More Interstellar Visitors?

Yesterday I talked about the latest interstellar intruder to come through our solar system. 2I/Borisov is thought to be a comet from another star system and this interstellar visitor is traveling on a fast trajectory past the Sun. By the end of the year, it will be well beyond Jupiter. So, astronomers are studying it now, while it’s still close enough to observe. The appearance of 2I/Borisov, along with the visit in 2017 from ‘Oumuamua (the first interstellar asteroid detected passing through our solar system), raises a lot of questions about how these objects get cast out on their long journeys.

Planet Formation and Interstellar Visitors

Planetary scientists spend a lot of time working to understand the formation of our own solar system. It turns out that planetary formation likely plays a role in the origins of alien interstellar objects. The general story is that the planets were built up from smaller objects called planetesimals. Those miniature worlds accreted from rocky materials and ice that existed in the infant solar system. This cloud of gas and dust, called the protoplanetary nebula, was the birthplace of the planets, moons, asteroids, and comets. All that material circled the Sun, and, over time, the planets formed and migrated to their present positions.

Of course, comets and asteroids migrated, too. They ended up in two regions in the solar system: the Kuiper Belt and the Oort Cloud. We already know about dwarf planets in the Kuiper Belt, and of course, there are populations of comets in both regions.

In the early solar system, the larger planetary objects swept out “clear lanes” in the dust cloud as they accreted. It turns out the planets that create those lanes may play a role in ejecting objects to space.

Making the Kuiper Belt and Oort Cloud

Planetary scientists want to understand how those comets and other bodies got “stuck” in such far-flung orbits. The details are still being worked out, but here’s the basic story. Interactions in the young solar system between larger worlds caused cometary particles to be “flung” out to the outer regions. Those regions are cooler places, where the icy bodies could more easily exist.

Could they have been “flung out” even farther? Maybe even out of the solar system altogether? Good questions. If they were, then there could be bits of our own solar system traversing the depths of space, just as 2I/Borisov and ‘Oumuamua are doing.

How Do Interstellar Visitors Get “Launched”?

A pair of astronomers at Yale University, Gregory Laughlin and Malena Rice, have been studying the situation. They are interested in the dynamics of just how interstellar objects could be ejected from their newly forming planetary systems. Their idea is simple. Travelers from a newborn solar system could be material ejected from large, newly formed planets in orbit around a star. Those planets, which would likely be orbiting farther away from their star, would have carved out pronounced gaps in the protoplanetary disks.

Far from being a quiet, peaceful place, those disks are very active. They’re made of very dense clouds of gas and dust, and all that stuff mixes and collides. The gravity of the star also disturbs the disk, as does the activity of magnetic fields. Some of that action helps build new planets. However, some of it could also cause gravitational interactions between large, distant planets and the material around them. Those actions could fling material out of the newly forming solar system at a very high speed.

If that happens, then the ejected bodies would travel through space. Eventually, they could pass through neighboring planetary systems, just like 21/Borisov and ‘Oumuamua have done.

So, can such actions be observed? It’s possible, as astronomers are finding out.

Intruder Homes are Out There

The two scientists wanted to see if their idea works out. First, they got time on the Atacama Large-Millimeter Array in Chile to look around at some nearby young planetary systems. ALMA is a linked collection of radio telescopes tuned to submillimeter wavelengths of light. That kind of light can slip through the dusty disks surrounding other stars and reveal information about their characteristics.

The astronomers looked at three protoplanetary disks, searching out large gaps made by distant planets as they plow through their birth disks.

Next, the pair applied what they observe to computer simulations of disks with gaps. They are coming up with some boundaries on the sizes of such objects being flung away from the systems they observe. It turns out, based on observations and simulations that giant planets with lengthy periods (the time they take to orbit their stars) and that create gaps, can gravitationally eject objects the size of ‘Oumuamua. That asteroid is somewhere between 100 and 1000 meters long and 35 to 170 meters across. That may be out our 2017 visitor got started on its long trip through the galaxy!

Implications of Interstellar Visitors

As I mentioned yesterday, close studies of these intruders from other star systems give clues to the conditions in their home systems. Thanks to the work by Laughlin and Rice, we now know that the protoplanetary disks themselves have information about the sizes, shapes, and numbers of those intruders they send out. Based on the work by this pair of scientists, it turns out we can expect MORE intruders in the coming years. With a new generation of telescopes able to study these interstellar travelers in more detail, it looks like we’ll be getting a whole new look at distant star systems and their planetary processes.

You can read more about Laughlin and Rice’s work in their paper.

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