lobes that spread in opposite directions, forming the ‘wings’ of the butterfly. A dark band of dusty gas poses as the butterfly’s ‘body’.

This band is actually a doughnut-shaped torus that’s being viewed from the side, hiding the nebula’s central star — the ancient core of a Sun-like star that energises the nebula and causes it to glow. The dusty doughnut may be responsible for the nebula’s insectoid shape by preventing gas from flowing outward from the star equally in all directions.

The new Webb image zooms in on the centre of the Butterfly Nebula and its dusty torus, providing an unprecedented view of its complex structure. The image uses data from Webb’s Mid-InfraRed Instrument (MIRI) working in integral field unit mode.

This mode combines a camera and a spectrograph to take images at many different wavelengths simultaneously, revealing how an object’s appearance changes with wavelength. The research team supplemented the Webb observations with data from the Atacama Large Millimetre/submillimetre Array, a powerful network of radio dishes.

Researchers analysing these Webb data identified nearly 200 spectral lines, each of which holds information about the atoms and molecules in the nebula. These lines reveal nested and interconnected structures traced by different chemical species.

The research team were able to pinpoint the location of the Butterfly Nebula’s central star, which heats a previously undetected dust cloud around it, making the latter shine brightly at the mid-infrared wavelengths that MIRI is sensitive to.

The location of the nebula’s central star has remained elusive until now, because this enshrouding dust renders it invisible at optical wavelengths. Previous searches for the star lacked the combination of infrared sensitivity and resolution necessary to spot its obscuring warm dust cloud.

The paper ‘How is cosmic dust, the raw material of rocky planets and a key ingredient for life, formed in space?’ by Mikako Matsuura et al. has been published in Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/staf1194. (open access)

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