Dying Star Devours Its Own Planet

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In the second episode of the Doctor Who rebooted series — starring Christopher Eccleston as the ninth doctor — the regenerated Time Lord takes his new “companion,” Rose, far into the future. Specifically, he takes her to the year five billion, to join a bevy of various alien leaders on board a space station, parked near the sun. They are there for a momentous occasion: the sun is about to expand into a red giant, and in the process, destroy the Earth.

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Science fiction? Well, yes, at least when it comes to the whole time travel and alien space station bit. But as far as the fate of the sun — and with it, the fate of our pretty blue planet — that’s firmly in the realm of science fact.

Astronomers have just announced the first observational evidence of an aging star destroying its own planet in the Astrophysical Journal.

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The sun’s energy comes from the nuclear reactions taking place in its core, specifically the conversion of hydrogen into helium. But that energy is not infinite. Eventually all the hydrogen will be converted into helium — in fact, as this article at Universe Today points out, that process speeds up, little by little, with a corresponding slight increase in the sun’s brightness, or heat:

In 1.1 billion years from now, the Sun will be 10 percent brighter than it is today. This extra energy will cause a moist greenhouse effect in the beginning, similar to the runaway warming on Venus. But then the Earth’s atmosphere will dry out as the water vapor is lost to space, never to return. In 3.5 billion years from now, the Sun will be 40 percent brighter than it is today. It will be so hot that the oceans will boil and that water vapor will be lost to space as well. The ice caps will permanently melt, and snow will be ancient history; life will be unable to survive anywhere on the surface of the Earth. The Earth will resemble dry hot Venus.

When all the hydrogen is turned into helium — roughly 6 billion years from now — the sun will no longer generate sufficient energy to counteract the pull of gravity. The core will contract inward, heating up the interior to sufficient temperatures to fuse helium into carbon. This spells doom for Earth, and all of its inhabitants, assuming any managed to survive that hot, dry Venus stage.

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As this new phase begins, the sun will puff outward and turn into a red giant, expanding its outer layers as the core collapses. Those outer layers will engulf the nearest planets, including Earth, which will plunge into the core and be vaporized.

The sun’s own lifetime will continue, however, until it ultimately cools into a white dwarf star.

In a trillion years, that remnant of our our sun will reach thermal equilibrium with the rest of the cosmos (just a few degrees above absolute zero).

That puffing-out stage, into a red giant, is what the Doctor Who episode depicts. And now an international team of astronomers has found evidence of a similar event when they studied a red giant star called BD+48 740.

They used spectroscopic analysis to determine the chemical composition of this star system, and found huge amounts of lithium. It’s a rare element, despite being one of the oldest (dating back to the Big Bang, 14 billion years ago), because it is usually destroyed by stars. So why is it so abundant here?

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Alexander Wolszczan (Penn State University) and his colleagues propose in their paper that the lithium was created when a planet-sized massive object spiraled into BD+48 740, heating up sufficiently while it was being “digested” to trigger the production of lithium.

It’s not just the spectroscopic evidence that points to the destruction of a planet. The team found another massive planet (larger than Jupiter) orbiting the star, with an equally rare highly elliptical orbit. This could also be caused by the destruction of a massive planet by the host star. That event would produce a burst of energy sufficient to toss the second massive planet into such a strange orbit.

“Catching a planet in the act of being devoured by a star is an almost improbable feat to accomplish, because of the comparative swiftness of the process,” co-author Eva Villaver (Universidad Autonoma de Madrid) said via press release. But astronomers can infer this happened by looking at stellar chemistry and orbits of other planets in the system

Or you can just park yourself on an alien space station billions of years from now, courtesy of a Time Lord, and wait. Just be sure you’re paying attention at the right moment.

Images: (top) A planet gets eaten by its red giant star. Credit: NASA. (bottom) The Sun photographed by the Atmospheric Imaging Assembly (AIA 304) of NASA’s Solar Dynamics Observatory (SDO). Credit: NASA/SDO