Largest extrasolar system found so far

[wautd]

Another Solar System Like our Own?

Artists impression shows the planetary system around the Sun-like star HD 10180. Credit: ESO/L. Calçada
There is another Sun-like star out there with an intriguing family of planets orbiting about and it could be the closest parallel to our own solar system that astronomers have found yet. European astronomers discovered a planetary system containing at least five planets, orbiting the star HD 10180, with evidence that two other planets may be present. If confirmed, one of those would have the lowest mass ever found.
“We have found what is most likely the system with the most planets yet discovered,” says Christophe Lovis, who led the team. “This remarkable discovery also highlights the fact that we are now entering a new era in exoplanet research: the study of complex planetary systems and not just of individual planets. Studies of planetary motions in the new system reveal complex gravitational interactions between the planets and give us insights into the long-term evolution of the system.”

To make this system even more intriguing, the team also found evidence that the distances of the planets from their star follow a regular pattern, as also seen in our Solar System. “This could be a signature of the formation process of these planetary systems,” said team member Michel Mayor.
HD 10180, is located 127 light years away in the southern constellation of Hydrus. The five confirmed planets are large, about the size of Neptune — between 13 and 25 Earth masses —with orbital periods ranging from between six and 600 days. The planets’ distances from the star ranges from 0.06 and 1.4 times the Earth–Sun distance.

“We also have good reasons to believe that two other planets are present,” said Lovis. One would be a Saturn-like planet (with a minimum mass of 65 Earth masses) orbiting in 2200 days. The other would be the least massive exoplanet ever discovered, with a mass of about 1.4 times that of the Earth. It is very close to its host star, at just 2 percent of the Earth–Sun distance. One “year” on this planet would last only 1.18 Earth-days.
“This object causes a wobble of its star of only about 3 km/hour— slower than walking speed — and this motion is very hard to measure,” says team member Damien Ségransan. If confirmed, this object would be another example of a hot rocky planet, similar to Corot-7b.
The team used the planet-finding HARPS spectrograph, attached to ESO’s 3.6-metre telescope at La Silla, Chile, and made observations of HD 10180 for six years.
The newly discovered system of planets around HD 10180 is unique in several respects. First of all, with at least five Neptune-like planets lying within a distance equivalent to the orbit of Mars, this system is more populated than our Solar System in its inner region, and has many more massive planets there. Furthermore, the system probably has no Jupiter-like gas giant. In addition, all the planets seem to have almost circular orbits.
With this new announcement, the total number of exoplanets found is 472.

So a solar system with 5 to possible 7 planets, one of which may also be the smallest exoplanet found so far. Good stuff

edit: better overview on Wiki

[kouchpotato]

This is a Sol-type star, but the planets are all within 1.5 AU as opposed to our Solar System where the planets are spaced out much further. Do we know why that solar system might have formed so bunched up?

[Junghalli]

Well keep in mind most of our methods are biased toward finding planets orbiting close to the star. There could be more planets farther out that we just don't know about.

[Vehrec]

This is a Sol-type star, but the planets are all within 1.5 AU as opposed to our Solar System where the planets are spaced out much further. Do we know why that solar system might have formed so bunched up?

IIRC, the vast majority of extrasolar planets known to exist are like this. Planetary systems like our own might actually be the exception, and we might have to find some explanation for why Jupiter didn't go crashing Sol-ward, devouring all in its path. However, if I were to guess, I would suspect that sometime during the accretion process, most of these systems wound up successively perturbing themselves inward closer to the star until their orbits finally stabilized.

[Temujin]

I remember hearing some speculation that our solar system might have had a few more proto-Jupiters that were either devoured by the Sun or expelled from the systems. Though I have a feeling that that would have had to happen very early on, otherwise the orbits of the current planets would have been disrupted, and I believe it's assumed that Earth hasn't strayed too far in its orbit path since it's creation.

[Sarevok]

No I will have to agree with Junghelli. Current extra solar planet finding methods are heavily biased towards big planets orbiting very close to their star. Untill our detection methods improve it is premature to draw the conclusion our own solar system is a rather rare type of star system.

[starslayer]

I agree with Sarevok and Junghalli on this one; I do this kind of work right now, and our detection methods simply aren't good enough, and our database not large enough to make any conclusions yet on the abundance of different kinds of solar systems. Hopefully we'll be able to in the next decade or so, as spectrometers capable of detecting Earth-sized and smaller planets in Earth-like orbits come online (my group is trying to design one right now, actually).

[Junghalli]

Note that there are two known Jupiter-like planets in outer orbits around stars within 20 light years of us.

Epsilon Eridani b
Gliese 832 b

That would suggest they're not actually all that rare, at least ~1/40 stars if you include Jupiter. It's possible it's just an unusual coincidence to have so many in such a small volume of space, but it would fit well with the idea that they seem so rare more because of observing method bias than anything else.

[CaptainChewbacca]

I'm not sure if anyone has proposed a mechanism for inward migration of gas giants, but their presence in a star system almost guarantees you won't have any sort of earth-like planets, and the odds of any large moons getting ripped away by gravity are even money.

[Junghalli]

I'm not sure if anyone has proposed a mechanism for inward migration of gas giants

I believe there are two common theories on how it happens:

1) Friction between the planet and the protoplanetary disk slows it down, causing it to spiral in.
2) Close encounters with another planet throw it into a different orbit.

1 may explain the hot Jupiters with circular orbits while 2 explains the eccentric gas giants.

but their presence in a star system almost guarantees you won't have any sort of earth-like planets

Not necessarily true, at least not if "earthlike" refers to terrestrial. Link to paper.

Such planets may have a lot more volatile compounds than our terrestrial planets though, so indeed they might generally not be particularly Earthlike.

and the odds of any large moons getting ripped away by gravity are even money.

I tried doing some simulations of moon survivability in planet-planet scattering myself with Gravity Simulator ... it was pretty depressing.

[Skgoa]

I tried doing some simulations of moon survivability in planet-planet scattering myself with Gravity Simulator ... it was pretty depressing.

Can you tell me more about that?

[starslayer]

Can you tell me more about that?

When two or more massive bodies and one or more bodies of negligible mass end up getting cozy, the little objects either end up crashing into something, or being ejected from the system at very high speed. There are very few instances where they stay in even a very eccentric elliptical orbit.

[Skgoa]

I graduated kindergarten ~fifteen years ago. Can I have the version intended for CS M.Sc. students who took rocket science as a minor, please? I.e.:
- do low orbit moons have higher survivability?
- what about more or less mass?
- how does the rotation of the planet system around the common center of gravity affect this?
- etc.

[starslayer]

Honestly that's the version I'd give to most anyone, but, since you asked nicely:

Any system with more than two bodies is not, in general, analytically solvable, so we turn to a computer. The easiest method of determining what happens is to start with a given set of initial conditions (i.e., the initial velocities and masses of each object, etc.) and calculate the force on each object at any given time from all the other ones. When you've done this, finding the acceleration is trivial (F = ma, after all), and you apply that by numerically integrating (to find velocity and position) and increment to the next time step. And then keep doing this. Now, since you're a CS guy, you can probably already find the flaw here: this method takes a ton of computing power and time. There are various elegant ways to both maintain accuracy and gain speed, but I don't remember them at the moment.

Now, when you do a whole lot of these trials with various systems and initial conditions, you find a funny thing: systems with more than two bodies in them, no matter the masses, are not usually stable, at least if they're allowed to get close to each other (and you can be pretty much guaranteed they will at some point). At least one of the objects usually ends up getting ejected, or two or more of them collide. Basically, when they get really close to each other, they tend to trade a lot of gravitational energy with each other. If one of the bodies is significantly less massive than another (say, moon vs. planet, or planet vs. star), the proportional transfer tends to be really one-sided, so even though conservation of momentum holds, our little moon will get a lot more momentum, relatively speaking, than the planet it stole it from will lose. The end result is it gets slingshotted out of the system at a rather rapid clip. It can also be swung directly into something else, or simply come too close anyway and be variously ripped apart, swallowed, etc. So moons tend to get the short end of the stick when two planets duke it out (and likewise, planets around close binaries or trinaries, etc.). To answer your direct questions:

do low orbit moons have higher survivability?

I honestly don't know. If I had to guess, I'd say they around the same or perhaps slightly better survivability, because while their odds of being ejected are lower, requiring a closer encounter between two planets, it's easier to for them to collide with their parent planet. I don't know if those two probabilities balance out or not.

what about more or less mass?

Generally, the more massive an object is, the safer it is.

how does the rotation of the planet system around the common center of gravity affect this?

Not really sure... Assuming you're talking about planets around a star, they'll all be orbiting the solar system barycenter, obviously, and this will be the largest component of their motion, but if they're close enough to affect each other in a significant way, it really won't matter so much at that moment. Note that "close enough" doesn't necessarily have to be all that close if the right conditions are present; I saw one guy's thesis presentation here at UCSC where his numerical simulations showed that Mercury could possibly be ejected by Jupiter in about 2 billion years if the right orbital resonances develop.

[Skgoa]

Thanks, that was very helpfull.

[Eternal_Freedom]

Mercury ejected by Jupiter? Not a pleasent thought. Does anyone know of any simulations showing what might happen to the rest of the planets if Mercury was ejected?