The Origin of Our First Interstellar Visitor Script (Patreon)

We were recently visited by a traveler from outside our solar system. This is the first time we’ve ever seen an object that came to us from interstellar space. Today on Space Time Journal Club we’ll see what mysteries it can unlock.

On October 19th this year astronomers spotted an 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 monitor the sky for moving or variable objects like asteroids or comets, It 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 we’ve now 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. Our focus will be a paper by a Dutch team from Leiden Observatory: Portegies Zwart, Pelupessy, Bedorf, Cai, Torres 2017. PZ17 for short. This paper, which is still under the process of being peer-reviewed, looks at the possible origins of the 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 Space Odyssey, hundreds of meters long. Its size and shape can’t be determined directly from 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 realised 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 that the surface to at least one meter depth 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 around the sun in ellipses as governed by Kepler’s Laws of Planetary Motion. The eccentricity of the orbit measures how stretched out the ellipse is. Eccentricity 0 is a perfect circle and eccentricities less than 1 are elliptical orbits, meaning the object is gravitationally bound to the Sun. The Earth, for example, has an eccentricity of .0167 giving us a nearly circular orbit, while Halley’s comet has eccentricity of .967- that’s an extremely stretched out ellipse.

Objects with eccentricies greater than 1 follow unbound hyperbolic paths. They never actually orbit the Sun; rather they are 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 about it 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 was well inside Mercury’s orbit. The escape velocity at that closest approach, or “perihelion”, is a little over 80 km/s. That means Ouamuamua has just 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? PZ17 investigate three hypotheses. The first is that it originated in our own Kuiper belt or Oort cloud. Now objects falling in from the Oort cloud or Kuiper belt, like comets, only pick up enough speed to bring them back to where they started. But perhaps Ouamuamua got some sort of gravitational kick from an unknown planetary body. PZ17 perform 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 Ouamuamua’s solar-system-escaping speed is that it gained that speed by falling into the solar system from outside. So hypothesis number 2 is that the object was ejected and flung towards us from a nearby star system. PZ17 performed a computer simulation to rewind the motion of both Oumuamua and the 3700 stars within 100 light years of the Sun. They found that the object passed through the Oort cloud of another star - the unpoeticly-named TYC4742-1027-1 - around 1.3 million years ago. But its speed relative to the 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 system throwing rocks at us. Uh, no, it’s never aliens.

The 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 hypothesised of the existence of such objects, and with good reason. In planet formation models, lots of chunks of matter, and even some whole 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. PZ17 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 helpfully give the Klingon translation, “mob nagh”.

Based on this one possible chunk of interstellar debris we’ve found so far, and the the volume of space scanned by Pan-STARRS, PZ17 extrapolate to estimate the density of the debris field. They get that, in order for us to have seen this one object in 5 years we’ve been watching, there must be roughly seven hundred trillion objects per cubic parsec in the solar neighborhood. That’s higher than the density of comet-like objects in the Oort cloud. Based on this, PZ17 predict that two to twelve 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 Atacaman Andes in northern Chile, with first light planned for 2019. It will photograph its entire night sky every couple of nights. LSST will be able to see objects around 14 times fainter than Pan-STARRS, so we should expect to find many more “solae lapides”.

Well then what of our visitor ‘Oumuamua? Is this the end of its story? Well, it will leave the solar system behind in roughly twenty thousand years, but it'll be invisible to our telescopes within a month or two. It’s trajectory will send it towards the constellation Pegasus, perhaps to find a new star there to visit. In the meantime it joins its countless interstellar cousins; orphaned planetary debris stretching across the reaches of space time.