Despite providing a nurturing home for planets for billions of years, stars sometimes turn treacherous and eat their children. With time, the evidence of that filicide sinks into the core of a star, never to be visible again.
But now, astronomers have found a way to catch murderous stars red-handed and figured out how long we have until the case grows cold.
When stars get hungry
Stars can potentially kill (although astronomers prefer to use the more polite term "engulf") their planets at a variety of stages in the stars' life cycles. At one end of the cycle, when typical sunlike stars are about to die, they swell and turn red, becoming giant or supergiant stars.
When this happens, any inner worlds unlucky enough to be too close will be consumed. The outer planets of the system may suffer as well, as the shifting gravitational landscape caused by the dying stars' convulsions can send planets careening into their parent star. This fate will befall our own solar system in about 4.5 billion years, when the sun will destroy Mercury, Venus and likely Earth.
But it's not just fits of old age that can destroy a planet. It also happens when stars are young. The early formation days of a solar system are an especially violent time. The protostar at the center grows in both temperature and density — but fitfully, occasionally throwing off massive tantrum storms of plasma.
Meanwhile, the planets begin evolving around the temperamental star. Accreting from smaller building blocks, the planetesimals crash into each other, gravitationally destabilize each other and generally roughhouse as they attempt to become full-fledged planets. Naturally, all this commotion ejects some material from the system, with other material flowing into the still-forming star.
A tale told of metals
This influx of planet-building material into the central star can be slow or fast. In some cases, a steady trickle of heavy elements makes its way to the star over millions of years. In that case, it's less a case of outright planetary murder and more like a slow strangulation of the ingredients needed to build more or bigger worlds around a star. In other cases, an entire planet crashes right into the star, disappearing completely in the blink of an eye.
The ferocious energy and searing temperature inside a star are more than enough to completely destroy a planet. And that's assuming the planet even survives entry and isn't torn to shreds by the gravitational tidal forces around the star as the planet passes close. Within only a few years, an Earth-like planet would be entirely consumed.
The only remaining evidence that a star killed one of its planets is an extra abundance of metals, meaning, in astronomers' context, any element heavier than helium. And these are the elements — like silicon, oxygen and carbon — that planets need to grow.
Over time, the engulfed metals will slowly slink their way to the heart of a star, simply because those elements are heavier than the hydrogen and helium that make up the vast bulk of a star. Astronomers can only determine what a star is made of based on what's on the surface, because the surface is the only part of the star to emit light. It's from the spectral fingerprint of that light that scientists can figure out what elements are inside a star. And so once the metals slink into the depths of the star, it will successfully hide any evidence of its past crimes from astronomers' prying eyes.
But how long does that cosmic cover-up take? Early estimates, based on the simple diffusion of metals within a star, said billions of years or more, meaning any star we could observe wouldn't be able to hide.
But a new study submitted for publication in the journal Monthly Notices of the Royal Astronomical Society takes a more nuanced approach. In an attempt to build a detailed accounting of how long the evidence can remain, the research team created dozens of simulated stars and simulated situations of how and when the stars could eat their planets. In their simulations, the astronomers accounted for the fact that metal-enriched material inside a star will have different densities than its surroundings and that this can set up currents that drag those metals down faster than gravity alone would.
Naturally, the answer is different for every star, as it depends on how massive the star is and how much planetary material it consumed. But in general, the astronomers found that stars hang on to metals in their surface for less than a billion years.
Thus, to find evidence of "planet-cide," astronomers have to catch a star younger than about a billion years old. Also, they can't just look at a single star in isolation, as it's impossible to tell if that star ate a planet (or more) or was simply born with that excess amount of metals. Instead, astronomers must look at binary systems in which one star has significantly more metals than its companion. In those limited cases — binary systems that are younger than a billion years old — astronomers can finally catch the criminal.