Although they have provided a nurturing home for planets for billions of years, stars sometimes turn treacherous and eat their children. Over time, the evidence of this filicide sinks into the heart of a star, never to be seen again.
But now astronomers have found a way to catch the killer stars red-handed and figure out how long we have until the case cools.
When the stars are hungry
Stars can potentially kill (although astronomers prefer to use the more polite term “gobble up”) their planets at various stages in the life cycle of stars. At one end of the cycle, when typical solar-type stars are about to die, they swell and turn red, becoming giant or supergiant stars.
Related: Scientists have a gruesome look at how stars like our sun eat their own planets
When this happens, any inner worlds unlucky enough to be too close will be consumed. The outer planets of the system may also suffer, as the shifting gravitational landscape caused by the convulsions of dying stars can send planets hurtling into their parent star. This fate will befall our own solar system in about 4.5 billion years, when Sun will destroy Mercury, Venus and probably Earth.
But it’s not just fits of old age that can destroy a planet. This too happens when the stars are young. The first days of training a solar system are a particularly violent period. The protostar at the center increases in both temperature and density – but intermittently, occasionally causing massive plasma storms.
Meanwhile, the planets begin to evolve around the wayward star. Accumulating from smaller building blocks, planetesimals crash into each other, gravitationally destabilize and generally jostle as they attempt to become full-fledged planets. Naturally, all this commotion ejects some of the material from the system, while others flow into the still-forming star.
This influx of planetary building materials into the central star can be slow or fast. In some cases, a steady stream of heavy elements is heading towards the star for millions of years. In this case, it’s less a case of outright planetary murder and more of a slow throttling of the ingredients needed to build more or bigger worlds around a star. In other cases, an entire planet crashes directly into the star, completely disappearing in the blink of an eye.
The fierce energy and searing temperature inside a star is more than enough to completely destroy a planet. And that’s assuming the planet even survives the entrance and isn’t torn apart by gravitational tidal forces around the star as the planet passes by. In just a few years, an Earth-like planet would be completely consumed.
The only remaining evidence that a star has killed one of its planets is an extra abundance of metals, that is, in the context of astronomers, any element heavier than helium. And these are the elements – like silicon, oxygen and carbon – that planets need to grow.
Over time, sunken metals will slowly work their way into a star’s core, simply because these elements are heavier than the hydrogen and helium that make up a star’s vast mass. Astronomers can only tell what a star is made of based on what’s on the surface, because the surface is the only part of the star that emits light. It is from the spectral fingerprint of this light that scientists can determine what elements are inside a star. Thus, once the metals have crept into the depths of the star, it will successfully hide any evidence of its past crimes from the prying eyes of astronomers.
But how long does this cosmic concealment last? Early estimates, based on the simple scattering of metals in a star, indicated billions of years or more, meaning that any star we could observe could not be hiding.
But one new study submitted for publication in the journal Monthly Notices of the Royal Astronomical Society takes a more nuanced approach. In an attempt to establish a detailed accounting of the shelf life of evidence, the research team created dozens of simulated stars and simulated situations of how and when the the stars could eat their planets. In their simulations, the astronomers took into account that the metal-enriched material inside a star will have different densities than its surroundings and that this can create currents that carry these metals away faster than gravity alone would.
Of course, the answer is different for each star, as it depends on its mass and how much planetary matter it has consumed. But in general, astronomers have found that stars cling to metals on their surfaces for less than a billion years.
So, to find evidence of “planet-cide,” astronomers need to catch a star that’s less than about a billion years old. Also, they can’t just look at a single star in isolation, because it’s impossible to tell if that star ate a planet (or more) or was just born with that excessive amount of metals. Instead, astronomers must look binary systems in which a star has many more metals than its companion. In these limited cases – binary systems less than a billion years old – astronomers can finally catch the criminal.
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