Apr 21 2020

The Strange Interstellar Comet

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Is our solar system similar to other solar systems? That’s actually a complex question with many layers. We know that there are different types of stars, varying mainly on their mass and age. We have a yellow sun, but a system around a red, orange, or blue sun is likely to be very different. We also know that at different relative locations in the galaxy the composition of the gas clouds out of which stellar systems form can be very different. One specific difference is known as “metallicity” – which refers to the amount of elements heavier than hydrogen or helium. Older stars were formed before a lot of heavier elements were made, so they have lower mellacity. This feature also varies within our galaxy, with higher metallicity closer to the center. And different galaxies have different mettalicity.

But what should we expect from a stellar system with a yellow sun at a similar location in our own galaxy? If the known variables are the same, should we expect the compositions of elements to also be roughly the same? This gets to the deeper scientific question of how typical our system is. Can we assume that the rest of the universe is similar to our tiny little corner of it? To counteract the hubris of humanity in thinking that we are somehow special, scientists try to follow the principle to assume that we are ordinary. But is that assumption always correct?

How can we even answer this question for stars that are light-years away? The primary method that we use is spectral analysis  (spectroscopy) Рan awesomely powerful tool that allows us to identify specific elements and chemicals simply from analyzing the light we see from it. You can do a spectral analysis in a lab on a sample, or you can do it with telescopes on distant objects. The method is actually fairly simply. You use a prism to spread out the light into its color spectrum (like a rainbow). You can then analyze the emission lines or absorption lines which are like a signature.

If you have pure white light, that will create a spectrum of EM radiation at every frequency from infrared to ultraviolet (depending on the source). When liquids and solids are heated enough they glow in a broad spectrum But when gases glow then tend to emit in very specific frequencies, depending on their chemical composition. These are called emission lines. Gases also will absorb specific frequencies of light that passes through them, leaving behind black absorption lines in the spectrum of the light.  So we can look at the light from a star and tell its chemical composition, but we can also look at the light passing through an interstellar cloud of gas and tell its composition Рeven across the observable universe. Pretty cool.

But if we want to know more about distant systems we are limited by that vast distance. We won’t be visiting other systems anytime soon. However, perhaps on occasion a bit of a distant stellar system may come to us. In 2017 astronomers detected the first confirmed interstellar object, Oumuamua. We could tell it was interstellar from its trajectory. At first astronomers thought it might be a comet, but when it got closer to the sun it did very little outgassing, and so was eventually classified as an asteroid (although a comet-like asteroid). This was the first an only interstellar encounter, and the question was – how common are they? We won’t know until we find others.

Then, in 2019, just two years later, we found a second interstellar object, 2I/Borisov. This one turned out to be a full comet, which was exciting. Comets are balls of ice, rocks, and dust. As they get closer to a star they heat up, and the ice sublimates into gas. We can then use spectroscopy to tell what kind of gas it is, and this should tell us something about the comet’s composition. Just as stars may differ depending on where they form withing a galaxy, objects orbiting a star may differ depending on where in the system they form. Star systems form out of a swirling cloud of gas, but that gas cloud is not uniform. The farther out from the center you get (especially after the star ignites) the colder it gets. This allows substances with higher melting points to stay as ice.

So we can tell roughly where a comet formed based upon its composition. But do these rules also apply to alien comets? Well, so far we have one point of datum – comet Borisov. What the astronomers have found, however, is interesting. Two teams, actually, independently studied Borisov and came to similar conclusions. They found it has a very high carbon monoxide (CO) content, 9-26 times higher than comets from our own system. What does this mean?

Well, CO is very volatile, so it sublimates quickly when comets get close to stars. So any comet with high CO content must have formed in the outer reaches of the system and stayed there. Perhaps it is coming in to the inner solar system for the first time, and has not yet lost its CO. But Borisov has a really high CO content, even higher than comets from our own system that have high CO. Why is this? We don’t know for sure, but it suggests that Borisov formed in the outer reaches of its system, perhaps in the equivalent of their Oort cloud or the farther parts of their Kuiper belt, and then was knocked away by interaction with something else, perhaps a dwarf planet or passing star. Borisov then wondered the galaxy for millions or perhaps even billions of years before straying into our system.

Perhaps Borisov comes from a red dwarf system, which makes sense for two reasons. First, there are more red dwarf out there than any other kind of star, by a large margin. About 80% of stars in our galaxy are red dwarfs. So, just playing the odds, it’s a pretty good bet it came from such a system. But it makes sense for another reason – red dwarfs are colder, so perhaps in general their comets have a higher CO content than comets that form around yellow suns. We have just one tiny glimpse, one puzzle piece, and so it is hard to form any firm conclusions. But these are reasonable hypotheses.

As an aside, Borisov also has a high hydrogen cyanide (HCN) content, but typical for comets in our system as well. This reminded me of the visit of Halley’s comet in 1910. At that time astronomers were able to detect using spectroscopy that the coma and tail of Halley’s comet contained HCN. This led French astronomer Camille Flammarion to surmise that the hydrogen cyanide would become infused into the Earth’s atmosphere and wipe out all life on Earth. This caused a bit of a panic, including panic buying of gas masks and exploitation by countless hucksters selling anti-comet pills. Scientists at the time, including the famous Percival Lowell, had to point out that the hydrogen cyanide was so rarified it was almost a vacuum, and posed no threat. But not everyone listened to the scientists over the charlatans.

All of this sounds distressingly familiar, over a century later.

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