Planet orbiting sun-like star suffers a sad fate |

Planet orbiting sun-like star suffers a sad fate

Ice is different from water because the molecules in a solid are bound together more strongly than they are in a liquid. Although a foothills lake may freeze over, it doesn’t stay frozen forever. Sooner or later, warm weather arrives. As the ice warms, the molecules within it vibrate faster and faster. Eventually they vibrate so fast they can’t hold together, and the ice melts.

The nucleus of an atom is composed of protons and neutrons, bound together. If the nucleus gets hot, it can vibrate so violently that the nucleons (protons and neutrons) can’t hold together, and the nucleus falls apart.

The force that holds nucleons together is over 100 million times stronger than the force that binds molecules in ice (which is why nuclear weapons are so much more powerful than chemical weapons; physicists call it “the strong force”), so the temperature needed to destroy a nucleus is higher than that needed to melt ice – the sort of temperature found in the heart of a star.

Just as ice melts at a lower temperature than iron, some nuclei fall apart at lower temperatures than others. One especially easily broken nucleus is a variety of the metal lithium. The reactions that produce sunlight in the center of the sun produce copious amounts of “lithium-six,” (Symbolized as 6Li, it’s called lithium-six because it has three protons and three neutrons, for a total of six nucleons).

Lithium-six cannot survive the 27 million-degree Fahrenheit temperature at the sun’s core.

Outside the core, the sun is cooler. Some distance out from the center, the sun begins convecting, the hotter stuff floating upward, cooler stuff sinking down.

If it hasn’t been destroyed, lithium-six can escape the sun’s hot center by convecting upward. Once at the surface, such a nucleus can be spewed out by the solar wind that blows off the sun. Blowing through the solar system, such nuclei collect within the gaseous bodies of the larger planets such as Jupiter and Saturn, where they can be stored, as if in a freezer, more or less forever.

With minor exceptions, we’re pretty much limited to studying the sun as it is right now. To understand how our sun might act if it were, say, older or younger, astronomers search the galaxy for other “sun-type” stars. Sun-type stars provide clues to how much brighter or dimmer our sun might have been in the past, or might be in the future.

Astronomers have never found 6Li in any sun-type star. Until now.

In a report in the journal Nature last spring, astronomers in Spain and Switzerland described a sun-type star whose spectrum (the rainbow made from its light) displayed the colors in which lithium-six is known to glow. But how could the easily-broken nucleus have survived inside the star?

It hadn’t. Rather, the astronomers reasoned, the 6Li had been added to the star.

As they revolve around their star, planets pull on one another, distorting their orbits. A planet might be tugged outward, away from its star, or it might be sent sailing inward.

Some time in the past, the report’s authors suggest, a planet rich in 6Li was sent spiraling inward, so close to its star that the star’s gravity was able to pull it all the way in. In the words of the report, the unfortunate planet, lithium-six and all, was “ingested.”

Alan Stahler is an amateur astronomer and amateur naturalist who trained as a biologist, gives private lessons, and teaches biology and geology at Bitney Springs Charter High School. His radio science programs can be heard at noon on alternate Tuesdays on KVMR, 89.5 FM. You can write him in care of The Union, 464 Sutton Way, Grass Valley, 95945.

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