Only the beginning
Nobody has a firm handle on how many such objects exist in our galaxy, but the numbers must be staggering. According to Stern, Jupiter, Saturn, Uranus, and Neptune combined probably ejected 10
13 to 10
14 objects larger than 1 km early in our solar system’s history, when it was still cluttered with debris left over from the planet-formation process. Multiply that by the 10
11 stars in the Milky Way, and one comes up with numbers like 10
24 to 10
25 objects larger than a kilometer. Smaller objects like 1I/‘Oumuamua must be orders of magnitude more plentiful. But our galaxy’s volume of space is so enormous that catching an interstellar interloper in the solar system is like finding a needle in a haystack.
When the 8-meter Large Synoptic Survey Telescope comes online around 2021, its deep reach over large swathes of sky will undoubtedly reveal scads of interlopers. “We have the potential to study a lot of these objects. However, to do this efficiently really requires larger follow-up telescopes, such as the 30- to 40-meter-class telescopes now being built,” says Weryk’s University of Hawaii colleague Karen Meech.
“When we have seen more of these objects, it will be interesting to see if most are asteroidal or cometary,” adds Stern. Because the giant planets in our solar system are far from the Sun, they ejected primarily icy (cometary) objects. If it turns out that most interstellar objects are rocky, that could indicate that most planetary system that eject small bodies have giant planets closer to their host stars.
Astronomers will also be interested in learning the size distribution of interstellar objects, and whether some of them have satellites. They will also learn about their rotation rates and reflectivity.
Stern emphasizes that these objects come from very distant stars. They have been orbitally mixed over billions of years, so they represent a statistical sample of what is coming out of all the planetary systems that exist in the galaxy.
“It’s a little bit like pollen in the wind,” explains Stern. “Even if you don’t have any trees in your yard that produce pollen, you’ll still see pollen blowing through the wind from faraway trees. By studying the pollen, you can learn about the population of trees that are generating it. Now we’re going to start getting the statistical sample of all the ejecta from all the other planetary systems in the Milky Way Galaxy.”
Interstellar objects have been roaming the frigid depths of space for eons, with few evolutionary forces acting upon them. In other words, they are objects that have been stored in deep freeze. “They are well-preserved examples of things made in other star systems,” says Stern. “For that reason, they’re going to be valuable as the population is studied.”
1I/‘Oumuamua is not the first interstellar object ever detected in the solar system per se; it’s the first
macroscopic object. Astronomers have previously detected cosmic rays, gas, and ions, and they have even collected interstellar dust grains.