Stealing from the Solar System: the effects of a stellar fly-by

Posted: August 18, 2018 by oldbrew in Celestial Mechanics, modelling, research, solar system dynamics
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The highly tilted orbit of Eris compared to the orbits of Ceres (light blue), Jupiter (maroon), Saturn (orange, Uranus (green), Neptune (blue), Pluto (olive, and MakeMake (red) [image credit: Fandom]

Could a ‘rogue’ star passing nearby have disturbed outer parts of the early solar system? Beyond Neptune things become somewhat different.

The outer reaches of our solar system harbor a number of mysterious features. Astrobites reports on whether a single stellar fly-by could help explain them all.

A star is born from the gravitational collapse of a cloud of gas and dust. Yet not all of the material ends up in the star, and instead forms a flat protoplanetary disk that surrounds the new star. Over time, the materials in this disk coalesce to form planets, moons, asteroids, and most other objects you might expect to find near a typical star.

Since protoplanetary disks are flat, the expectation is that all of the planets and objects orbiting a star that formed out of a protoplanetary disk should orbit on a single plane. So when we find stars with planets that orbit at multiple different inclinations, this raises questions.

A recent astrobite discussed such a case, where an exoplanet was observed orbiting on a completely different plane than the other exoplanets in that same system. But we needn’t look that far to find deviations like this — our very own solar system exhibits several features that don’t line up, so to speak.

Inclinations and Eccentricities and Truncations, Oh My!

For the first 30 AU around the Sun (until right around where Neptune orbits) things are relatively “well-behaved”: most planets’ orbital inclinations only differ from each other by 1–2 degrees, and no planet has an inclination of more than 8 degrees. But beyond Neptune, in the outer solar system, orbital inclinations are considerably higher.

Pluto, recently demoted from planet to dwarf planet, is one example; its orbital inclination is more than 17 degrees. The same trend exists for orbital eccentricities, which tend to be significantly larger for objects beyond 30 AU compared to those inside the 30 AU cutoff.

A similar pattern also exists in our solar system’s surface density profile, which can be obtained by smoothing out the cumulative mass of solar system objects (planets, moons, asteroids, etc.) to approximate what the Sun’s protoplanetary disk might have looked like. The surface density profile gradually declines until ~30–35 AU, where it drops abruptly by a factor of nearly 1,000 (a phenomenon often referred to as disk truncation). Coincidence? Perhaps not.

Astronomers seek to develop theories that can help explain these peculiar features in our solar system. One proposed explanation claims that the existence of a yet undiscovered faraway planet (sometimes called Planet 9) could cause these effects on the outer solar system. A second possible explanation, involving a supernova going off near the solar system in its early days, was covered in another recent astrobite.

Today’s paper offers yet another explanation — what if a star flew by our Sun early on, stealing a bunch of the outer material from the Sun’s protoplanetary disk with it, and throwing what was left into inclined and eccentric orbits?

Continued here.

  1. oldbrew says:

    3. The Kuiper Cliff

    Why does the Kuiper Belt suddenly end? The Kuiper Belt is a huge region of the Solar System forming a ring around the Sun just beyond the orbit of Neptune. It is much like the asteroid belt between Mars and Jupiter, the Kuiper Belt contains millions of small rocky and metallic bodies, but it’s 200-times more massive. It also contains a large quantity of water, methane and ammonia ices, the constituents of cometary nuclei originating from there (see #4 above). The Kuiper Belt is also known for its dwarf planet occupant, Pluto and (more recently) fellow Plutoid “Makemake”.

    The Kuiper Belt is already a pretty unexplored region of the Solar System as it is (we wait impatiently for NASA’s New Horizons Pluto mission to arrive there in 2015), but it has already thrown up something of a puzzle. The population of Kuiper Belt Objects (KBOs) suddenly drops off at a distance of 50 AU from the Sun. This is rather odd as theoretical models predict an increase in number of KBOs beyond this point. The drop-off is so dramatic that this feature has been dubbed the “Kuiper Cliff.” [bold added]
    – – –
    Planets beyond Neptune

  2. Graeme No.3 says:

    It must be due to man made Climate Change. Screams hysterically and hops backward until Christmas.

  3. p.g.sharrow says:

    Just what makes them think that the proto system nebula was a simple disk? Only the gravity drag of the spinning accreting star would create a disk of the nearby material of the nebula. The nebula would be material in chaos that developing gravity would drag into organization. As the proto disk is formed, eddies in it would form the interior planets.
    Our solar system is a 3rd generation creation from the materials spewed out from previous star novas, so why can’t existing planets be among the debris?
    Somehow these “scientists” start with simple creations and then make them complex. In the real Universe complex chaos tends to become organized into more simple things…pg

  4. E.M.Smith says:

    The “star truncated the disk” solution looks like a solution to an already discovered thing. From their link per supernovae:

    The shockwave that sweeps through the system can strip material away from the protoplanetary disk completely. Just like we saw with the radiation, this stripping affects the outer edge of the disk more than the inner disk. The result is therefore a truncated disk, in which the outer edge has

    So just looking for more ways it could have been done or finding ways to keep the grants coming?

  5. ivan says:

    Maybe Clifford D. Simak had the right idea in his 1973 short story Construction Shack. 🙂

  6. J Martin says:

    Didn’t the binary research institute people have that as one of the side effects ?

  7. oldbrew says:

    J Martin – yes, worth a mention.
    – – –
    Then there’s Sedna – a bit smaller than Pluto, and ‘way out there’.
    Semi-major axis 506.8 AU, perihelion 76 AU

    If Sedna formed in its current location, the Sun’s original protoplanetary disc must have extended as far as 75 AU into space.[46] Also, Sedna’s initial orbit must have been approximately circular, otherwise its formation by the accretion of smaller bodies into a whole would not have been possible, because the large relative velocities between planetesimals would have been too disruptive. Therefore, it must have been tugged into its current eccentric orbit by a gravitational interaction with another body.[47] In their initial paper, Brown, Rabinowitz and colleagues suggested three possible candidates for the perturbing body: an unseen planet beyond the Kuiper belt, a single passing star, or one of the young stars embedded with the Sun in the stellar cluster in which it formed. [bold added]

  8. Tim Spence says:

    I think it’s entirely possible that a large roaming planet could traverse the solar system (and most likely has already) without colliding with anything or disturbing the force much, because the solar system is 99% empty. For something else to disturb the force from outside, sufficiently to re-incline outer orbits by 5 to 10º would have to be a heck of a force, but over billions of years clearly possible, that’s astronomy.

    Isn’t our galaxy colliding with Andromeda according to recent news?

  9. J Martin says:

    Andromeda collision. If Andromeda were brighter it could be seen to take up as much space in the night sky as the moon. I’m hoping to live long enough to witness the collision between the milky way and the Andromeda galaxy.

  10. oldbrew says:

    By the time Andromeda meets the Milky Way the Sun will be near the end of its working life.