IRA FLATOW: This is Science Friday. I’m Ira Flatow. All right everybody, mark your calendars. October 19, 2017, the day astronomers caught sight of our first ever visitor from outside our solar system. OK, fine. It wasn’t an alien spacecraft, but it was still interstellar. A piece of space rock speeding through our cosmic neighborhood like a stranger through town. And unlike our own meteorites, we won’t be seeing this one ever again.

How do we know this object came from beyond our solar system? And what did we learn from it while it was here, fleetingly here? Joining me to break down the interstellar visit with me is our space rock expert, Meenakshi Wadhwa, is Director of the Center for Meteorite Studies at Arizona State University. Welcome to Science Friday.

MEENAKSHI WADHWA: Hi, Ira. Glad to be here.

IRA FLATOW: So we had a visitor come to our cosmic neighborhood. How do we know it was a visitor, not just one of our own meteorites?

MEENAKSHI WADHWA: Well, we believe that it came from outside our solar system based mainly on speed and its trajectory, but mainly on the trajectory. So it has a hyperbolic orbit, which seems to suggest that it came from another star system altogether.

IRA FLATOW: Hyperbolic means what?

MEENAKSHI WADHWA: Well, so when you think of all the planets and asteroids and comets that formed within our own solar system, they all have these elliptical orbits that are closed loops, which means that these objects are gravitationally bound to the sun. But when you have an hyperbolic object, when you’ve got an object that’s got an hyperbolic object, the orbit doesn’t close in on itself and basically indicates that the object is not gravitationally bound to the sun. So it had to be coming from somewhere else from outside of our solar system.

IRA FLATOW: So would it be correct in saying if you didn’t see it, you missed it, it’s gone?

MEENAKSHI WADHWA: Exactly. Exactly. It’s actually booking its way outside our solar system, just as we speak.

IRA FLATOW: Huh. Do we know where it came from?

MEENAKSHI WADHWA: Well, so when it was observed based on its trajectory, it seems to be coming from the direction of the star Vega, which is in the constellation Lyra. But that’s an apparent sort of direction that it came from, but it may not have originated there basically. Ultimately, we really don’t know where it originated and how long it’s been traversing interstellar space.

IRA FLATOW: So is this the first object of any size to come into our solar system from outside?

MEENAKSHI WADHWA: Well, so the interesting thing that this is the first macro object. So we know that there’s actually interstellar dust that makes its way into our own solar system, and we’ve actually captured that. Spacecraft, like Stardust, have captured interstellar dust. And there’s actually interstellar dust in meteorites too. But this is the first object that’s a macro in size. You can actually estimate that it was about maybe I think 160 meters in diameter is what’s estimated at the current time.

IRA FLATOW: Were we are able to tell what it was made out of and see if it was different from the stuff we have here?

MEENAKSHI WADHWA: Well, so that’s another really great question, and I think there have actually been some optical studies. So they’ve looked at the spectrum of this particular object very briefly, and it looks very similar to some objects in our own solar system, some Kuiper belt objects, for example. It has a reddish color to it, but that information is very, very limited. And I would love to have a sample of it in my laboratory, of course, but that’s not going to happen any time soon.

IRA FLATOW: Well, how do you know there isn’t one on earth somewhere? That’s–

MEENAKSHI WADHWA: Well, another really good question, of course. We have lots of meteorites in our collections, and we know what they’re made of. An interstellar rock, now what wold that look like? That’s a really great thought exercise, but we know that we have interstellar grains in meteorites. And we can actually differentiate them from other just solar system matter based on their chemical signatures. And so, my assumption is that we would be able to tell based on the chemical signatures if we had a rock that was so just different from everything else that we have in our collections, we’d know it. And we haven’t actually seen a whole rook with a distinct composition like that.

IRA FLATOW: Well, you’re a meteorite collector, right? You like–

MEENAKSHI WADHWA: Exactly.

IRA FLATOW: Well, how excited would you be to find a rock?

MEENAKSHI WADHWA: I would love for a day if we had an interstellar meteorite to study in our laboratory. That would be amazing.

IRA FLATOW: What are the odds you think that another one will come by in our lifetime?

MEENAKSHI WADHWA: Actually, it’s surprising that we haven’t seen more of these types of objects because we think that the probability of objects being ejected from other solar systems is actually quite high. Just like we believe that from our own solar system there’s asteroids and cometary objects being ejected from our own solar systems. Particularly, during the initial phases of solar system formation when the giant planets were forming, interaction of the gravity of these giant planets with some of these objects basically did throw out a lot of material out into interstellar space. So I would have expected to have seen these types of objects more often actually, and probably we will in the future. It is my hope that we will.

IRA FLATOW: I mean, when you think about it, that’s where our whole solar system must have come from someplace else too, right?

MEENAKSHI WADHWA: Well, sure. I mean, we originally were formed from a cloud of gas and dust, and the dust obviously was formed around other stars somewhere. And we’re an aggregate of all that material, as our other star systems around us. They’re aggregates of all kinds of interstellar dust and gas that formed in the planets. Clearly, it’s similar processes. But we know from spectroscopic observations of other stars that, in fact, there’s quite a lot of variability in the chemical compositions of different stars. And so when you think about rocks being formed in different solar systems, they may or may not have the same kind of composition that we have for rocks in our system, from meteorites in our own solar system.

IRA FLATOW: Well, this is exciting. Last week, we learned about all this gold coming from a collision of neutron stars. Now–

MEENAKSHI WADHWA: Yes, wasn’t that exciting?

IRA FLATOW: Now hearing about the rock coming through our solar system. It’s a lot of fun stuff to talk about.

MEENAKSHI WADHWA: Absolutely, This has been a bonanza for planetary scientists and astronomers these last couple of months.

IRA FLATOW: Well, Dr. Wadhwa, thank you for taking time to be with us today.

MEENAKSHI WADHWA: Thank you.

IRA FLATOW: Dr. Wadhwa is Director of the Center for Meteorite Studies at Arizona State University in Tempe.

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