‘Oumuamua: a shred of its former self

One time I ordered a beer then straight away spilled it all over the bar.  There was just a tiny sip left in the glass.  Not my finest hour (frowny face).

Remember ‘Oumuamua, the cigar from another star?  That weird-shaped rock flying through the Solar System? Like the sip of beer left in my glass, it might only be a shred of what it once was.

‘Oumuamua’s trajectory through the Solar System. Credit: Wikipedia.

Here is what we think we know about ‘Oumuamua.  All our information comes from measurements of the object’s brightness (which oscillates dramatically) and its spectrum (which is reasonably-well characterized):

  • It has a stretched-out shape. Exactly how stretched-out is debated, but it’s probably cigar-shaped with a long axis 5 to 10 times longer than the two shorter ones. The dimensions are ballpark: 230 m × 35 m × 35 m (800 ft × 100 ft × 100 ft).
  • It spins in about 7 hours but in an irregular way, not along any of its main axes.  This is called tumbling and is rare among asteroids and comets.
  • Its spectrum is similar to those of primitive, organic-rich objects in the Solar System (certain classes of comets and water-rich asteroids) but without showing any water.
A tumbling model for ‘Oumuamua (left) that produces variations in brightness (right) like the observations.  Credit: Wikipedia.

In a previous post I presented a simple origins story for ‘Oumuamua. It went like this: ‘Oumuamua was born in a planet-forming disk around a young star.  That same disk also formed two or more gas giant planets.  The orbits of the gas giants became unstable and they underwent a violent instability.  A huge pile of planetary leftovers — called planetesimals — were launched by the giant planets into interstellar space.  ‘Oumuamua is one of those planetesimals.  Here is a simulation of this process (the star is at zero (not shown), the big black things are gas giants, small colored dots are planetesimals and the growing rocky planets):

There is a new twist to this story that fixes a small but important problem.  Let me explain.

For decades we have expected to find rogue interstellar planetesimals like ‘Oumuamua.  After all, we’ve already found rogue planets (see here), and there should be a whole lot more small guys out there than big ones.  But we want to know how many are out there, because that number matters (at least to people like me in the planet formation business).

If we know how efficient we are at finding interstellar objects like ‘Oumuamua, we can estimate how abundant they are.  We can compare it to the number of stars.  Finally, assuming ‘Oumuamua to be a member of a population of such bodies, we can figure out how much mass in planetesimals each star ejects on average.

To do this last part, think of ‘Oumuamua as a pea in a distribution of interstellar peas and pumpkins.  There are 1000 peas for every pumpkin.  So, by number you should expect to see peas, but one pumpkin has much more mass than a thousand peas.  So, if our calculation tells us that there are a million peas, then we know there must be a thousand pumpkins. If we know the mass of a pumpkin then we’re golden.

From this simple calculation I find that, in order to explain the detection of ‘Oumuamua, each star must eject hundreds to thousands of Earth masses in planetesimals!  That is such a big number that it makes no sense!  A typical planet-forming disk only contains a total of between about ten and a few hundred Earth masses, and it can’t all be ejected! What is going on?

I was confused so I changed approach.  I took a closer look at the behavior of planetesimals as they are kicked out of their planetary systems. Planetesimals are kicked around by giant planets 10-100 times or so on their way to getting ejected.  But about one in every hundred planetesimals gets really close to a giant planet during this process. So close that it should be tidally disrupted.  Disruption happens when the gravity on one side of an object is different enough from the gravity on the other side that is is literally torn to pieces.

We have seen tidal disruption in action.  Back in 1992 comet Shoemaker-Levy 9 flew too close to Jupiter and was torn to shreds:

NASA Hubble Space Telescope image of the fragments of comet Shoemaker-Levy 9. From Wikipedia.

Those pieces eventually fell back onto Jupiter in 1994 (some on my 17th birthday!)

About 1% of planetesimals ejected into interstellar space undergo such close encounters that they must have disrupted like comet Shoemaker-Levy 9. This leads to a new thought: what if ‘Oumuamua is not a planetesimal, but a fragment of a disrupted planetesimal?

‘Oumuamua as a fragment may explain some of its puzzling characteristics.  First, it is really stretched-out.  Tidal disruption is the process of stretching something out until it breaks, so it’s not a big surprise that the shreds should be stretched-out.  Its weird tumbling might also be explained by disruption. And who knows, maybe that type of event could have dried ‘Oumuamua out as well.

If ‘Oumuamua is a fragment it also fixes the problem I pointed out before, that to explain ‘Oumuamua’s detection every star needs to eject hundreds of Earth masses in planetesimals.  Let’s go back to thinking of interstellar planetesimals as peas and pumpkins. When planetesimals form, for every thousand peas there is one pumpkin.  As those objects were ejected into interstellar space, 99% of them did not change and have the same 1000-to-1 pea to pumpkin ratio.  But remember, 1% of planetesimals were torn to pieces.  So, let’s take 1% of the pumpkins and shred them into pea-sized pieces.  That’s a *lot* of peas.  It’s so many that the 1000-to-1 pea to pumpkin ratio is now much higher.  If we take that into account, each star only needs to eject about one Earth’s worth of planetesimals to explain ‘Oumuamua’s detection.  That is easy to explain. Problem solved.

Here is the main idea:

Our origins story for ‘Oumuamua (see here), using images from NASA and ESO/L. Calçada.

So there you go: ‘Oumuamua may be a fragment of a planetesimal that was torn to pieces by a gas giant before being ejected into interstellar space.  Boom!

Our (updated) paper can be downloaded here.

Questions?  Comments?  Words of wisdom?


16 thoughts on “‘Oumuamua: a shred of its former self

  1. Laughlin and Batygin ( https://arxiv.org/abs/1711.02260) found that a planetesimal was most likely to get the gravitational kick necessary to eject it from a planetary system from a giant planet beyond the iceline. Would the close approach needed for tidal disruption (and the larger change in velocity the fragments produced underwent) change where in its original planetary system ‘Oumuamua was most likely ejected?

    1. …or (seeing now that the paper addresses this for a giant planets orbiting outside the iceline) where the planet ejecting it most likely orbited?

      1. Good question! The closer-in the gas giant, the more encounters are needed to statistically eject a planetesimal, so the odds of having a disruption event go up. This goes in the direction of increasing the contribution from rocky planetesimals. However, those rocky planetesimals are higher-density and so may be harder to disrupt. In the end, I don’t know which effect wins, whether fragments are likely to be more cometary or asteroidal. However, 2 things are clear: 1) closer-in gas giants should generate more tidal disruption events; and 2) denser gas giants (with tidal radii that are as far as possible beyond the physical radius) will also cause more tidal disruption.

  2. Something new that fits with one of your earlier posts:

    2004 EW95: A phyllosilicate bearing carbonaceous asteroid in the Kuiper Belt

    “The spectrum bears striking resemblance to those of some C-type asteroids, suggesting that those objects may share a common origin with 2004 EW95.”

  3. This caught my attention: “Yet a modest fraction (roughly 1/4 to 1/3) of fragments should not have become inactive and should instead become active during their passage close to the Sun”

    How would that translate into the fraction that are observed given the greater visibility of the active objects?

    1. Ooh, that is a great question! There are a couple of factors. You’re right that still-active objects would be much brighter and easier to detect. But outgassing can impart strong enough acceleration to make it hard to back out an object’s v_infinity, making it uncertain whether that object is actually of interstellar origin. For example, there are around a dozen known objects on formally hyperbolic orbits but with small enough v_infinity that their true origin is uncertain.

      Add those up and …. I don’t know what a survey would really deduce.

  4. Some recent speculation about `Oumuamua suggested that was is an alien light sail:

    This was prompted by the detection of non-gravitational acceleration,

    And the lack of visible activity which resulted in it being classified as an asteroid.

    I’m going to guess that the lack of visible activity just means no visible dust.

    A paper I came across a while back claimed that the dust activity of comets meant that they had low tensile strengths which indicated they formed via gravitational instabilities which results in slow impact velocities.


    Now for a bit of wild speculation (though not as wild as the alien light sail)

    If `Oumuamua is a fragment of a larger body it would have been subject to stronger compressive forces and thus have higher tensile strengths. Out gassing then could occur without visible dust activity.

    Of course there are probably other explanations.

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