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For a long time, astronomers believed that our second alien interstellar visitor*, Comet 2 / I Borisov, was rather boring: it looked pretty much like every little comet in our own solar system.
However, now it seems that it was special: it was a virgin, almost completely unchanged since its formation, which means ours was the first solar system he visited closely.
Isn’t that nice?
2 / I Borisov was discovered by amateur astronomer Gennady Borisov at the end of August 2019, and it wasn’t long before it was deemed very interesting: literally alien, a comet from another star who had spent countless years traveling through interstellar space before passing the Sun.
It passed the Sun in December 2019, about 300 million kilometers from it, or just inside the orbit of Mars, before returning to a hyperbolic orbit, a path that is unrelated to the Sun ( unlike planets which have orbits or comets which can be almost parabolic). Astronomers have used telescopes around the planet (and above) to observe it, hoping to see something weird, something weird about it – that’s from another star – and what they found is … it’s a comet.
I mean, it looked like any other comet in our own solar system. If it hadn’t been howling on an interstellar path, it wouldn’t have been special at all.
It was a bit disappointing. I was hoping for kryptonite, or vibranium, or at least a small protomolecule.
However, this is what direct images and spectra have told us. It proves polarization tells a different story.
As I recently described for a very different type of object (a supermassive black hole engulfing Earth’s mass or two every day):
Light can be thought of as a series of waves carrying energy away from an object. Normally, these waves are oriented at random; if you imagine one like the string of a guitar, you can pluck it to vibrate up and down, or left and right, or at any angle.
Some phenomena, however, send out aligned waves – like strumming a guitar, the strings all vibrate horizontally. It is said that the light waves thus aligned are polarized.
You may already be familiar with its concept: light reflecting off metal or glass can be polarized. Polarized sunglasses have filters that contain molecules all aligned in one direction, so that when polarized light reflected from horizontal surfaces hits them, it gets blocked. Only polarized light aligned with the molecules passes through.
Dust also polarizes light. The solid part of the comet is called core, which are basically frozen ice cream and dust. As it gets closer to the Sun, it heats up, ice turns into gas, and dust is blown from the core with it, forming the hazy coma and the long tail.
The sunlight reflected from these tiny dust grains is polarized, which can say a lot about the grains, which in turn tell us about the comet’s makeup.
Observing Borisov with the Very Large Telescope, the scientists found that the polarization of the dust was strange. The way the light was polarized in different colors didn’t match the way any other known comet in the solar system polarizes light … except one: Hale-Bopp.
Hale-Bopp was a huge comet (nucleus 30-40 km wide; Borisov is less than 1 km) that passed through the inner solar system in the 1990s. It was very bright, easily visible to the naked eye and has been extensively studied. Its structure and composition indicated that it had originally formed very far in the solar system, at the edge of interstellar space, and that at most it had only passed through the system once. internal solar. It was a virgin comet.
And Borisov matches it. This indicates that she is also a virgin, likely ejected into the galaxy by an encounter with a giant planet in her own home solar system before she had a chance to get close to her star. It also means that it probably did not pass close to other stars until it was close enough to our Sun.
Other observations of the comet have also revealed something else. Most comets emit roughly the same mass of dust as gas when they heat up, kilogram per kilogram. But when Borisov passed the Sun, it was blowing about 200 kilograms of dust per second, while emitting only about 60 to 70 kg / s of gas. Such a high dust-to-gas ratio is extremely unusual for local comets, but it corresponded to one. Yeah. Hale-Bopp.
Incidentally, the amount of gas and dust thrown by Borisov as it passed the Sun means that it shrank by about two meters. I think that’s just a good thing to know.
This next part is subtle. Using the ALMA millimeter / submillimeter grating, astronomers discovered that Borisov’s dust grains were large, probably 1mm or larger, but compact, not fluffy (so more like icy particles than snowflakes). Again, this is unusual because local comets are fluffy. This implies that it formed in the inner part of its home solar system: at first, many pebble-sized pieces of rock and ice floated around and could slowly touch a growing comet, compacting it.
However, they also saw that the amount of carbon monoxide and water it emitted drastically changed between before it passed through the Sun and after it passed, implying that the comet is heterogeneous, as if it were made up of pieces composed differently.
This implies that a part formed near its star, and others farther away. This in turn means that the material from the inner part of this alien solar system has been mixed well with elements from further afield, possibly due to the gravitational influence of the giant planets. This gives astronomers insight how another planetary system was formed. This is quite difficult information to obtain otherwise, because we are light years away from other systems. And it was delivered to us directly!
We expect to see more interstellar objects like Borisov overtaking us, perhaps relatively often. We can learn more about them, and maybe even visit them; the European Space Agency is building a mission called Comet Interceptor to do just that. It will achieve very high acceleration and be able to catch up with the fast moving objects that visit the galaxy as a whole.
We won’t be sitting hundreds of millions of miles away for this. We will have front row seats. What will we learn then?
*The first was’ Oumuamua, which is probably a large chunk of nitrogen ice.
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