We are currently being visited by a traveler from outside our solar system.

This is the first time we've ever seen an asteroid like object that came to us from interstellar space.

Today, on Space Time Journal Club, we'll see what mysteries it can unlock.

12 00:00:29,530 --> 00:00:32,020 On October 19th this year, astronomers spotted an unusual object moving rapidly away from the sun.

The discovery was made by Pan Starrs, the panoramic survey telescope and rapid response system, a group of telescopes that constantly monitors the sky for moving or variable objects like asteroids or comets.

This object was initially thought to be a new comet.

But after 34 days of follow up observations, and a bit of orbital mechanics, it became abundantly clear that we were looking at the first object ever observed that came from outside our solar system.

The mysterious object has been dubbed 1I, the first object in a brand new International Astronomical Union class, with I standing for interstellar.

1I also scored a more poetic name.

Oumuamua, from the Hawaiian meaning a messenger from afar arriving fast.

Now this object was first spotted several weeks ago.

But by now, we've had time to study it with many of the world's great telescopes.

For this Space Time Journal Club, we'll be discussing what we've learned about Oumuamua since its discovery.

Our focus will be on a paper by a Dutch team from Leiden Observatory, Portegies Zwart, et al., 2017, PZ 17 for short.

This paper, which is still in the process of being peer reviewed, looks at the possible origins of this strange object.

Before we get to its origins, let's look at the object itself.

It's pretty weird.

It's highly elongated, like a cigar or the monolith from "2001: A Space Odyssey."

Hundreds of meters long, it's size and shape can't be determined directly from the images.

It just appears as a faint dot.

Even to our most powerful telescopes.

However, it varies in brightness with an irregular period.

And astronomers realized that this is consistent with an elongated body with a tumbling motion.

We've seen such tumblers in our solar system.

And it suggests a major collision in the object's past.

Oumuamua doesn't have a tail like a comet, so its surface isn't vaporizing in the sun's radiation.

That suggests the surface to at least one meter depth is rocky like an asteroid rather than icy like a comet.

Or it could be the flickering exhaust jets of an alien scout ship returning to report a planet ripe for invasion.

But no, it's never aliens.

The weirdest thing about Oumuamua is its motion.

Every previously observed asteroid-- indeed, every previously observed everything in our solar system-- moves in elliptical orbits as governed by Kepler's laws.

The eccentricity of the orbit measures how stretched out the ellipse is.

Eccentricity 0 is a perfect circle.

And eccentricity is less than one, our elliptical orbits.

Meaning the object is gravitationally bound to the sun.

The earth, for example, has an eccentricity of 0.0167, giving us a nearly circular orbit.

While Halley's comet has an eccentricity of 0.967.

That's an extremely stretched out ellipse.

Objects with eccentricities greater than 1 follow unbound hyperbolic paths.

And they never actually orbit the sun.

Rather, they're deflected by the sun's gravity as they pass by.

Oumuamua has an eccentricity of 1.2, meaning its hyperbolic path will take it out of the solar system.

Another way to think of this is in terms of escape velocity.

That's the velocity an object would need to have to escape a gravitational field.

Oumuamua had a maximum speed of 87.7 kilometers per second at its closest approach to the sun, which is well inside Mercury's orbit.

The escape velocity at that closest approach was a little over 80 kilometers per second.

That means Oumuamua has enough speed to climb out of the sun's gravitational well and escape back to interstellar space.

So where did this object come from?

And why is it moving so fast?

PZ 17 investigate three hypotheses.

The first is that it originated in our Kuiper belt Oort cloud.

Now objects falling in from these regions, like comets, only pick up enough speed to bring them back to where they started.

But perhaps Oumuamua got some sort of gravitational kick from an unknown planetary body.

PZ 17 performed computer simulations to find the frequency of such kicks sending objects close to the earth at such high speeds.

They conclude that it's exceptionally unlikely.

The simplest explanation for Oumuamua's solar system escaping speed is that it gained that speed by falling into the solar system from outside.

So, hypothesis number two is that the object was ejected and flung towards us from a nearby star system.

PZ 17 performed more computer simulations to rewind the motion of both Oumuamua and the 3,700 stars within 100 light years of the sun.

They found that the object passed through the Oort cloud of another star, the unpoetically named TYC4742-102701 around 1.3 million years ago.

But its speed relative to that star would have been over 100 kilometers per second, higher than the escape velocity at the distance of closest approach to that star.

So Oumuamua was probably only a visitor to that solar system too.

Unless of course there are aliens in that solar system throwing rocks at us?

No, it's never aliens.

A third possible origin for Oumuamua is that it's been traveling for a very very long time.

It may come from a vast population of random debris floating around in interstellar space.

People have hypothesized about the existence of such objects.

And with good reason.

In planet formation models, lots of chunks of matter, and even some planets, get ejected from the relatively violent protoplanetary disk.

Given how many stars there are, there should be a ton of asteroidal objects floating around in interstellar space.

Our sun, as it moves around the galaxy, passes through this field of debris.

PZ 17 conclude that this is the most likely origin for Oumuamua.

They dubbed this type of unbound non cometary object a lonely rock, or sola lapis.

In their paper, they hopefully give the Klingon translation, mob nagh.

Based on this one possible chunk of interstellar debris that we've found so far, and on the volume of space scanned by Pan Starrs, PZ 17 extrapolate to estimate the density of the debris field.

They get that in order for us to have seen this one object in the five years we've been watching with Pan Starrs, there must be roughly 700 trillion objects per cubic parsec in the solar neighborhood.

Now that's higher than the density of comet like objects in the Oort cloud.

And based on this, PZ 17 predicts that two to 12 of these interstellar objects should pass through our solar system inside Earth's orbit every year.

The main reason we've only spotted one so far is that most don't get close enough to the earth for Pan Starrs to detect.

Oumuamua got pretty close.

Only 18 million kilometers at closest approach.

But even then, Pan Starrs was only just able to spot it.

Anything smaller or more distant would be missed.

The good news is that future telescopes will be much more sensitive.

For example, the Large Synoptic Survey Telescope, LSST, which is currently under construction in the Anacondian Andes in northern Chile, with first light planned for 2019.

It'll photograph the entire night sky every few nights.

LSST will be able to see objects around 14 times fainter than Pan Starrs.

So we should expect to find many more of these sole lapids.

What, then, of our visitor, Oumuamua?

Is this the end of its story?

Well, it'll leave our solar system behind in roughly 20,000 years.

But, it'll be invisible to our telescopes within a month or two.

Its trajectory we'll send it towards the constellation Pegasus, perhaps to find a new star system there to visit.

In the meantime, it joins its countless interstellar cousins, orphaned planetary debris, stretching across the reaches of space time.