The story of Oumuamua the first visitor from another star system Karen J. Meech
NASA’s always on the lookout
for possible asteroid collision hazards,
so the Pan-STARRS telescope
is scanning the sky every night.
Each morning, candidate objects
are examined by Pan-STARRS staff
and usually discovered to be no big deal.
But on October 19, 2017,
Pan-STARRS spotted an object
moving rapidly between the stars,
and this time the usual follow-up
measurements of position and speed
showed something completely different.
By October 22nd, we had enough data
to realize that this object
wasn’t from our solar system.
Holy cow.
That’s when I got the phone call,
the phone call that all solar system
astronomers are waiting for.
Let me tell you how exciting this was.
(Laughter)
NASA’s been expecting to see
an interstellar comet
pass through the solar system
since the 1970s,
but until now, we’d never seen anything.
Our own solar system is huge,
so even getting a package
from the nearest star system
4.4 light years away
would take over 50,000 years.
So this is a really big deal.
The interstellar visitor
entered our solar system
from above the plane of the planets,
coming from the direction
of the constellation Lyra,
and it passed closest to the Sun
on September 9th,
passing inside the orbit of Mercury.
Now this isn’t a particularly
close approach or unusual distance.
It’s just much easier
to see objects close by.
On October 14th,
before we discovered it, it made
its closest approach to the Earth,
within about 15 million miles.
This is really close
by astronomical standards.
Now rather than call this
by its unwieldy catalog name,
we briefly called it “Rama,”
after the cylindrical spacecraft
that passed through the solar system
in Arthur C. Clarke’s classic
science fiction story in 1973.
But this wasn’t quite right either,
so in honor of it being discovered
by a telescope in Hawaii,
we consulted two experts
on Hawaiian culture –
a Hawaiian navigator and a linguist –
to propose a name.
And they suggested “‘Oumuamua,”
which means scout or messenger
from the distant past reaching out to us.
Now this discovery
was important for many reasons,
but to me the most significant
is for what ‘Oumuamua can tell us
about the past of our solar system.
The process of the birth of a new solar
system and the growth of planets
can be a violent and messy business.
Leftover icy and rocky debris
gets ejected from the new solar system
as the giant planets migrate
through the dusty disk
out of which they’re formed.
Now have you ever felt an emotional chill,
something that’s so exciting
that a shiver runs up and down your spine?
Or something that’s
very emotionally moving?
Well this was it for me.
This was my wow moment.
We actually had a piece of material
from another solar system
coming close enough for us to observe.
So what would you like to know
about ‘Oumuamua,
the very first visitor
from another star system?
Well, I could think of a million things,
but there’s what you want
and what you can have,
and ‘Oumuamua was moving away
and fading very rapidly.
In the span of about a week,
it had dropped in brightness
by a factor of [10].
So this is about all the time
we were going to have
to study it easily.
So we had to distill the process
of getting telescope time –
normally a very competitive,
peer-reviewed proposal process
that can take up to months –
down to less than a few days.
So began a “polite”
competition for resources.
OK, let me not mince words.
It was a fierce battle.
We dropped everything,
working around the clock,
trying to craft
perfectly crafted proposal words
to send to the observatory directors.
Well, good news. We got the time.
Now, from a perfectly
selfish point of view,
the first thing we might like to know
is how massive ‘Oumuamua is.
Because after all,
it passed very close to the Earth,
and we didn’t know about it
until afterwards.
How bad would this have been
had it not missed the Earth?
Well, the impact energy
depends on the square
of the velocity times its mass,
and the mass depends
on how big it is and what it’s made of.
So how big is ‘Oumuamua,
and what’s its shape?
Well, we can get this from its brightness.
Now, if you don’t believe me,
think of comparing the brightness
of a firefly in your backyard
to the navigation lights
on a distant airplane.
You know the airplane is much brighter –
it just appears faint
because it’s so far away.
We’re also going to need to know
how reflective
the surface of ‘Oumuamua is,
and we don’t have any clue,
but it’s reasonable to assume
it’s very similar to small asteroids
and comets in our solar system,
or in technical terms,
something between the reflectivity
of charcoal and wet sand.
Nowadays, most of the big telescopes
are used in what’s called a service mode,
meaning we have to carefully develop
all the instructions
and send them to the telescope operator,
and then anxiously wait
for the data to come back,
praying to the weather gods.
Now I bet most of you don’t have careers
that critically depend on whether or not
it’s cloudy last night.
Well, we weren’t going to get
any second chances here.
Because the weather was great,
‘Oumuamua decided not to be.
Its brightness wasn’t constant.
Now here we see ‘Oumuamua
racing between the stars.
It’s centered in the middle.
The stars are trailed out because
the telescope is following its motion.
It started faint and then it got brighter,
fainter, brighter, and fainter again,
as sunlight is reflected off
of four sides of an oblong object.
The extreme brightness change
led us to an unbelievable
conclusion about its shape.
As shown in this artist’s impression,
‘Oumuamua is apparently
very long and narrow,
with an axis ratio of about 10 to one.
Assuming it’s dark,
this means it’s about half a mile long.
Nothing else in our
solar system looks like this.
We only have a handful of objects
that even have an axis ratio
bigger than five to one.
So we don’t know how this forms,
but it may be part of its birth process
in its home solar system.
‘Oumuamua was varying in brightness
every 7.34 hours,
or so we thought.
As more data started
to come in from other teams,
they were reporting different numbers.
Why is it the more
we learn about something,
the harder it gets to interpret?
Well, it turns out that ‘Oumuamua
is not rotating in a simple way.
It’s wobbling like a top.
So while it is rotating
around its short axis,
it’s also rolling around the long axis
and nodding up and down.
This very energetic, excited motion
is almost certainly the result
of it being violently tossed
out of its home solar system.
Now how we interpret the shape
from its brightness
depends very critically
on how it’s spinning,
so now we have to rethink
what it may look like,
and as shown in this beautiful painting
by space artist Bill Hartmann,
we think that ‘Oumuamua
may be more of a flattened oval.
So let’s get back to the energetics.
What is it made of?
Well, ideally we would love
to have a piece of ‘Oumuamua
into the laboratory,
so we could study it in detail.
But since even private industry
can’t manage to launch
a spacecraft within a week
to something like this,
astronomers have to rely
on remote observations.
So astronomers will look at how the light
interacts with the surface.
Some colors may get absorbed,
giving it a chemical fingerprint,
whereas other colors may not.
On the other hand, some substances
may just reflect more blue
or red light efficiently.
In the case of ‘Oumuamua,
it reflected more red light,
making it look very much like the organic
rich surface of the comet recently visited
by the Rosetta spacecraft.
But not everything that looks reddish
has the same composition.
In fact, minerals that have
tiny little bits of iron in the surface
can also look red,
as does the dark side
of Saturn’s moon Iapetus,
shown in these images
from the Cassini spacecraft.
Nickel-iron meteorites,
in other words, metal,
can also look red.
So while we don’t know
what’s on the surface,
we know even less
about what’s on the inside.
However, we do know
that it must at least be strong enough
to not fly apart as it rotates,
so it probably has a density
similar to that of rocky asteroids;
perhaps even denser, like metal.
Well, at the very least,
I want to show you
one of the beautiful
color images that we got
from one of the ground-based telescopes.
All right, I admit,
it’s not all that spectacular.
(Laughter)
We just don’t have the resolution.
Even Hubble Space Telescope
doesn’t present a much better view.
But the importance of the Hubble data
was not because of the images,
but because it extended
our observations out
to two and a half months
from the discovery,
meaning we get more positions
along the orbit,
which will hopefully let us figure out
where ‘Oumuamua came from.
So what exactly is ‘Oumuamua?
We firmly believe it’s likely to be
a leftover archaeological remnant
from the process of the birth
of another planetary system,
some celestial driftwood.
Some scientists think
that maybe ‘Oumuamua formed
very close to a star
that was much denser than our own,
and the star’s tidal forces
shredded planetary material
early in the solar system’s history.
Still others suggest that maybe
this is something that formed
during the death throes of a star,
perhaps during a supernova explosion,
as planetary material got shredded.
Whatever it is, we believe
it’s a natural object,
but we can’t actually prove
that it’s not something artificial.
The color, the strange shape,
the tumbling motion
could all have other explanations.
Now while we don’t believe
this is alien technology,
why not do the obvious experiment
and search for a radio signal?
That’s exactly what
the Breakthrough Listen project did,
but so far, ‘Oumuamua
has remained completely quiet.
Now could we send
a spacecraft to ‘Oumuamua
and answer this question once and for all?
Yes, we do actually have the technology,
but it would be a long
and expensive voyage,
and we would get there so far from the Sun
that the final approach trajectory
would be very difficult.
So I think ‘Oumuamua probably
has many more things to teach us,
and in fact there might be
more surprises in store
as scientists such as myself
continue to work with the data.
More importantly,
I think this visitor from afar
has really brought home the point
that our solar system isn’t isolated.
We’re part of a much larger environment,
and in fact, we may even
be surrounded by interstellar visitors
and not even know it.
This unexpected gift
has perhaps raised more questions
than its provided answers,
but we were the first to say hello
to a visitor from another solar system.
Thank you.
(Applause)
Jedidah Isler: Thanks, Karen.
I of course enjoyed
that talk very much. Thank you.
As I recall, we found it
pretty late in its journey towards us.
Will future technologies like
the Large Synoptic Survey Telescope
help us detect these things sooner?
Karen Meech: Yeah. We’re hoping that
we’ll start to see a lot of these things,
and ideally, you’d love to find one
as it’s approaching the Sun,
because you want to have time
to do all the science,
or even more ideal,
you’d get a spacecraft ready to go,
parked somewhere in the L4 or L5 position,
somewhere near Earth,
so that when something comes by,
you can chase it.
JI: Awesome, thanks so much.
Let’s thank Karen again.
(Applause)