What Is A Metal?

What a chemist sees:

What an astronomer sees:

Scientists among you will get the joke. For others, you should know that hydrogen and helium make up almost all the matter in the universe. It is thought that planet formation depends in part on the relatively teeny amount of heavier materials produced in stellar interiors and supernovae.

So UT-Austin astronomers finding planets circling a ten-billion-year-old star is a little surprising. (Though it’s not clear to me we should be surprised by anything we find in space.)

It’s thought that stars of generations preceding our sun were significantly lacking in elements heavier than helium. Ten or so billion years ago, fewer supernovae and giant stars had blown off their chemical products into the cosmos. (In other words, no iron cores, no silicate rocks, no sand, not even any oxygen, ice, or carbon for the most ancient baby solar systems.)

Star HD 155358, twice as old as our sun, contains one-fifth the quantity of “metals.” Is that enough to kick-start planet formation? Or are these big balls of hydrogen and helium without rock and metal cores?

Let the astronomers explain it for us:

“There are two competing planet-formation models,” (Michael) Endl said. Those models are known as the “core accretion model” and the “disk instability model.”

Both models start with a rotating cloud with a star forming at its center. As it rotates, the cloud flattens into a disk. Over time, dust in the disk begins to clump together to form the seeds that will eventually become planets. Where the two models differ is in terms of timescale.

In the core accretion model, a Jupiter-like planet forms in a two-step process. Over about a million years, a proto-planetary “core” several times the mass of Earth forms through gravitational accumulation of solid materials. When it reaches this mass, it has enough gravity to then pull huge amounts of gas onto itself. Over several million more years, it grows into a gas giant planet.

This model relies on large amounts of heavy elements to be present in the disk – and, of course, in the star- to form the cores, Endl said.

“Most of the planets found using the radial velocity technique are found around metal-rich stars,” he said. “That argues for the ‘core accretion’ model. Many astronomers in this field agree that the higher fraction of planets around metal-rich stars is supporting evidence for the core-accretion model.”

“Having this process happen to form not just one, but two, planets around a star that had so little solid material available for planet-building is quite remarkable.” William Cochran said.

The competing model of planet formation is called the disk instability model. It argues that the rotating disk of gas and dust around the forming star becomes unstable very soon after the disk forms, causes the disk to break into giant clumps. Gravity within each clump can cause the gas to collapse under its own gravity, forming giant planets in only several hundred years.

“Gas giant planets formed this way might not have any solid core at all,” Endl said.

Of course, a radical like me would ask why it has to be one way or the other. What if both models are in play?

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