Exoplanets Are Sacrificing Moons to Their White Dwarf Overlords

An artist’s impression of a planet, comet and debris field surrounding a white dwarf star (NASA/ESA)

As if paying tribute, exoplanets orbiting white dwarfs appear to be throwing their exomoons into hot atmospheres of these stellar husks.

This fascinating conclusion comes from a recent study into white dwarf stars that appear to have atmospheres that are “polluted” with rocky debris.

A white dwarf forms after a sun-like star runs out of hydrogen fuel and starts to burn heavier and heavier elements in its core. When this happens, the star bloats into a red giant, beginning the end of its main sequence life. After the red giant phase, and the star’s outer layers have been violently ripped away by powerful stellar winds, a small bright mass of degenerate matter (the white dwarf) and a wispy planetary nebula are left behind.

But what of the planetary system that used to orbit the star? Well, assuming they weren’t so close to the dying star that they were completely incinerated, any exoplanets remaining in orbit around a white dwarf have an uncertain future. Models predict that dynamical chaos will ensue and gravitational instabilities will be the norm. Exoplanets will shift in their orbits, some might even be flung clear of the star system all together. One thing is for sure, however, the tidal shear created by the compact white dwarf will be extreme, and should anything stray too close, it will be ripped to shreds. Asteroids will be pulverized, comets will fall and even planets will crumble.

Stray too close to a white dwarf and tidal shear will rip you to shreds (NASA/JPL-Caltech)

Now, in a science update based on research published late last year in the journal Monthly Notices of the Royal Astronomical Society, astronomers of the Harvard-Smithsonian Center for Astrophysics (CfA) have completed a series of simulations of white dwarf systems in an attempt to better understand where the “pollution” in these tiny stars’ atmospheres comes from.

To explain the quantities observed, the researchers think that not only is it debris from asteroids and comets, but the gravitational instabilities that throw the system into chaos are booting any moons — so-called exomoons — out of their orbits around exoplanets, causing them to careen into the white dwarfs.

The simulations also suggest that as the moons meander around the inner star system and fall toward the star, their gravities scramble to orbits of more asteroids and comets, boosting the around of material falling into the star’s atmosphere.

So there you have it, planets, should your star turn into a white dwarf (as our sun will in a few billion years), keep your moons close — your new stellar overlord will be asking for a sacrifice in no time.

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