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View Full Version : Question about binary systems, Jovian planets, and comet impacts

Somes J
2010-Jul-03, 08:37 PM
This is something I've been wondering about. I know Jovian planets supposedly play an important role in shielding Earth from comet impacts. Now, in a relatively close binary system like Alpha Centauri there isn't room for such a planet, unless maybe it orbits both stars, but would the binary star possibly serve an equivalent function? I would think that a binary star at, say, 20 or 40 AU out would be quite efficient at ejecting any comets with orbits that crossed into the inner solar system, and this would effectively protect inner terrestrial planets from comet impacts, hence complex life might have a good chance of being able to develop in such a solar system even without a Jovian planet present.

Is that reasonable, or am I wrong?

Shaula
2010-Jul-04, 12:09 AM
The problem with binary systems is that if the stars are that close then the orbit of the inner planet has to be solved as a three body problem. There are no simple, stable solutions to this (I think I saw an article ages ago implying that there were a few very nearly stable ones but they were nothing like circular orbits!). So basically while the star would shield the inner system planets could not form or remain there. The reason they say Jovian is that it is that it is small enough to not disturb the inner planets enough to eject them but large enough to help clear the area of smaller bodies.

Somes J
2010-Jul-04, 01:11 AM
The problem with binary systems is that if the stars are that close then the orbit of the inner planet has to be solved as a three body problem. There are no simple, stable solutions to this (I think I saw an article ages ago implying that there were a few very nearly stable ones but they were nothing like circular orbits!). So basically while the star would shield the inner system planets could not form or remain there. The reason they say Jovian is that it is that it is small enough to not disturb the inner planets enough to eject them but large enough to help clear the area of smaller bodies.
According to Solstation simulations indicate a planet can form around Alpha Centauri:

"On January 15, 2010, a team of astronomers released the results of computer simulations indicating that kilometer-size planetesimals can form and accrete into rocky Earth-size planets around Alpha Centauri B despite gravitational perturbations from Alpha Centauri A. The binary system, however, does not offer "favorable" conditions for the formation of gas giants, like Jupiter and Saturn. Star B was chosen for analysis because efforts are underway to detect of an Earth-like planet in or near its habitable zone (between 0.5 and 0.9 AU) using available technology -- more discussion below (Xie et al, 2010; and Jessica Griggs, New Scientist, January 29, 2010)." - Link (http://www.solstation.com/stars/alp-cent3.htm).

Gamma Cephei, which is a binary system with a minimum seperation of 12 AU, has a planet in an eccentric orbit with a distance of 1.7-2.5 AU (link (http://www.solstation.com/stars2/errai2.htm)).

If the Gamma Cephei planet can survive, I think it's reasonable to think a planet orbiting at ~1 AU of a star like Alpha Centauri could also survive.

Hungry4info
2010-Jul-04, 01:28 AM
Somes J,

The multiplicity and separation of the system would reduce the amount of material each star has to make planets out of. Once they get incorporated into planets, there's not a lot of other asteroids to bombard the planets.

In any event, neither α Cen A or B have been reported to have an infrared excess associated with an asteroid belt. So the system is quite clean as far as that is concerned.

So I'd say you're being reasonable.

Edit:
However a problem arises in that there isn't as efficient a method of getting water to those planets. Remember a Kuiper belt is out of the question around either star. Any water would have to be found in asteroids and comets hanging out beyond the two stars. If Proxima Centauri is bound to the system, then perhaps it can perturb such objects and send them toward the inner planets.

Somes J
2010-Jul-04, 03:00 AM
However a problem arises in that there isn't as efficient a method of getting water to those planets. Remember a Kuiper belt is out of the question around either star. Any water would have to be found in asteroids and comets hanging out beyond the two stars. If Proxima Centauri is bound to the system, then perhaps it can perturb such objects and send them toward the inner planets.
Deuterium ratios suggest that no more than ~10% of the water in our oceans may be of comet origin (link (http://www.thelivingcosmos.com/TheOriginofLifeOnEarth/OriginOfWaterOnEarth_12May06.html)), so this may not be a problem as long as the stars are far enough apart that some material survives beyond the snow line (that being the other obvious source of water for the inner system). I also think lack of water may not be as big of a problem as it's often made out to be. Our oceans are much deeper than they need to be in order to be world-spanning; you could have quite respectable seas on a planet with a fraction of our water.

EDG
2010-Jul-04, 05:59 AM
The problem with binary systems is that if the stars are that close then the orbit of the inner planet has to be solved as a three body problem. There are no simple, stable solutions to this (I think I saw an article ages ago implying that there were a few very nearly stable ones but they were nothing like circular orbits!). So basically while the star would shield the inner system planets could not form or remain there. The reason they say Jovian is that it is that it is small enough to not disturb the inner planets enough to eject them but large enough to help clear the area of smaller bodies.

There'd be a significant region in the system in which planets can't form or are ejected from the system, but if you're talking about a companion star 30 AU from the primary (assuming a circular orbit) then you could still potentially get planets forming within about 8.2 AU of each star, and you could potentially (if it's even possible at such distances) get planets forming beyond about 72 AU from the centre of mass (this is according to the formulae in "Long-Term Stability of Planets in Binary Systems (Holman, Matthew J.; Wiegert, Paul A.) The Astronomical Journal, Volume 117, Issue 1, pp. 621-628") - it all depends on (a) the eccentricity of the stars' orbits around their centre of mass, and (b) the mass ratio of the stars. If one star is more massive than the other, then the limit within which you can find planets around the more massive star expands outwards, and the limit around which you can find planets around the less massive star contracts inwards.

However, it seems that it's not just down to this - according to the "Debris Disks In Main Sequence Binary Systems" paper by Trilling et al. (in Astrophysical Journal), it looks like stars with a separation between 3 and 50 AU don't have any dust disks, which implies that planets aren't forming there. If stars are within 3 AU of eachother then they seem to have dust orbiting both stars (not individually), and if they are beyond 50 AU then dust can be found around one of the stars (presumably beyond a larger distance - say 500 AU - you can get dust around each star individually?). If that's true (and I'm not sure how accurate this is, it seems to be based on a rather small number of observations) then there seems to be some other process that prevents dust disks from forming that isn't down to orbital dynamics - it could be due to radiation from the stars, maybe?

I haven't really reconciled the implications of the two papers. For now, when I'm making my own fictional systems, I assume that the Trilling et al. dust disk paper is accurate, and then use the other Holman et al paper to figure out possible orbits given that.

Somes J
2010-Jul-04, 06:37 AM
However, it seems that it's not just down to this - according to the "Debris Disks In Main Sequence Binary Systems" paper by Trilling et al. (in Astrophysical Journal), it looks like stars with a separation between 3 and 50 AU don't have any dust disks, which implies that planets aren't forming there.
Well, there's at least one exoplanet that orbits a binary star with a minimum seperation of 12 AU: Gamma Cephei Ab (http://www.solstation.com/stars2/errai2.htm), so at least sometimes planets can form in such systems.

astromark
2010-Jul-04, 06:47 AM
I am of the informed opinion that both Alpha A and B could have terrestrial planets at Earth type distances from either star...

Gas giants at Jupiter and Saturn distances are consumed or flicked away... Thats a tested theory.

Until we know that some or none such planets exist I will stay balanced on the fence... leaning towards, might be....

This subject has been complicated by some recent contradictions : That having a gas giant seeming to sweep the chaff out of the system.

May not be an advantage. It could just as easily draw objects into a crossing path.

With a binary system you have the greater gravity wells. So a safety margin I can understand. I think. Not all the cards are on the table yet.

whimsyfree
2010-Jul-04, 06:56 AM
According to Solstation simulations indicate a planet can form around Alpha Centauri:

"On January 15, 2010, a team of astronomers released the results of computer simulations indicating that kilometer-size planetesimals can form and accrete into rocky Earth-size planets around Alpha Centauri B despite gravitational perturbations from Alpha Centauri A. The binary system, however, does not offer "favorable" conditions for the formation of gas giants, like Jupiter and Saturn. Star B was chosen for analysis because efforts are underway to detect of an Earth-like planet in or near its habitable zone (between 0.5 and 0.9 AU) using available technology -- more discussion below (Xie et al, 2010; and Jessica Griggs, New Scientist, January 29, 2010)." - Link (http://www.solstation.com/stars/alp-cent3.htm).

Gamma Cephei, which is a binary system with a minimum seperation of 12 AU, has a planet in an eccentric orbit with a distance of 1.7-2.5 AU (link (http://www.solstation.com/stars2/errai2.htm)).

If the Gamma Cephei planet can survive, I think it's reasonable to think a planet orbiting at ~1 AU of a star like Alpha Centauri could also survive.

The problem is forming in the first place. Some simulations are for and some are against. 90% of the people here are going to listen to the former only.

EDG
2010-Jul-04, 07:52 AM
Well, there's at least one exoplanet that orbits a binary star with a minimum seperation of 12 AU: Gamma Cephei Ab (http://www.solstation.com/stars2/errai2.htm), so at least sometimes planets can form in such systems.

Well, that's good to know :). I threw the numbers for Errai into the equation from the Holman paper and it works out at least - it says that for a system where M1 = 1.6 Solar masses, M2 = 0.4 solar masses, e=-0.44, and a=22.2 AU (to get an orbital period of about 74 years, and perihelion of 12.43 AU and aphelion of 31.97 AU) the furthest a planet can be from star A is 4.07 AU. So the planet there is well within that distance at least.

Somes J
2010-Jul-04, 07:08 PM
The problem is forming in the first place. Some simulations are for and some are against. 90% of the people here are going to listen to the former only.
The Gamma Cephei planet must have formed somehow, although I suppose it's possible that it's a captured rogue or that the configuration of the binary has changed since it formed (the latter strikes me as plausible, given that the zone for forming a gas giant in the first place around a 1.6 solar mass star with a companion that gets within 12 AU is probably pretty narrow if it exists at all).

Doing some Google searching, Gamma Cephei Ab isn't a complete anomaly, there's at least one other planet that exists in a relatively close multiple system (average seperation 12.3 AU according to Extrasolar Visions): HD 188753 Ab (http://www.extrasolar.net/planettour.asp?PlanetID=305). But in that case there's strong evidence the star system's configuration has changed since the planet formed (a gas giant simply shouldn't have been able to form with the present configuration).

whimsyfree
2010-Jul-05, 03:53 AM
The Gamma Cephei planet must have formed somehow, although I suppose it's possible that it's a captured rogue or that the configuration of the binary has changed since it formed (the latter strikes me as plausible, given that the zone for forming a gas giant in the first place around a 1.6 solar mass star with a companion that gets within 12 AU is probably pretty narrow if it exists at all).

Doing some Google searching, Gamma Cephei Ab isn't a complete anomaly, there's at least one other planet that exists in a relatively close multiple system (average seperation 12.3 AU according to Extrasolar Visions): HD 188753 Ab (http://www.extrasolar.net/planettour.asp?PlanetID=305). But in that case there's strong evidence the star system's configuration has changed since the planet formed (a gas giant simply shouldn't have been able to form with the present configuration).

HD 188753 Ab No such planet in exoplanet.eu or any other reliable site. AFAIK disconfirmed.

Somes J
2010-Jul-05, 06:05 AM
HD 188753 Ab No such planet in exoplanet.eu or any other reliable site. AFAIK disconfirmed.
I tried looking for something to confirm that, I found a mention of it being disconfirmed in a discussion on the Extrasolar Visions board (http://solar-flux.forumandco.com/extrasolar-news-and-discoveries-f2/many-new-harps-discoveries-t13-30.htm) and Wikipedia (yes, I know...) lists it under dubious and disproven candidates (http://en.wikipedia.org/wiki/List_of_unconfirmed_extrasolar_planets#Dubious_and _disproven_candidates). It appears you are right.

Pity, it would have been an interesting system.

Hungry4info
2010-Jul-05, 07:11 AM
It was reported here:
An extrasolar giant planet in a close triple-star system (http://www.nature.com/nature/journal/v436/n7048/abs/nature03856.html)

The planet was a pest to planet-formation theories.
Constraints on the Formation of the Planet Around HD188753A (http://arxiv.org/abs/astro-ph/0507356)
Cluster Origin of Triple Star HD 188753 and its Planet (http://arxiv.org/abs/astro-ph/0509490)

More RV observations were gathered to constrain its orbit better and alas, no convincing evidence for the planet came out of the data.
No evidence of a hot Jupiter around HD 188753 A (http://arxiv.org/abs/astro-ph/0702574)

Thus ends the story of HD 188753 Ab.

Hungry4info
2010-Jul-05, 07:20 AM
For another example of a semi-close binary system with a planet host, there's Gliese 86, a K dwarf primary and a white dwarf separated at 21 AU. The primary has a m sin i = 4 MJ companion in a ~15 day orbit.

whimsyfree
2010-Jul-05, 08:48 AM
For another example of a semi-close binary system with a planet host, there's Gliese 86, a K dwarf primary and a white dwarf separated at 21 AU. The primary has a m sin i = 4 MJ companion in a ~15 day orbit.

None of these examples provides much hope for finding an Earth-like planet in such a system.

Hungry4info
2010-Jul-05, 09:29 AM
Especially having a white dwarf in the system. A significant amount of evolution and mass rebalancing has occurred at Gliese 86.

whimsyfree
2010-Jul-13, 01:14 AM
Especially having a white dwarf in the system. A significant amount of evolution and mass rebalancing has occurred at Gliese 86.

True. Super-Jovians in highly eccentric orbits aren't encouraging either.