NCCR PlanetS: Capturing Alien Comets

[bystander]

Capturing Alien Comets
National Centre of Competence in Research PlanetS | 2019 Dec 19

Comet Borisov in front of a distant background spiral galaxy imaged by the Hubble Space Telescope on November 16, 2019. (Credit: NASA, ESA, and D. Jewitt (UCLA))

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There should be interstellar comets hiding in our Solar system after making a journey of many lightyears. Maybe we have already seen one but believed it was a “normal” comet formed in the Solar system. ...

Comets that suddenly light up in the sky, only to disappear again after some weeks or months, have fascinated humanity for centuries. Where do these exotic objects come from? According to the most popular theory proposed by Dutch astronomer Jan Oort, during a very early phase of the Solar system’s formation the giant planets scattered objects into the outer regions far away from the Sun. There, the icy rocks and dust particles formed a kind of cloud. Passing stars may then scatter these objects back into the inner Solar system where we observe them as comets. Coming from the Oort cloud these long-period comets often need many more than 200 years for one orbit around the Sun.

“We present a second potential origin for such comets,” says Tom Hands, postdoc at the Institute for Computational Science of the University of Zürich: “They can be captured out of interstellar space in the relatively recent past.” Two interstellar visitors made headlines in the past years. In 2017, the first such object was detected, an asteroid-like body later named Oumuamua. In August 2019 amateur astronomer Gennady Borisov found a comet that came from interstellar space and will leave the Solar system again. Oumuamua and Comet Borisov are both leftovers of planet formation in other Solar systems, in the same way our comets and asteroids are thought to be the leftovers of planet formation in our Solar system. ...

Capture of Interstellar Objects: A Source of Long-Period Comets ~ Tom Hands, Walter Dehnen

• Monthly Notices of the RAS: Letters (online 18 Dec 2019) DOI: 10.1093/mnrasl/slz186
• arXiv.org > astro-ph > arXiv:1910.06338 > 14 Oct 2019

[saturno2]

How do we know than these comets come from outside
The Solar System?

[BDanielMayfield]

How do we know than these comets come from outside
The Solar System?

So far, the only way to tell is by such objects having a speed so fast that they just pass in and out of our system without being bound to the Sun.

But that didn't really answer saturno2's question. How do we even know that any comet that is bound to our system was originally formed in our system? The paper herein discussed proposed that some of the Sun's family of comets were captured by Jupiter's gravity. Such objects would loose enough momentum to then become bound to the Sun. Then indeed it would be very hard to tell if such a comet's origin was interstellar.

[BDanielMayfield]

Here are more details from the above report that bystander posted:

The results of the simulations now published in the Monthly Notices of the Royal Astronomical Society (MNRAS) reveal that in a small minority of cases, the trajectories of the objects are altered enough by Jupiter that they become bound to the Solar system. “Although the capture probability is small, there could be anywhere from a few hundred to hundreds of thousands of these alien comets orbiting the Sun,” says the astrophysicist. Captured objects are typically on orbits very similar to those of long-period comets that humanity has observed for centuries, suggesting that they are hiding in plain sight. “If we could identify one, we would have a real possibility to study the composition of material formed in other Solar systems in close detail,” says Hands.

ESA has recently selected a mission called Comet Interceptor which is designed to make a fly-by of a long-period or interstellar comet. The University of Bern will provide the camera system and the mass spectrometer for this mission and will search for differences between these objects and those comets known to have their origins in our Solar System.

In an earlier paper published in May 2019, Hands and colleagues studied how close interactions between stars in their birth cluster affect the comets and asteroids formed around each star. They found that objects may become liberated and left “free-floating” in the galaxy, or alternatively “stolen” by other stars. This led them to suggest that the Oort cloud might be populated partially by objects that were formed around other stars but then captured by the Sun in its birth cluster billions of years ago (see: Stolen comets and free-floating objects). This latest study investigates the capture of free-floating asteroids and comets, which may have been liberated from their parent star by the mechanism demonstrated in the earlier study.

[neufer]

How do we even know that any comet that is bound to our system was originally formed in our system? The paper herein discussed proposed that some of the Sun's family of comets were captured by Jupiter's gravity. Such objects would loose enough momentum to then become bound to the Sun. Then indeed it would be very hard to tell if such a comet's origin was interstellar.

Scattered disc objects have a good chance of slipping in front of Jupiter (moving in roughly the same direction & at a slow relative speed of ~6km/s) and being slowed down into periodic comets. Oort cloud & alien objects (moving in random directions & at faster relative speeds) are much less likely to be so affected.

<<The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objects (SDOs) have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units. These extreme orbits are thought to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune.

Although the closest scattered-disc objects approach the Sun at about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered objects among the coldest and most distant objects in the Solar System. The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects traditionally called the Kuiper belt, but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the Kuiper belt proper.

Because of its unstable nature, astronomers now consider the scattered disc to be the place of origin for most periodic comets in the Solar System, with the centaurs, a population of icy bodies between Jupiter and Neptune, being the intermediate stage in an object's migration from the disc to the inner Solar System. Eventually, perturbations from the giant planets send such objects towards the Sun, transforming them into periodic comets. Many objects of the proposed Oort cloud are also thought to have originated in the scattered disc. Detached objects are not sharply distinct from scattered disc objects, and some such as Sedna have sometimes been considered to be included in this group.>>

[saturno2]

Thanks for the explanations