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How exoplanets can violently die

Like the Greek god Cronus, some stars are not kind to their children.
WolszczanRedGiantPlanet
Red giant stars can expand far enough to swallow their planets; this artist's impression shows a doomed Jupiter-like world as its growing star fast approaches.
NASA

Like an aging relative, a planet can suffer a slow decline, spiraling gradually into its star. Or its death can be quick, a relative eyeblink as its sun abruptly changes. Understanding these deaths can help scientists better glimpse the life and evolution of individual planets, as well as how long-lasting other systems might be.

The most recent addition to the dying planets club is KELT-16b, a world nearly three times as massive as Jupiter orbiting its sun in less than one Earth-day. The planet is one of only six worlds with extremely close orbits lasting less than a day that can be observed moving between Earth and their bright host stars, making them vulnerable to strong tidal forces.

“What makes KELT-16 unusual is that we have a very precise measure of the system age and state evolution, and so we can pinpoint when this is likely to occur,” Keivan Stassun of Vanderbilt University in Tennessee says by email. Stassun and his colleagues identified the hapless world, which will probably be shredded in the next half million years. “It is imminent on astronomical time scales.”

The end of the world

While thousands of worlds have been spotted around other stars in the last decade, only a handful have been glimpsed at the end of their lifetime. Most of them are spotted at the end of a star’s lifetime, their material scattered across the stellar surface. Others have been found in the middle of their stories, during the bulk of their star’s lifetime, remaining stable over long timescales.

“What we are missing is the beginning of the story: planets like KELT-16b when the systems are very young and the host star is still extremely hot,” Stassun says. While such observations include worlds that will eventually become stable, they will also include those that didn’t make it and wind up quickly vaporized by their sun.

Enter the Kilodegree Extremely Little Telescope (KELT). With one instrument in Arizona and another at the Sutherland Astronomical Observation Station in Africa, the project is dedicated to hunting for exoplanets around bright stars. When it comes to finding planets orbiting hot young host stars, “the KELT project is hot on the trail, and it will not be long before such examples are discovered,” says Thomas Oberst, lead author on the paper announcing KELT-16b. The research was published in the Astronomical Journal.

Orbiting close to their stars, hot Jupiters are the first in line to be destroyed. If the planet is less dense than its star, its material can move to the sun over the course of its lifetime. If the world is more than five times denser than the star, however, it can wind up swallowed whole, according to Oberst. Figuring out how long it takes for the star to pick its planet apart can be “stubbornly difficult to measure,” he says.

Understanding planets whose orbits are shifted can help astronomers to better understand the star itself, as well as how many hot Jupiters can form early in the life of a planetary system.

“Although we now have many examples of what solar systems can look like, a complete picture requires understanding how often planets don’t survive,” Stassun says. “In other words, we need information about ‘planetary mortality’ in order to make complete sense of the planet census.”

Destroyer of worlds

Not all dying planets are killed off by close orbits. Some suffer from the aging process of their star. Near the end of its lifetime, stars like the Sun swell up into massive red giants that consume the closest planets and shift the orbits of those farther away. When the Sun goes through the process in 5 or 6 billion years, it will quickly devour Mercury and Venus. Whether or not Earth will be immediately consumed remains under debate, but if it survives, its dance along the edge of the Sun will render it uninhabitable.

Destroyed planets “should be pretty common,” Eva Villaver, of the Autonomous University of Madrid, says by email. “This will happen especially for those planets in close orbits.”

In 2012, Villaver and her colleagues identified debris in the composition of a red giant star that revealed that it had recently consumed one of its planets. They also found a surviving planet whose eccentric orbit suggested that it once had a companion.

Eventually, red giant stars shed their outer layers and shrink down to white dwarfs, no longer undergoing fusion but still warm. Because white dwarfs have hydrogen and helium surfaces, heavier elements quickly sink down. When astronomers spot other elements on the surface of a white dwarf, they know it can’t be from the dying star. Instead, it allows them to see the inside of planets and debris leftover.

But in 2015, astronomers received their first glimpse of a planet around the white dwarf WD 1145+017. By using a process known as the transit method, they found debris passing between Earth and the dying star as it orbited. Material on the white dwarf suggested that the object, smaller than Ceres, was being shredded and dumped onto the star.

The tiny world probably started out far from the host star. As the star transitioned into a red giant and again into a white dwarf, the changes would have affected the orbits of any surviving planets, which in turn tossed the planet inward. When Andrew Vanderburg, a graduate student at the Harvard-Smithsonian Center for Astrophysics, spotted the dying world, it was in the process of being vaporized, part of its material already pulled into the white dwarf.

“We have known for about a decade that planets probably are disrupted by white dwarfs,” Vanderburg says via email. “But transiting objects around WD 1145+017 were the smoking gun for this theory.”

Like hot Jupiters in the disruption process, material found on white dwarfs can provide insights into the futures of most planetary systems.

“The process we see happening around WD 1145+017 is most likely very common,” Vanderburg says via email. “We see evidence for this kind of thing happening in 30 to 50 percent of all white dwarfs, which means that this process of small rocky bodies being kicked inwards from distant orbits around dead stars and being crushed, vaporized, and accreted onto the white dwarfs is almost universal. This is the end fate of almost all planetary systems.

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