Cosmic 'accident' explains why we don't see silicon on Jupiter and Saturn

For years, scientists wondered why silicon, a common element, rarely shows up in the upper atmospheres of giant planets. The answer is taking shape thanks to a strange neighbor that sits about 50 light years away, roughly 294 trillion miles from Earth.

That object is a brown dwarf nicknamed The Accident, a body that is heavier than a planet yet too light to act like a star.

It is unusual in many ways, and it turned out to hold a missing piece about silicon in worlds like Jupiter and Saturn.

What the new evidence shows

Lead author Jackie Faherty, an astrophysicist at the American Museum of Natural History (AMNH), and colleagues used the James Webb Space Telescope (JWST) to examine the object’s atmosphere, and they reported a clear detection of silane in a peer reviewed study.

The Webb spectrum shows a strong feature near 4.55 micrometers with an abundance near 19 parts per billion, enough for a firm identification without exotic assumptions.

That detail matters because silane had long been predicted to appear in giant planet atmospheres but had remained out of reach. Seeing it here links chemistry to the history of the object rather than to a single odd observation.

“Sometimes it’s the extreme objects that help us understand what’s happening in the average ones,” said Faherty.

Her team laid out how composition, clouds, and vertical mixing shape what shows up near the top of a giant world’s atmosphere.

Why silicon hides in gas giants

In atmospheres where oxygen is plentiful, silicon quickly combines with it to make heavier silicates that form deep clouds.

Those clouds sink below lighter layers, so silicon near the top is hard to spot with telescopes that gather infrared light.

That path keeps silane scarce in cool giants like Jupiter and Saturn, and it helps explain past blank searches on distant exoplanets.

The new detection shows what appears when that oxygen pathway slows down or runs short.

Silicon does not vanish, it changes partners. When oxygen ties it up into silicate grains, molecules that contain silicon are locked away far below the layers we can observe.

Thin supply of oxygen

The Accident likely formed 10 to 12 billion years ago, when the universe had fewer heavy elements than it does today.

With less oxygen in its atmosphere, more silicon was free to pair with hydrogen, so silane had a better chance to sit higher where Webb could see it.

The same analysis indicates the object’s heavy element content is far below the Sun’s, and its motion points to the Milky Way’s halo.

Age and composition fit the picture of a world that began with a thinner stock of oxygen bearing elements.

Brown Dwarfs as test cases

Brown dwarfs are often easier to observe than distant giant planets because there is no nearby star to drown out their light.

They also sit in a middle zone of mass and temperature that overlaps with conditions on many giant planets scientists want to understand.

These objects are not massive enough to sustain hydrogen fusion in their cores, so they cool with time, a point described by NASA.

That cooling, paired with a lack of glare from a host star, gives observers a clear view of the gases and clouds in their atmospheres.

The Accident first came to light in 2020 in the Backyard Worlds: Planet 9 project, where volunteers scan infrared data for moving objects. Dan Caselden spotted the source and follow-up work identified unusual colors and motion.

Early observations hinted that it was very faint and low in heavy elements compared to typical brown dwarfs. Those traits motivated later use of Webb to read out the spectrum in detail.

Silicon and future planet hunts

Detecting silane in one object does not mean it will appear everywhere.

It does show that silicon chemistry depends on how much oxygen and other elements were present when a world formed, and on how gases move between deeper and higher layers.

This new result also gives observers a specific target, a feature near 4.55 micrometers, and a ballpark abundance near a few dozen parts per billion.

Future work can scan other old, low metallicity objects for the same signature and then compare those spectra to what we see on Jupiter and Saturn.

Finding or not finding silane will tighten the models that predict which molecules rise and which sink in giant worlds. The work here turns a curious outlier into a guide for reading the hidden layers of planets across the galaxy.

The study is published in Nature.

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