How do we know that ʻOumuamua is interstellar and not otherwise? I fail to understand the logic that the extraordinary speed of the object proves it to be of interstellar origin, and that our sun's gravity failed to capture it.

  • $\begingroup$ Wikipedia gives several links to follow : origin of Oumumua. You need to know what escape velocity is to properly understand why speed matters. Your second question is just ridiculously broad. $\endgroup$ Nov 25, 2018 at 8:17
  • $\begingroup$ Your question should be "Why do scientists believe that Oumumua's origin is likely to be beyond the solar system?" "Know" is too strong of a word here, and so your question is a bit leading as currently phrased. Oumumua is also misspelled, and lacks capitalization. $\endgroup$
    – uhoh
    Nov 25, 2018 at 9:06
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    $\begingroup$ @uhoh - I do believe that know is indeed the correct term here. $\endgroup$ Nov 25, 2018 at 9:14
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    $\begingroup$ @uhoh Guilty as charged! My question should have been the way you said it should have! Makes much sense that way! "Know" indeed is very strong word here but I had to use it because I have come across the idea several times reading about oumuamua, that no other object within our solar system have been known to travel near speeds of the object under consideration. The thought that came to me first was if we have amassed enough knowledge about our solar system to be confident about the impossibility of any new object to be able to travel near the speed of oumuamua! $\endgroup$
    – asabhish
    Nov 25, 2018 at 9:51
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    $\begingroup$ It's hard to see how this can be answered. The reason it is sure to have an interstellar origin is its very high speed, much larger than an object simply falling from the Oort cloud. If you could say what your objection to this is, then perhaps it would make a question. $\endgroup$
    – ProfRob
    Nov 25, 2018 at 9:52

2 Answers 2


I'll start with the second question first:

Also, do we exactly know about each and every object inside our solar system?

Each and every object? Of course not. That however is irrelevant to the question at hand. The vast majority of objects in our solar system are very, very small. Quite literally as small as dust. But because they're so small, they have essentially zero impact on the orbits of objects that are not as small as dust.

How do we know that ʻOumuamua is interstellar and not otherwise?

Because it's moving so quickly. ʻOumuamua's huge 26 kilometers per second excess means that is not possible that ʻOumuamua originated within the solar system.

I'll start with the two body problem, e.g., the Sun and some other object interacting gravitationally. The Newtonian gravitational interaction between two objects is a conic section: Either a circle, an ellipse, a parabola, or a hyperbola. The total mechanical energy (kinetic energy plus gravitational energy) for objects following a circular or elliptical trajectory is negative. Such objects are gravitationally bound to one another. Hyperbolic trajectories on the other hand have a positive mechanical energy. Parabolic trajectories form the boundary cases, where the total mechanical energy is exactly zero. Objects on parabolic or hyperbolic trajectories are not bound. They visit once and then they're gone. The excess velocity of an object on a parabolic trajectory (zero excess velocity) or a hyperbolic trajectory (positive excess velocity) is a constant of motion in the two body problem.

The N-body problem makes things a bit dicier. Long-period comets are believed to have originated from within the solar system. These comets appear because something perturbed their very long-period orbits. The perturbation make those long-period comets dive well inside Neptune's orbit. A few of those long-period comets appear to be following an unbound trajectory (non-negative excess velocity). A chance close encounter with a planet can make that happen.

This raises the question: Could ʻOumuamua be yet another long-period comet whose orbit has been perturbed to an extent that puts it on a hyperbolic trajectory? This would not be the first case of an object with an over-unity eccentricity. The answer to this question is no. A close encounter with any of the four known giant planets, or even with the conjectured Planet IX, or even multiple encounters, could not have given ʻOumuamua the excess velocity that has been observed for it.

  • $\begingroup$ Your explanation makes partial sense to me! Going through it made me realise that I need to study way more than I know to understand the concept of oumuamua and such things! But thank you anyways for the effort you put! Please allow me to do some more homework and cross question you for better understanding! Meanwhile.... Apologies and thanks at the same time!! $\endgroup$
    – asabhish
    Nov 26, 2018 at 18:00
  • $\begingroup$ A close encounter with any of the four known giant planets, or even with the conjectured Planet IX, or even multiple encounters, could not have given ʻOumuamua the excess velocity that has been observed for it. Can you provide a reference/evidence for that claim (which I don't dispute)? $\endgroup$
    – Walter
    Jun 21, 2019 at 16:05

It's not that much about the speed, but about the orbital eccentricity. Wikipedia gives a good explanation:

Based on observations spanning 34 days, ʻOumuamua's orbital eccentricity is 1.20, the highest ever observed. An eccentricity exceeding 1.0 means an object exceeds the Sun's escape velocity, is not bound to the Solar System and may escape to interstellar space. While an eccentricity slightly above 1.0 can be obtained by encounters with planets, as happened with the previous record holder, C/1980 E1, ʻOumuamua's eccentricity is so high that it could not have been obtained through an encounter with any of the planets in the Solar System. Even undiscovered planets in the Solar System, if any should exist, could not account for ʻOumuamua's trajectory nor boost its speed to the observed value. For these reasons, ʻOumuamua can only be of interstellar origin.


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    $\begingroup$ I'm not going to downvote, but it's all about velocity. For one thing, that's how an object's orbit is calculated. The two key measures that go into determining a non-cooperative object's orbit are range (the distance between the object and a ground station) and even more importantly, range rate. Angular position provides a very noisy third and fourth measurements. Multiple such measurements must be made so as to provide sufficient data for orbit determination. Secondly, excess velocity speaks more to what is or is not possible from a planetary encounter than does eccentricity. $\endgroup$ Nov 26, 2018 at 16:47
  • $\begingroup$ @DavidHammen OK, I agree with you. There's no high eccentricity without high velocity. $\endgroup$
    – stackzebra
    Nov 26, 2018 at 18:01

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