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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.
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.
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[INAUDIBLE] Leonard asks whether a particle
can have momentum higher than its mass
times the speed of light.
In other words, shouldn't there be an upper limit to momentum
if the speed of light is limited?
Well actually, momentum can be arbitrarily high,
even when speed is limited to the speed of light.
The equation for momentum, P = M times V,
only works at low speeds approaching the speed of light.
And you have to divide that MV by the Lorentz factor.
That factor, the square root of one
minus V squared on C squared, approaches 0
as velocity approaches the speed of light.
That means momentum approaches infinity
for any object with mass that's approaching the speed of light.
William Smith wrote a good question.
How can a photon's frequency be generalized as momentum?
Does frequency then include information
about direction of motion?
Well we generalize frequency as momentum
because the Heisenberg uncertainty principle
applies to momentum in general.
In the case of matter, uncertainty in momentum
can manifest in both velocity and mass.
But photons have constant speed and no mass.
So that uncertainty is all in their frequency.
And yes, also in their direction of motion, which is
separate to frequency.
That last fact explains the increasing spread
in the direction of photons after they
pass through a narrowing slit.
By increasing our certainty in the location
of the photon passing through the slit,
we increase the uncertainty in its momentum and hence,
its direction of motion after it exits the slit.
[INAUDIBLE] 777 asks, if you try to do a Fourier
transform of PBS Space Time, do you get PBS infinite series?
Actually, I think you need an infinite series of PBS Space
Time to get a Space Time localization of PBS
infinite series.
But I'm not certain.