29Jul2018

A Dead Star Is Eating Its Own Planets

Astronomers have found a star that’s eating its own planets. That might sound weird, but trust me, this gets a lot weirder. The star was found in the massive database of observations taken by the European Space Agency’s Gaia telescope

Its a spacecraft that has been sweeping the sky since 2014, measuring with incredibly accuracy the positions, motions, and colors of something like a billion stars. Yes, billion.

By doing this it can measure the parallax of the stars, the apparent motion caused by the motion of the Earth around the Sun. This is actually how the distances to stars were first determined, but Gaia is doing it with unprecedented accuracy. With the distance, brightness, and colors of each star accurately found, a host of other properties can be found, including its mass, diameter, and more.

The star in question is called Gaia J1738–0826 (the name is based on its coordinates in the sky), and it’s a white dwarf. These kinds of stars are what’s left over after a “normal” star like the Sun dies. For billions of years such stars live out their lives, merrily creating helium by fusing hydrogen atoms deep in their centers. The helium builds up, inert, taking far higher pressures and temperatures to fuse itself.

Eventually the star runs out of usable hydrogen to fuse. By this time there’s so much helium that the pressure inside the star’s core is very high, and it gets very hot. This extra heat makes its way out of the core and into the star’s outer layers, which respond as any gas does when you heat it: They expand and cool. When this happens the star becomes what we call a red giant.

Artwork depicting a white dwarf that’s destroyed its inner planets, which are now raining down on it. Credit: Jon Lomberg / Gemini Observatory

Artwork depicting a white dwarf that’s destroyed its inner planets, which are now raining down on it. Credit: Jon Lomberg / Gemini Observatory

Because it’s so much bigger, it’s also a lot less dense. The outer layers feel less gravity from the star, but are also getting hit with a huge amount of light and heat from below. This causes the gas to start to flow away from the star in a stellar wind, like the solar wind from the Sun. After a few hundred million years, so much of the outer parts of the star have escaped that the core itself is exposed, hot and tiny, about the same size as the Earth. Mind you, roughly half the original star’s mass is squeezed into this ball, so it’s ridiculously dense, and has immense gravity on its surface. This is a white dwarf.

Here’s where things get even more interesting. If the star had planets in close orbits around it, they may have been cooked when the star became a red giant, literally finding themselves inside the star. They would have rapidly spiraled down even closer, accelerating their destruction. Even after the star’s envelope was blown away, the planets’ woes were not done. They now faced the intense gravity of the white dwarf. If they were close enough to this über-dense beast, the gravity of the star could rip them apart. What’s left is a belt of rubble, like an asteroid belt, orbiting the star, slowly falling down to the surface, where the debris gets eaten by the tiny dead star.

Sounds like sci-fi, right? Yeah, but we’re actually seeing this happen. White dwarfs are very dense, but they’re so hot the material making them up still acts in many ways like a gas. It can flow, so denser stuff on the surface will sink rapidly, leaving behind the lighter. White dwarfs like this are mostly helium with some hydrogen, and helium is heavier than hydrogen. This means the surface is in many cases almost pure hydrogen.

When we take a spectrum of such a star — breaking the light up into thousands of individual colors — hydrogen leaves behind a very easily identifiable fingerprint. But Gaia J1738–0826’s spectrum is contaminated: It has features clearly identified as being from calcium!

An artist’s rendering of a comet (which is very similar to an asteroid) getting torn apart near a white dwarf. Many white dwarfs have rings of dust around them as well, more evidence of a planetary system orbiting them. Credit: NASA/JPL-Caltech

An artist’s rendering of a comet (which is very similar to an asteroid) getting torn apart near a white dwarf. Many white dwarfs have rings of dust around them as well, more evidence of a planetary system orbiting them. Credit: NASA/JPL-Caltech

That’s a big deal. Calcium is much heavier than hydrogen, and would sink below the surface immediately. The fact that we see any at all means it must have a steady source of it falling onto the surface… and the most likely source of calcium is the debris from destroyed planets. In other words, the star is eating its own planets after tearing them apart, and we’re watching the meal.

The amount seen on the star implies the calcium is falling onto it at a rate of about 2.6 tons per second. That’s not a huge amount, but that’s just calcium. It’s only one element making up planets, and given the amount of calcium usually seen in planets, this implies a total consumption rate of all material of about 160 tons per second.

If that sounds like a lot, remember that planets are big. A star like this could eat them at this rate for a long, long time. In fact, the star probably died and became a white dwarf well over a billion years ago, so this has been a very long meal indeed.

Gaia J1738–0826 is not the first white dwarf seen snacking on its planets, either. Lots of them have been found… including van Maanen’s star, which is a great story: Its spectrum was found to have calcium as well as iron and magnesium in it too, but that was unexplainable at the time of its discovery in 1917. It would be decades before astronomers understood white dwarfs, and the implications of these elements in its spectrum. But, because of this, van Maanen’s star was actually the first star seen to have evidence of a planetary system (even though the star was eating them), 75 years before the first actual exoplanet was ever found!

So this is an interesting class of star scientifically, as well as just being incredibly cool. White dwarfs are a bit difficult to find in general because they’re intrinsically faint, and fade to obscurity with distance. Gaia, though, is sensitive enough to find many thousands of these dead stars, and a decent fraction of them (around 15%, from previous studies) may turn out to be only mostly dead. How many zombie stars slowly devouring their cooked and ground-up planets are still waiting to be discovered?

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