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Most comets are thought to have originated great distances away, traveling to the inner solar system from the Oort Cloud. But new computer simulations show that many comets – including some famous ones – came from even farther: they may have been born in other solar systems. Many of the most well known comets, including the Hale Bopp Comet (above), Halley, and, most recently, McNaught, may have formed around other stars and then were gravitationally captured by our Sun when it was still in its birth cluster. This new finding solves the mystery of how the Oort cloud formed and why it is so heavily populated with comets.
Comets are believed to be leftovers from the formation of the solar system. They are observed to come to the solar system from all directions, so astronomers have thought the comet’s origin was from the Oort Cloud, a giant sphere surrounding the solar system. Some comets travel over 100,000 AU, in a huge orbit around the sun.
But comets may have formed around other stars in the cluster where the sun was born and been captured gravitationally by our sun.
Dr. Hal Levison from the Southwest Research Insitutue, along with Dr. Martin Duncan from Queen’s University, Kingston, Canada, Dr. Ramon Brasser, Observatoire de la Côte d’Azur, France and Dr. David Kaufmann (SwRI) used computer simulations to show that the Sun may have captured small icy bodies from its sibling stars while still in its star-forming nursery cluster.
The researchers investigated what fraction of comets might be able to travel from the outer reaches of one star to the outer reaches of another. The simulations imply that a substantial number of comets can be captured through this mechanism, and that a large number of Oort cloud comets come from other stars. The results may explain why the number of comets in the Oort cloud is larger than models predict.
While the Sun currently has no companion stars, it is believed to have formed in a cluster containing hundreds of closely packed stars that were embedded in a dense cloud of gas. During this time, each star formed a large number of small icy bodies (comets) in a disk from which planets formed. Most of these comets were gravitationally slung out of these prenatal planetary systems by the newly forming giant planets, becoming tiny, free-floating members of the cluster.
The Sun’s cluster came to a violent end, however, when its gas was blown out by the hottest young stars. These new models show that the Sun then gravitationally captured a large cloud of comets as the cluster dispersed.
“When it was young, the Sun shared a lot of spit with its siblings, and we can see that stuff today,” said Levison.
“The process of capture is surprisingly efficient and leads to the exciting possibility that the cloud contains a potpourri that samples material from a large number of stellar siblings of the Sun,” said co-author Duncan.
Evidence for the team’s scenario comes from the roughly spherical cloud of comets, known as the Oort cloud, that surrounds the Sun, extending halfway to the nearest star. It has been commonly assumed this cloud formed from the Sun’s proto-planetary disk. However, because detailed models show that comets from the solar system produce a much more anemic cloud than observed, another source is required.
“If we assume that the Sun’s observed proto-planetary disk can be used to estimate the indigenous population of the Oort cloud, we can conclude that more than 90 percent of the observed Oort cloud comets have an extra-solar origin,” Levison said.
“The formation of the Oort cloud has been a mystery for over 60 years and our work likely solves this long-standing problem,” said Brasser.
“Capture of the Sun’s Oort Cloud from Stars in its Birth Cluster,” was published in the June 10 issue of Science Express.
Source: Southwest Research Institute
Good and interesting article. Thanks.
This article could be made better with a bit more quantitative info. What is the average separation of stars in an star-forming cluster? And, you state that the Oort Cloud extends to 1/2 the distance to the nearest star. Do you really mean about 2 lyrs?
Am I missing something here? The Sun starts out with a tiny number of cometary cores and gains an order of magnitude above that from its siblings.
However unless the Sun system is very different from its siblings then the Sun should have started with a similar number of cores. So, what is it that gave the siblng stars so many cores that the Sun could catch ten times as many as it already had?
Do the models suggest anything about the type of star that would generate such a huge excess of cores? Since all the siblings started from the same collapsing cloud and so should have similar compositions I suppose the main difference would be mass. Could initial angular momentum affect the evolution? Suggestions please?
Wow, … that is something unexpected.
@ sfwrtr:
I cheated and looked at the wikipedia article. Indeed estimates may vary, but tidal effects from other stars that could decide the Oort cloud outer edges are claimed at ~ 100 000 – 200 000 AU, so 1.5 – 3 ly.
Tidal effects or in other words residual forces should go as 1/r^3 for an 1/r^2 force like gravity. For an unbound object like a passing star to affect a bound Oort object it would then have to pass close.
But considering that the nearest stars are now ~ 4 ly out a somewhat closer distance seems like a reasonable guess to me. Especially if we started out in a dense cluster.
Hopefully those who actually know something in the area will jump in and help out.
@ ESA:
The paper is behind a paywall, so I can only use the article.
I don’t see the problem you pose, as I understand it, though. If most systems in our origin cluster donate a common pool of comets among them and later recapture them as the cluster disperse, they would retain more comets than if they had been solitary systems. (So they could each have been no different from the Sun as regards number of cometary cores.)
By the famous synchronicity of the web, UT describes the accretionary phase, and how at least that part has passed a test:
“”Once trapped in the star’s magnetic field, the gas is being funneled along the field lines arching out high above and below the disk’s plane,” Eisner explained. “The material then crashes into the star’s polar regions at high velocities.”
In this inferno, which releases the energy of millions of Hiroshima-sized atomic bombs every second, some of the arching gas flow is ejected from the disk and spews out far into space as interstellar wind.
“We want to understand how material accretes onto the star,” Eisner said. “This process has never been measured directly.”
Eisner’s team pointed the telescopes at 15 protoplanetary disks with young stars varying in mass between one half and 10 times that of our sun.
“We could successfully discern that in most cases, the gas converts some of its kinetic energy into light very close to the stars” he said, a tell-tale sign of the more violent accretion scenario.
“In other cases, we saw evidence of winds launched into space together with material accreting on the star,” Eisner added. “We even found an example – around a very high-mass star – in which the disk may reach all the way to the stellar surface.””
W00t!
Some comets coming from sister systems isn’t at all shocking.
ESA: This would also mean, some “comets” which originally formed with our system were picked up by other solar systems.
This doesn’t mean all systems have approximately the same number of comets. This will depend on exactly “when” they formed in comparison to neighbors, how much gas and debris was available in their area, and just how much mass was gained during the process. This is where models can have problems. You can have a very massive star with relatively few planets, and a less massive star with many planets.
How space-time is warped within a birthing system affects what elements end up where and how long they are available. (I.e. what happened to all the nitrogen in our system?)To make things confusing, planets don’t maintain their original orbits.
As more and more metals become available, the process of creating a solar system will likely change in the future.
Although nobody can really be certain, it is likely the Oort cloud extends to nearly 2LY from the Sun. It seems to get larger all the time. It may extend so far out, as our system moves within the galaxy, we drop some Oort cloud items and pick others up.
It actually wouldn’t shock me to find out some planetessimals are swapped between systems. A star forming region isn’t exactly the most stable place you’ll ever encounter.
When quite a few large objects get near each other, it bends space-time enough to destabilize the area. Enough so, orbital trajectories are changed. It really doesn’t take a lot to change a highly elliptical orbit into a trajectory which will now slingshot an object away (i.e. get another massive object in the area to increase accelleration)
A recent paper by Vadim Bobylev looked into close approaches by known stars over several million years (past and future) and found nine candidates. Gleise 710 was found to have a very good chance (86 percent) of plowing into the Oort Cloud within the next 1.5 million years. While the paper did not address the possibility of an Oort Cloud surrounding Gleise 710, many comments on the paper did raise this point. Check out the post and comments on this topic at Physics arXiv Blog: http://www.technologyreview.com/blog/arxiv/24917/ (also contains a link to the paper!)
Not really, if the current ideas are true AFAIU them. Star gravity, radiation and magnetic fields sets up a “distillery” chain with the protoplanetary ring as component. It aggregates dust but it also creates and moves molecules around. [My reference is “Lectures in Astrobiology” by Barbier et al.]
Torbjorn…
Your “Distillery” comment is exactly what I am describing. Gravity has quite a bit to do with it, creating the different levels (or chambers) within the towers. Some items end up at certain levels, and react at given temperatures/pressures. A very good reason there are so many differences among the planets in our solar system.
Think of the changes in gravity the same as levels in a distillery tower; which is likely where the largest planets end up (more gravity collects more elements during the given time), along with the elements trapped at those levels. The only difference being, there is no pattern of levels; the heaviest elements will not always be at the bottom of the tower…so to speak.
I haven’t seen magnetic forces really cause any differences in models. This force seems to be more of an effect than a cause. Not all elements create a magnetic field, yet they all have gravity. Although the thought of two large planets repelling each other due to magnetic fields being strong enough to overcome gravity is something I wouldn’t mind playing with in a simulation.
Radiation… depends exactly what you mean by this. Since most elements don’t give off any radiation, and it doesn’t really become abundant until the star begins its nuclear reaction. Even then, I don’t think there is enough energy from radiation to change orbits or a heavy objects velocity.
However, it may work as a ‘fan’ to move free elements which are near the sun outwards…mixing some things up a bit.
Aodhhan, frankly I have no idea what you are saying here.
I gave my reference; I dunno if the material can be found on the web.
Here goes in a nutshell:
The accretion-ejection model of Shu et al (1997) describes how the large scale structures of the magnetic field channel protoplanetary ring gas and dust toward the star and outwards under rotation. ~ 90-70 % of the material is not accreted but forms the “X-wind”.
That is build on by Gounelle et al (2001) which posits that X-rays and particle radiation from the star polar magnetic reconnection regions hits the appropriately named “X-region” behind the ring shield closest to the star. Neutral dust ejected from this region rains ballistically back on the disk.
In all of this churning and radiation, as well as later UV radiation from the energetic newborn star, radioactive isotopes, molecules and dust are created and dispersed.
From this follows the following problems with your claims:
– Gravity is not a static cell maker in this model.
– This has nothing directly to do with later planetary formation or planetary magnetic fields which mainly happens after the accretionary phase. Note the later UV influence though.
– The existence of magnetic fields make a huge difference.
– Elements don’t have to be magnetic to respond to magnetic fields, it is enough that they are ionized.
– Radiation, mostly X-ray (but also some protostar IR of course), is abundant long before the protostar ignites.
As for planets repelling each other due to magnetic fields, such dipole fields goes like 1/r^3 as opposed to gravity’s 1/r^2 and would interact weakly. And don’t even think about electrostatic fields.
[As comparison, the rotation of Earth’s magnetic field generates a whooping ~ 40 C (!) spread over Earth volume, I got that from an astrophysicist seminar. Ta-fucking-da!]
While looking into the particulars of the forthcoming encounter with Gliese 710 I found that a possible ‘comet-swap’ encounter with Algol occurred about 7 Myr ago. Compared to Gliese 710 (dwarf type dM1 or K7 V, mass ~0.6 Msolar), Algol is a triple star system with a total mass ~5.8 Msolar. According to a 1999 study by Garcia-Sanchez, about 7 Myr ago Algol passed within ~2.6pc of the Sun. This was seen as the second largest potential perturber of the Oort Cloud in the time frame examined (+- 10 Myr). During this close encounter some exchange of Oort Cloud materiel may have occurred (the paper offers no mention of this though). The upcoming approach of Gliese 710 is also detailed. The paper “Stellar encounters with the Oort Cloud based on Hipparchos data” is available here: http://iopscience.iop.org/1538-3881/117/2/1042/pdf/1538-3881_117_2_1042.pdf
What would be the size of a cometary reservoir around a system like Algol? Demonically large. 🙂
Am hoping to see C. McNaught this weekend… WX permitting. Would be #42 para moi… I likes comets!