Sarah Al-Ahmed: This is one of those stories that just lights the imagination on fire. Interstellar objects provide one of our only opportunities to be able to really observe material coming into our Solar System from somewhere else entirely, and I think people are right to be really excited about this.
Bryce Bolin: It’s just mind-blowing. You have all these theories about how many of these things exist out there and what their properties could be like, but the difference between theory and what we actually observe can be quite large. So for ‘Oumuamua, we think it’s most likely a comet because it has large non-gravitational perturbations like a comet. Comets “defy the laws of gravity” — their orbits can’t be explained by gravity alone because they emit gases, and this has a non-negligible momentum on their trajectory.
Comets outnumber asteroids by quite a lot in our star system, and in extrasolar star systems, we think they should as well. You’d think that what’s going to be ejected into space and seen somewhere else in the galaxy is going to be a comet, but we didn’t know.
And so when ‘Oumuamua was defying the laws of gravity, that kind of gave us a sigh of relief. The other two interstellar objects, 2I/Borisov and 3I/ATLAS, are clearly comets. So it seems that so far all the interstellar objects are comet-like material, which checks with our expectations of planet formation and what we think the material around other stars should be like.
Sarah Al-Ahmed: What kind of things have you been studying during the limited amount of time that we’ve known about the subject?
Bryce Bolin: For 3I/ATLAS, our first look at it was using broadband colors, getting optical wavelength colors. We also observed it with the Keck Telescope to get near-infrared colors and spectra. We can see possibly the emission of cometary gases like cyanogen, diatomic carbon, and triatomic carbon, and this can be indicative of the devolatilization of the comet. These molecules first originate as ices in the comet, and then the heat from the Sun causes the ices to sublimate. They undergo complex reactions while they’re in the coma of the comet, and one of the products can be cyanogen, as well as the triatomic carbon.
We’re also going to get some James Webb observations of these objects. I’m not formally a part of that team, but James Webb will be able to observe the comet in wavelengths well beyond the visible and near-infrared, enabling us to detect the emission of cometary gases directly.
Sarah Al-Ahmed: Some things I’ve seen online suggest that this object could be much older than other things in our own Solar System. What do we know about where this object came from and its potential origins?
Bryce Bolin: My understanding is that this is actually pretty difficult to find out. It turns out that the galaxy is quite complicated. It’s not like you have all the stars in the galaxy orbiting one central thing like a star, a supermassive black hole. They do in a way, but the gravitational potential is quite variable in the galaxy because it’s huge and has irregularities — molecular clouds, spiral arms, stellar clusters. And so the way that this affects the orbits of stars and interstellar objects that they float around in the galaxy is quite difficult to understand. Over millions of years, a object could encounter spiral arms and all sorts of things that can really throw off its trajectory in the galaxy. And so I think it’s really hard to say anything about an interstellar object’s trajectory beyond maybe one or two galactic orbits — which, for our Sun, takes about a million years. For something billions of years old, like I’ve seen some papers state, its trajectory and origins are pretty hard to pin down.
Sarah Al-Ahmed: Do we have a general idea of the way that this thing might be passing through our Solar System?
Bryce Bolin: 3I/ATLAS is actually going to get quite close to the orbit of Mars, which could be an interesting opportunity for spacecraft at Mars to try and observe this thing. I’m a fan of it. I think they should give it a try.
3I/ATLAS will be observable until the end of August, early September-ish, and then it goes into solar conjunction, meaning the Sun will block the view of 3I/ATLAS from Earth. But when Earth gets around the other side of the Sun, we’ll be able to see it again.