Moons like Earth's might be common

[GrandMasterTerwynn]

Relatively speaking

Moons like Earth's could be more common than we thought
By Jason Palmer Science and technology reporter, BBC News
Earth-like planet as seen from rocky moon artwork The interactions that form comparatively large moons like our own were thought to be rare
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About one in 10 rocky planets around stars like our Sun may host a moon proportionally as large as Earth's, researchers say.

Our Moon is disproportionately large - more than a quarter of Earth's diameter - a situation once thought to be rare.

Using computer simulations of planet formation, researchers have now shown that the grand impacts that resulted in our Moon may in fact be common.

The result may also help identify other planets that are hospitable to life.

A report outlining the results will be published in Icarus.

Last year, researchers from the University of Zurich's Institute of Theoretical Physics in Switzerland and Ryuja Morishima of the Laboratory for Atmospheric and Space Physics at the University of Colorado in the US undertook a series of simulations to look at the way planets form from gas and smaller chunks of rock called planetesimals.

Our own moon is widely thought to have formed early in the Earth's history when a Mars-sized planet slammed into the Earth, resulting in a disc of molten material encircling the Earth which in time coalesced into the Moon as we know it.

The team used the results from their initial study as the input to a further "N-body simulation" to find out the likelihood that large-scale impact events could form large satellites in the same way.

Their results showed that there is about a one in 12 chance of generating a system comprising a planet more than half the Earth's mass and a moon with more than half that of our Moon (taking into account the errors in the simulation, the full range of probabilities was between one in 45 and one in four).
Stabilising influence

Sebastian Elser of the University of Zurich said the new estimates for the likelihood of Moon-sized satellites could inform the hunt for extrasolar planets.

Such large moons can confuse the measurements that spot the planets, and knowing that large satellites may be common could make the measurements easier.
Moon formation artwork The cataclysmic impact that resulted in the Moon still presents a number of computational mysteries

Also, our Moon has served to stabilise the tilt of the Earth's axis - or its obliquity - which could otherwise have varied drastically over relatively short time scales. That in turn would wreak drastic changes to the way heat from the Sun is distributed around the planet.

It thus can be said that the Moon's presence made a more stable environment in which life could evolve, Mr Elser said.

"Checking for the possibility of an obliquity-stabilising moon is a good thing if you're trying to find out how many habitable worlds are out there in the galaxy," he told BBC News. "But it's surely not the only one and not the most important."

Eiichiro Kokubo is a planet formation expert who has published widely on the mechanics behind the development of both the planets in our Solar System and the Moon.

He called the result an "interesting estimate" but cautioned that there are several as-yet unknown parameters "which greatly affect lunar formation and evolution and thus the probability of hosting a large moon".

He told BBC News that, for example, it is still impossible to put numbers to the effects of a planet's initial spin before impact, or how the disc of material is formed and evolves after it.

"I think we should take the paper as a trial calculation based on what we know about formation of terrestrial planets and moons today," he said.

A fascinating find. If giant moons like the one orbiting Earth are as common as possibly one in every ten rocky planets getting one, it has interesting implications for things such as the commonness of life more advanced than blue-green algae.

[Serafina]

Indeed.

By the way, the reason why they might be so common is fascinating by itself.
Basically, much more planet-sized objects form during the formation of a star system than previously thought. However, most of these planets do not achieve a stable orbit around their star and get flung off after some million years. Those planets then drift trough space without orbiting a star - that makes them very hard to detect, but recent estimates based on gravitational measurements suggest that there are between 200 and 400 billion of them in our galaxy (and then there are the planets orbiting stars). That's about twice the amount of stars in our galaxy.

Now because so many planets form during stellar formation, the odds of the colliding are pretty good, especially if you factor in their gravitational attraction. We previously thought that only a few planets form and that the rest is debris that drifts into the Oort Cloud or get's sucked up by gas giants (which still happens, Jupiter probably swallowed a few smaller planets) - now that we know there are plenty more our Earth stops being special. Once again.

[Guardsman Bass]

That's good to hear.

The Earth-Moon system seems relatively stable, even if the Moon is slowly drifting farther out in terms of orbit. How stable would multiple-moon systems be around an Earth-sized planet? Would the moons have to be drastically smaller, like how Mars' two moons are nothing more than captured asteroids?

[madd0ct0r]

i think (guess really) that the bigger the moons, the less likely the orbit will be stable.

Assume a randomly generated planet and moon system. For it to be stable in the long term, those two moons can interfere with each others orbit, but it must be cyclic, returning to the start point eventually. Out of all of the possible interactions, these ones seem like a tiny proportion and are therefore unlikely (but i can't prove it).

The bigger the moon is, the bigger the area of space it can effect significantly, so the bigger each moon is, the more likely they'll affect each other significantly. Then we have to check whether that interaction falls into the cyclic class or not. If not, you are likely to have something interesting and permanent happen.

One argument against this could be convergence. If the system ain't stable, things will change and the system might move into a more stable configuration. The closer to stable the system is, the slower things change, asymptotically approaching a stable cyclic solution. The tide-locked moons of Jupiter are a good example here.
Then you have to calc the number of possible paths leading to the stable solution, and the number of paths were something interesting happens en-route.

[cosmicalstorm]

There have been a number of findings like this one recently. The extraordinary number of planets in the galaxy, the tendency of organic molecules to form more and more complex patterns when subjected to early earth conditions and so on. The great silence gets more and more troubling for every year that passes.

[Alerik the Fortunate]

As troubling as the great silence is, we should remind ourselves that we've only been looking for a period so short that it might not be considered by alien civilizations to be too long a time to wait for the punchline of some cosmic joke. Indeed we may still be looking with blind eyes.

[Surlethe]

It's pretty awesome that the BBC is linking to arXiV. Here are the links.

Initial study: http://arxiv.org/abs/1007.0579

We present results from a suite of N-body simulations that follow the accretion history of the terrestrial planets using a new parallel treecode that we have developed. We initially place 2000 equal size planetesimals between 0.5--4.0 AU and the collisional growth is followed until the completion of planetary accretion (> 100 Myr). All the important effect of gas in laminar disks are taken into account: the aerodynamic gas drag, the disk-planet interaction including Type I migration, and the global disk potential which causes inward migration of secular resonances as the gas dissipates. We vary the initial total mass and spatial distribution of the planetesimals, the time scale of dissipation of nebular gas, and orbits of Jupiter and Saturn. We end up with one to five planets in the terrestrial region. In order to maintain sufficient mass in this region in the presence of Type I migration, the time scale of gas dissipation needs to be 1-2 Myr. The final configurations and collisional histories strongly depend on the orbital eccentricity of Jupiter. If today's eccentricity of Jupiter is used, then most of bodies in the asteroidal region are swept up within the terrestrial region owing to the inward migration of the secular resonance, and giant impacts between protoplanets occur most commonly around 10 Myr. If the orbital eccentricity of Jupiter is close to zero, as suggested in the Nice model, the effect of the secular resonance is negligible and a large amount of mass stays for a long period of time in the asteroidal region. With a circular orbit for Jupiter, giant impacts usually occur around 100 Myr, consistent with the accretion time scale indicated from isotope records. However, we inevitably have an Earth size planet at around 2 AU in this case. It is very difficult to obtain spatially concentrated terrestrial planets together with very late giant impacts.

Latest result: http://arxiv.org/abs/1105.4616

The Earth's comparatively massive moon, formed via a giant impact on the proto-Earth, has played an important role in the development of life on our planet, both in the history and strength of the ocean tides and in stabilizing the chaotic spin of our planet. Here we show that massive moons orbiting terrestrial planets are not rare. A large set of simulations by Morishima et al., 2010, where Earth-like planets in the habitable zone form, provides the raw simulation data for our study. We use limits on the collision parameters that may guarantee the formation of a circumplanetary disk after a protoplanet collision that could form a satellite and study the collision history and the long term evolution of the satellites qualitatively. In addition, we estimate and quantify the uncertainties in each step of our study. We find that giant impacts with the required energy and orbital parameters for producing a binary planetary system do occur with more than 1 in 12 terrestrial planets hosting a massive moon, with a low-end estimate of 1 in 45 and a high-end estimate of 1 in 4.

[Crossroads Inc.]

i think (guess really) that the bigger the moons, the less likely the orbit will be stable.

Assume a randomly generated planet and moon system. For it to be stable in the long term, those two moons can interfere with each others orbit, but it must be cyclic, returning to the start point eventually. Out of all of the possible interactions, these ones seem like a tiny proportion and are therefore unlikely (but i can't prove it).

The bigger the moon is, the bigger the area of space it can effect significantly, so the bigger each moon is, the more likely they'll affect each other significantly. Then we have to check whether that interaction falls into the cyclic class or not. If not, you are likely to have something interesting and permanent happen.

One argument against this could be convergence. If the system ain't stable, things will change and the system might move into a more stable configuration. The closer to stable the system is, the slower things change, asymptotically approaching a stable cyclic solution. The tide-locked moons of Jupiter are a good example here.
Then you have to calc the number of possible paths leading to the stable solution, and the number of paths were something interesting happens en-route.

I am curious what would you think of an Earth sized planet that had as it's moon somethign as large as Mars? Could such an orbit remain stable? Would it have tidelocked seas?

[Sarevok]

Untill astronomy improves enough to accurately resolve individual satelites for exosolar planets we cant say for sure. Sure the simulations say one thing but there is no observational evidence to back it. All we got is our own solar system and here inner system terrestial planets are sorely lacking in large satelites.

Infact the article itself includes a link to another article talking about how Earths moon maybe a rarity.

[Surlethe]

If I might butt in ...

I am curious what would you think of an Earth sized planet that had as it's moon somethign as large as Mars? Could such an orbit remain stable?

Of course, but it would be a binary planet instead of a planet/satellite. The center of mass of an Earth-Mars system would be a good tenth of the way from Earth to Mars; by contrast, the center of mass of the Earth-Moon system is a mere 1% of the way from Earth to the Moon. The Earth's surface is about 2% of the way from Earth to the Moon.

Would it have tidelocked seas?

I don't even know what this means.

Sure the simulations say one thing but there is no observational evidence to back it. All we got is our own solar system and here inner system terrestial planets are sorely lacking in large satelites.

The simulations are based on sound mechanics and thermodynamics and our best models of how solar systems form. Until we get some observational evidence (and we'd have to resolve some rocky planets first), this is a very good educated guess.

[Sarevok]

Detecting the mere existence of exosolar moons, let alone performing measurements on them is going to be heck of a challenge. I am not sure the task is even possible with present day technology. We may require interstellar probes or one of those gigantic space based telescope ideas.

[Crossroads Inc.]

If I might butt in ...

I am curious what would you think of an Earth sized planet that had as it's moon somethign as large as Mars? Could such an orbit remain stable?

Of course, but it would be a binary planet instead of a planet/satellite. The center of mass of an Earth-Mars system would be a good tenth of the way from Earth to Mars; by contrast, the center of mass of the Earth-Moon system is a mere 1% of the way from Earth to the Moon. The Earth's surface is about 2% of the way from Earth to the Moon.

Would it have tidelocked seas?

I don't even know what this means.

I remember reading a long time ago that if two similar body masses orbited, they would rotate in an equal way so that one side would have perpetual High tide and the other perpetual low tide. So they would be locked in place. That was what I was mostly wondering about. It does make me wonder about how such a "binary" planet system would behave in terms of things like the phases of the other planet or eclipses. Especially if said mars sized body was the same distance as Earths moon.

[GrandMasterTerwynn]

If I might butt in ...

I am curious what would you think of an Earth sized planet that had as it's moon somethign as large as Mars? Could such an orbit remain stable?

Of course, but it would be a binary planet instead of a planet/satellite. The center of mass of an Earth-Mars system would be a good tenth of the way from Earth to Mars; by contrast, the center of mass of the Earth-Moon system is a mere 1% of the way from Earth to the Moon. The Earth's surface is about 2% of the way from Earth to the Moon.

Would it have tidelocked seas?

I don't even know what this means.

I remember reading a long time ago that if two similar body masses orbited, they would rotate in an equal way so that one side would have perpetual High tide and the other perpetual low tide. So they would be locked in place. That was what I was mostly wondering about. It does make me wonder about how such a "binary" planet system would behave in terms of things like the phases of the other planet or eclipses. Especially if said mars sized body was the same distance as Earths moon.

Owing to the much more significant combined mass of such a system, a supergiant moon like that would probably orbit closer, since they'd become tidally locked pretty early on (the Moon is receding from the Earth for reasons explained quite concisely here.)

So, what would happen? Well, the hemisphere of the planet facing away from the giant moon would never see it, owing to that whole tidally-locked thing. The moon-facing hemisphere would see the moon go through its phases over the course of a single day. Since the moon would take up much more celestial real-estate than our Moon, eclipses would be a frequent occurrence.

There would still be tides, but they'd be the lower-magnitude tides induced by the parent star. You would expect, however, that sea level will be notably higher directly under the giant moon and on the exact opposite side of the planet (as the gravitational pull of the moon would tend to try to 'squash' the planet, making it wider at the equator than at the poles.)

[Crossroads Inc.]

Interesting, that is somethign I did not know. So in such a world, one side of the lanet would not see the moon AT ALL?

That would make the early civilizations rather interesting. Imagine spending thousands of years in a civilization and then as they branch out, suddely finding a giant object floating in the sky that was never there before.

[U-95]

Very interesting; I've downloaded the preprints to read'em when I've more time.

Sounds good for the possibilities of (complex) life existing out there; the problem is that these are just numerical simulations and until we're able to find terrestrial planets and those moons (perhaps in some decades if everything goes fine and technology advances and lets us detect smaller and smaller bodies) we cannot know the truth; before, it seemed that a largue moon as the ours was quite rare -the bad thing of knowing just one 100% Earth-like planet: the ours- and -of course- surely no simulations predicted that Hot Jupiters could exist.