As our sun dies, it will explode outward and destroy Mercury, Venus, and Earth. But the solar system could rebuild, with new planets cobbled together from the scraps of those dead worlds.
When sun-like stars exhaust their fuel, they expand outward into a type of star known as a red giant, which in turn eventually expels all its hydrogen and helium. That leaves behind a white dwarf, a planet-sized mass of electron matter.
But then, something weird happens – all that planetary material doesn’t just burn away. Often, it forms a disc of debris close to the white dwarf. at Pennsylvania State University and his colleagues found that new planets could arise out of that orbiting material.
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“You’re more likely to find something the size of the moon or Mars rather than the size of the Earth,” he says. But the larger the planets are that get destroyed, the larger their progeny could be.
Just enough dust
The team built a model based on studies of how materials clump in the rings of Saturn, because the debris discs around the white dwarfs resemble Saturn’s system, albeit with more rocky material than ice.
When there’s enough debris, the dust clumps up into increasingly larger objects and migrates outward. At a certain point, an object might gain enough mass to move out to its own orbit instead of slipping back toward the so-called Roche limit where the star rips apart any object that comes too close.
In theory, a destroyed super-Earth would have enough mass to create about an Earth-mass planet. “Naively, I would expect these planets to look pretty similar to the precursor planets, really,” Shannon says. Smaller, Earth-sized planets might be destroyed and become something close to Mars or the Moon in size.
While the white dwarf could have a habitable zone – a region where temperatures wouldn’t freeze or fry any possible life – it would lack the kind of gases that make up atmospheres. That would limit the amount of water on these remade worlds, though some could still arrive on comets.
Monster worlds
Shannon says this lack of gases could lead to an unusual condition: if there were enough debris from several destroyed planets, it could combine to make something up to about 10 Earth masses. In most circumstances, anything that big would be a gas giant a little smaller than Uranus. But such a planet could end up rocky simply because there’s no gas to draw in as an atmosphere – something that is unlikely around normal stars.
The K2 Kepler mission identified at least one dead star, called WD 1145+017, that has larger asteroids in its debris disc. This could mean that the white dwarf is taking the first step toward making its Frankenstein’s monster planets.
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Read more: Saturn’s moons could reassemble after a cosmic smash-up
