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Astrotweeps: David @astrotweeps
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Good time-zone dependent morning, everyone! Today I'm going to talk about the research I did for my PhD project, looking at the remnants of planetary systems at white dwarfs. Lets talk about the end of the world(s)!
As I mentioned yesterday, white dwarfs are the left over cores of stars that have run out of hydrogen and helium to power nuclear fusion in their cores. Every star less than about 8 times the mass of the Sun will follow the top track here, ending up as a white dwarf.
What we're left with is a tiny, dense ball of carbon and oxygen (the products of helium fusion), surrounded by a layer of helium then, 80% of the time, a thin layer of hydrogen. There are some variations, but that's what your typical white dwarf is like.
But what about the planets? Do they survive the transformation of their star into a white dwarf? And can we detect signs of them now?

(The answer to both is yes, otherwise my thesis would have been rather short.)
In fact, the statement that actually got me interested in this area was my future supervisor speculating that, in 8 billion years time, someone in another star system could detect Mars transiting the white dwarf left by the Sun.
Why not Earth? Weeeeeeellll, that's the bad news
A couple of references before we go on: Schroder+Smith on how the evolution of the Sun effects the Earth… and if you want a real deep dive, here's Dimitri Vera's review…
As the Sun runs out of fusion fuel, the changes in its core will cause it to expand. By about 7.5 billion years from now, it will have reached the size of the Earth's orbit. Mercury and Venus will have been swallowed up, and the Earth...


It's uncertain.
The white dwarf left at the end of this will only have about half the mass of the Sun. Where does the rest of the mass go? As it expands, material begins to blow of its surface. Eventually this will form a planetary nebula. These have nothing to do with planets, but are pretty.
The steady mass loss will cause the Earth's (and other planets) orbit to expand. So the Earth will be slowly moving away from the Sun as the Sun expands towards it. Counteracting that will be increased tidal forces, drawing the Earth back in.
So the Earth's survival is dependent on three uncertain things: How much/how quickly will the Sun expand? How much/how quickly will it loose mass? How strong are the tidal forces?
So far the models suggest that the Earth is probably engulfed by the Sun, but its right at the point where the models aren't precise enough to give a definite answer. So it might survive. It will be a bit...melty...
But don't worry! This is all 7.5 billion years in the future! Life on Earth probably has a billion years left at most. We will all be long gone, along with any sign that we ever existed.

Enjoy your lunch!
Back to white dwarfs. After the Sun has completed its evolution, what we will be left with is a tiny white dwarf, then a huge cleared out gap, followed by Mars and the giant planets- all on orbits twice as large as before.
But what about other white dwarfs? Do they still have planetary systems? If they do, can we detect them, and can we learn new things about planets from them? To answer, let's go back 100 years...
screeching to a stop at this thread I wrote last year, which says everything I was going to say next. Thanks past me!
So building on that thread, here's a bit more about how we actually go about measuring the chemistry of the disrupted asteroids at white dwarfs. Our aim is to be able to measure the chemistry in detail and compare it to the Earth. Here's one I made earlier...
How do we get there? First, we need some spectroscopy! What kind depends on what elements we want to measure and where in the spectrum they leave their various fingerprints- which changes for white dwarf with different temperatures.
As you can see in the pie chart, Earth is mostly made of oxygen, silicon, magnesium and iron, so we want at least those four elements. Magnesium we can observe from the ground, but for the others we need ultraviolet spectroscopy, which means Hubble!
On that note, a minor tragedy: My PhD supervisor gave me this Hubble 25th anniversary chocolate when I was awarded my first Hubble observing program. It has not done well on its travels :(
Here are some example Hubble spectra of polluted white dwarfs. The wide dip on the left is absorption from hydrogen. All of the other lines are either bits of asteroids or interstellar material between us and the white dwarf.
(the blue and green lines are showing the positions of oxygen and carbon absorption, as those were the elements I was working on when I made that plot. From this paper:… )
Once we have the spectra, we need to fit them with model atmospheres. I let cleverer people than me do this bit. Essentially, we're trying to simulate the atmosphere of the white dwarf... find the right combination of temperature, surface gravity and abundance of each chemical that would produce a spectra like the one we see. Here's a pretty successful result- the red model atmosphere nicely fits the grey spectrum
Now we have the amount of each metal in the white dwarf atmosphere, we need one last thing - how quickly does each metal sink down into the white dwarf out of view? Say we have debris with the same amount of Si and Mg...
Mg sinks out more slowly than Si, so if we don't account for that then it will look like the debris had more Mg than Si.

Want to know sinking timescales for elements in your favourite white dwarf? The Montreal white dwarf database has a handy tool here
So with spectra, model atmospheres and sinking timescales, we've measured the chemistry of a poor, unfortunate asteroid that got too close to the white dwarf. What now? Well, as I showed earlier, we can compare it with the Earth and see if the chemistry is different
In this case, there's a lot more iron than the Earth. We interpreted as an object with a relatively large iron core compared to the Earth, something like asteroid Psyche or Mercury
On that note, one thing we really can't measure is how big the asteroids accreting on the white dwarfs are. Estimates range from ~10km to the size of Pluto! Of course they will be all different sizes, but knowing the size distribution would be helpful
Ideally though, we'd like to look at a large sample of debris measurements and compare it with the Solar system - exploring questions like what are rocky planets, on average, made of? What are the extremes? Is the Earth normal and does it matter for life?
My collaborators and I made the first stab at this a couple of years ago, with a search for "carbon planets". These are hypothetical objects where oxygen is swapped out for carbon as the main rock-forming element, and the tracer would be a C/O ratio greater than ~1 in the debris
(that's why I had C and O marked out in those spectra above. We didn't find any evidence for carbon planets. Which is good, as without oxygen its hard to make water- carbon planets would probably be lifeless.
eventually we'd like a full map of the chemical diversity of rocky exoplanets, in the same way that we have for meteorites in the Solar system. Hubble is busy observing more and more white dwarfs, so watch this space!
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