Making Mini-Oort Clouds

A new set of simulations shows that systems with so-called "hot Jupiters" might also have mini-Oort clouds detectable by today's space telescopes, giving astronomers a new potential tool for finding exotic extrasolar systems.

A newborn stellar system is a messy place. Infant planets compete to accrete gas and material, gobbling up anything that comes close. Sometimes these newly formed planets will pass near each other, gravitationally ricocheting into a new orbital configuration. Today’s space telescopes could detect the clouds of icy bodies produced by these close encounters, pointing observers to certain types of planetary systems.

The solar system's Oort Cloud contains trillions of icy objects and extends to perhaps 5 trillion miles (50,000 astronomical units) from the Sun. Simulations suggest that extrasolar systems with "hot Jupiters" may have similar, but much smaller comet clouds that would nevertheless be detectable by space telescopes.
Don Davis
A new study by Sean Raymond (Universitè de Bordeaux) and Phillip Armitage (University of Colorado) examined the effects of planet interactions on the remaning planetesimals in simulated solar systems. Each simulation started with 3 gas giants and a disk of smaller bodies. Raymond and Armitage tracked the interactions over time, measuring the position and velocity of every object in the simulation. They chose a variety of initial masses and locations for their planets and ran thousands of simulations. They found that for a small fraction of systems, a cloud of small bodies would form about 100 to 1,000 times farther away from the central star than Earth’s distance from the Sun.

These clouds are 1,000 times smaller than the solar system’s Oort Cloud, the flock of small bodies thought to be our source of long-period comets. Nevertheless, the mini-Oort clouds share many Oort Cloud characteristics: they're roughly sphere-shaped (rather than contained in a disk) and the individual objects take a long time to complete their highly elliptical orbits. Their origin is similar to the Oort Cloud’s too. When gas giants graze the planetesimal disk, the ensuing gravitational mosh pit adds energy to the planetesimals, flinging them to more elliptical and inclined orbits. Some of the objects are ejected from the system completely.

In most of the simulations, only one gas giant remains while the other two are completely ejected. The surviving giant migrates inward, becoming a “hot Jupiter,” scorchingly close the central star. With the gas giants out of the picture, the planetesimals settle into new orbits quite different from their original trajectories.

The interaction between planetesimals in their new orbits would produce dust that would be visible with the Spitzer and Herschel Space Telescopes. If Raymond and Armitage are correct, then the presence of hot Jupiters and mini-Oort clouds would be correlated. It is now up to observers to test this prediction and see if the gravitational madhouse that is a newborn solar system really does produce comets and mini-Oort clouds around other stars.

15 thoughts on “Making Mini-Oort Clouds

  1. Peter

    A mini-Oort cloud sounds plausible. I am still not convinced that our “giant” Oort cloud is real. After all, a more-or-less random distribution of comets in interstellar space is going to produce a spherical distribution. How can we be sure that Oort cloud comets are not from interstellar space? Can astronomers really distinguish between a comet in a 10,000 year elliptical orbit and an unbound hyperbolic one? In its 4.6 billion year journey through the galaxy, the sun has had many "close" encounters with other stars. How could comets 50,000 astronomical units away not be tidally disrupted in such encounters, and cast into interstellar space?

  2. Bruce

    Has Peter the contrarian struck again!? So now you’re trying to rob my distant cousin Oort of the thing that made him famous? My Dutch half stubbornly rebels. While it’s true that some comets are interstellar visitors, isn’t the standard thinking that since comets are transitory objects, without the Oort Cloud’s existence the number of comets would have petered out long ago? (Sorry, couldn’t resist :) You contend that “the sun has had many ‘close’ encounters” in it’s lifetime, but is that really so? Rounding things off, one AU is 1.5×10^11 meters, so 50,000 AU is 7.5 x 10^15 meters, which divided by a lightyear of 9.46×10^15 meters is about .79 ly. Our closest stellar neighbor, the Alpha Cen system system which is now about 4.21 ly distant is destined to have it’s closest approach in about 27,700 years when it will close to 3.26 lys. So that next ‘close encounter’ will still be over 4 times the radius of the questioned Oort Cloud radius. Therefore I question your questioning of the Oort Cloud’s existence Peter. I don’t think close encounters of the stellar kind are close enough nor frequent enough to do what you are suggesting.

  3. Peter Kontrarski

    That may have been their reasoning, but the fact that long period comets haven’t petered out supports both pictures: a local spherical cloud and a diffuse galactic cloud. Not to mention, all the simulations include a portion of the objects simulated being kicked into interstellar space in random directions, so there’s a source for the galactic cloud. Finally, the sun was presumably born in an open cluster like the Pleiades, so at one time had close neighbors, regardless of its wanderings since. Anyway, I am just asking if the issue has ever really been settled? In my understanding, Oort proposed the model, it worked, and that was that. If somebody else has proposed the galactic cloud model, and it’s been studied and rejected after extensive debate and observations, like the steady-state model, I am ignorant of it. Not arguing for a galactic cloud and against Oort‘s, just asking if there definitive evidence for one over the other?

  4. Rod

    Bruce et al. These are some interesting comments posted on this report. Consider that all long-period and short-period comets are transient or short-lived when compared to the 4.6 billion years old Sun model age, are the comets observed much more recent in origin, i.e. not billions of years ago? If the comets are much younger in origin you do not need an Oort Cloud to explain the observations.

  5. Bruce

    Richard, I think that there may be two gentlemen named Peter posting here. The first one’s last name starts with W I would wager if I did that sort of thing. Mr. Kontrarski does indeed sound like a very thoughtful, non-confrontational sort though. The world needs both types of thinkers, because without open minded (that does not mean gullible) people the contrarians (who are sometimes found to be right!) would never be listened to.
    Rod (@Rod seems more confrontational than I want to be), your question leads me to wonder, how can we measure cometary ages and/or origins? Is it possible to detect isotopic differences in the spectra of meteors, most of which come from known comets?

  6. Rod

    Bruce, the problem I pointed out concerns how many perihelion passages a long-period or short-period comet can survive. Each perihelion passage a comet will lose mass, many breakup too. The 4.6 billion years old Sun, it is tough to show comets can survive for so long a time span. Thus you need an Oort cloud to avoid time problems with comets and the age of the solar system. Without the Oort cloud, comets point to a recent origin, not 4.6 billion years ago.

  7. Peter

    The recent-origin requirement that Rod points out means the theory requires both a hypothetical cloud and a recent disturbance to account for the observed comets. It seems just as reasonable to start with a known cloud, that of comets cast into interstellar space during planet formation throughout the galaxy, and guess that one of them was drifting through the Milky Way for billions of years until it crossed Sol’s path at less than escape velocity ten million years ago. The accepted model is a hypothetical cloud-plus-disturbance, and the contrary view is a known cloud-plus-capture. The comments have been interesting, but I am assuming there’s not enough data to tell which model is correct.

  8. Bruce

    Everyone should be able to agree that all short-period comets are transitory. However, Rod’s statement asking ‘how many perihelion passages a long or short period comet can survive’ prompted me to research our prime example: Halley’s Comet. I submit the following facts and suppositions re Halley’s; Average period of about 75.3 years, current mass of 2.2E14 kg, thought to have been in it’s present orbit for from between 16 to 200 thousand years, before that being in a long period orbit, and before that possibly being a member of the questioned Oort Cloud. It’s also thought that Halley’s could possibly have lost from 80 to 90 percent of it’s original mass, but in spite of all that, it may still be around for up to 10 million more years! I know 10 million is short compared to 4.6 billion years, but it’s still not what I usually think of when I hear the word transitory. Imagine how long a long period comet might endure if it never passes deeply into the inner solar system. Think too of how surprisingly durable comets can sometimes be, taking last year’s sun grazing survivor Lovejoy as an example. Sorry cousin Oort, but my conviction that there needs to be a big honkin’ pile of future comet makers hanging out yonder in your cloud has wavered a little. I guess I’m not that stubborn after all, relatively speaking.

  9. Rod

    Bruce et al. Again some good observations posted on the Oort cloud and life time of comets. If we look at the definition of science like this:

    A brief summary of the meaning of science can be developed from the Latin scientia, Greek gnosis, and Hebrew madda and yada. Science based upon investigation, observation; knowledge that is “certain and evident”.

    The Oort cloud and survival times of comets are tough to fit into a description of certain and evident :) Bruce here is a reference to short period comet lifetimes:

    "The median lifetime of all known short-period comets from the current time to ultimate destruction or ejection is approximately 4.5 x 10^5 years." The long-term dynamical behavior of short-period comets, http://adsabs.harvard.edu/abs/1994Icar..108…18L, March 1994.

  10. Aqua4U

    I’ve found 45 comets with my little 4" scope! (I like!) w/#46 due any day now! (Am building a 12 1/2" Newtonian w/Ger.eq.) My favorites comets include Comets Halley, Holmes(Oort) and Shoemaker Levy 9(Oort). But Comets Halley, Hyakutaki, Hale-Bopp and McNaught which possibly originated in other solar systems were the most interesting! Captured by our Oort cloud and only eventually gravitationally ‘flung’ inward… they must be incredibly old(er)?

    At present Comet Hergenrother appears to be disintegrating rather like 3P/Schwassmann-Wachmann? Are these ‘younger’ comets? or debris from a much larger body?

  11. Joe S.

    Comets from an Oort cloud vs. from interstellar space can be distinguished by their velocities and observed orbit. I was taught that we see lots of comets with speeds indicating nearly parabolic motion (predominantly from Oort cloud) and few, if any, with hyperbolic speeds (interstellar). I don’t have the numbers on this, though. If, at any point in the orbit, r, the kinetic energy (mv^2/2) exceeds the potential energy (GMm/r), then the comet is hyperbolic (v>sqrt(2GM/r), where M is solar mass and m is comet mass.

  12. Bruce

    See, its been several days now, but no one can come forward to dispute the soundness of Joe S’s argument. Very well done sir! I love it when a question can be so effectively answered with verifiable mathematical logic. I think you have restored the Oort Cloud to its rightful place in the solar system Joe. I should have never doubted at all. (James 1:6)

  13. Joe S.

    I think the Oort cloud now is believed to have formed as the gas giants migrated inward (and Uranus and Neptune possibly swapped places). The migration was caused by gravitational interaction with each other and the material in disk itself. Their orbits came into resonance with different crowded regions of the inner solar system, eventually ejecting most of those comets. Many simply escaped, but others ended in very long period orbits, so they spend most of their lives way out there – making the Oort cloud. It was this same stirred up period of our solar system that caused the late heavy bombardment, evidence for which is seen on the Moon and Mercury (4.1 to 3.8 Gyr ago). Of course, there are competing theories (for example, our Sun exchanging cometary material with other stars prior to 4 Gyr ago while we were still in proximity to our stellar nursery).

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