BAN #401: Baby planets around dying stars?

14 February 2022 Issue #401

February 14, 2022

Subscribers never die; they create new subscribers around them (hint hint)

Blog Jam

[Another planet for Proxima? From Thursday’s article. Credit: ESO/L. Calçada]

Astro Tidbit

A brief (or in this case not really so brief) synopsis of some interesting astronomy/science news

Well now here’s something I never would have thought of: Can planets form around dying stars?

That’s a weird thought. After all, we know planets form around young stars: A cloud of gas and dust collapses, forms a swirling disk, material falls to the center and creates a star, and material around it clumps up to form planets. That was a hypothesis for a long time but now we physically see this happened all over the place. We have tons of evidence that this formation scenario not only works but actually happens.

However, that doesn’t mean that’s the only way they can form. We also know they can form around stars that have literally exploded as supernovae, condensing from the material that wasn’t blown away. But what about stars more like the Sun, that die in less catastrophic circumstances? They can have disks of material around them when the star becomes a red giant, so can planets form from them?

A team of astronomers thinks it may be possible, and that they may have evidence of it, too (link to paper). To be clear they can’t prove planets form from these disks, but instead show that they might. Still, this is pretty cool.

[Diagram of a binary system with one star already having become a red giant and blown a disk of material around the two; a gap or cavity in the disk could be due to a planet there eating up the dust. Credit: N. Stecki]

When a star like the Sun — say, stars from roughly half its mass up to about 8 solar masses — runs out of hydrogen to fuse, its core contracts and heats up. This extra energy gets dumped into its outer layers, which expand and cool. It cools because even though more energy is being generated, there’s so much more volume that the energy per cc drops and it cools off. The star becomes a red giant.

When the core gets hot enough it starts to fuse helium, which doesn’t generate as much energy, so the outer layers contract and heat up. After a while it runs out of helium, and once again the core gets extremely hot, and the star swells up again. This time though a lot more energy is dumped into the outer layers, so they swell up even bigger. This kind of star is called an asymptotic red giant or an asymptotic giant branch star due to the way they appear on the Hertzsprung-Russell diagram, a graph used to measure the evolution of a star.

After that star ejects a lot of its outer layers, the core is exposed to space, and now called a white dwarf: It just kind sits there being hot and tiny and slowly cooling over the next hundred billion years or so.

[Actual observation of rings of dust around the very young star AS 209 which indicates a planet is forming there. Credit: ALMA (ESO/NAOJ/NRAO)/ D. Fedele et al.]

But, if the star is a binary system, in a decently close orbit around a second star, things are different. The revolution of the two stars around each other is a source of centripetal acceleration (or centrifugal force, which is essentially the same thing). That’s a force outward in the plane of their mutual orbit. So when the dying star starts blowing off its outer layers, that material gets ejected preferentially in that plane… creating what can be a fairly dense disk of material around it. The star is no longer an asymptotic giant branch star at this point since its outer layers are gone, so it’s called a post-asymptotic giant branch star. Makes sense.

Here’s where the new work comes in. The material ejected is made of both gas and dust; the dust is made up of teeny grains of silicaceous or carbonaceous material. Rock and soot. The dust is warmed by the star and emits infrared light. We know how much infrared light should be coming from a star, but sometimes we see a lot more infrared than expected. Astronomers call this an IR excess, and it’s a dusty disk’s calling card. And it’s been seen around a lot of post-asymptotic giant branch stars (called post-AGB stars for short). Dust closer in is warmer, and dust farther out is cooler, and so the color of infrared light we see from such a star depends on the distribution of the dust in the disk.

In about 10% of these post-AGB stars with IR excess, we don’t see nearly as much emission from the hotter dust. This implies there isn’t as much dust close in to the star. We see disks like these around young stars, where there’s a wide gap between the star and the inner edge of the disk. There are a couple of ways these can form, but one way is to have a planet in that gap, sweeping up material as it forms.

[Artwork of a planet forming in a star’s disk. Credit: NAOJ]

That’s interesting. Could this mean that the post-AGB stars with gaps in their disks have planets there? Yes! In fact, studies have shown that a big gas giant planet can create higher gas pressure in the disk outside its orbit. The details are complicated, but this can trap dust grains there. Dust from the inner disk gets blown out by the star, then is trapped by the planet to be farther away. However, gas is still free to flow, and the gas in the inner disk flows onto the star. This gas is mostly hydrogen and helium so when it falls back onto the star it increases the amount of these light elements, and this is exactly what’s seen in these post-AGB stars themselves: Lots more lighter elements and much fewer heavy ones than you’d expect.

Again, evidence that there’s a planet there messing things up. But — and this is a big but — that planet may have formed when the star was young, and just stuck it out long enough to affect the disk.

What the scientists doing this work think, though, is that it’s possible this planet formed after the star started dying. There’s enough material in the disk to easily make a planet, and the disk can last for a few hundred thousand years, and recent work indicates that’s enough time to form a massive planet. And even if the planet was formed long before, the increased gas pressure in the outer disk can still form a new planet.

Either way, new planet. Around a dying star. Whoa.

So that’s what I meant earlier; they’ve found circumstantial — circumstellar, haha — evidence a planet can form in the disk of a post-AGB star, but it’s difficult to prove they actually do.

One way to figure that out is to go and get very high resolution images of these disks, perhaps using ALMA, a telescope that is very good at get exquisite observations of warm dust. If a planet is forming farther out ALMA might actually be able to see it. Observations like that haven’t been done yet, but I would be very interested indeed to see them.

If the astronomers are right it would be pretty dang nifty. Baby planets around a star dying of old age! Amazing. The science fiction possibilities alone are cool.

We see planets around all kinds of stars, from tiny red dwarfs up to gigantic powerhouses. We see them in binary systems, and maybe even in trinary systems with three stars. As I said earlier we also see them orbiting pulsars, the remains of long-dead massive stars that exploded as supernovae; the last place in the Universe I’d expect to find any.

The point is, planets really seem to like forming wherever the circumstances are even close to letting them do so. Maybe even around post-AGB stars, and maybe soon we’ll have proof.

Et alia

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