## Tuesday, October 16, 2012 ... /////

### Planet with 4 stars found by armchairs astronomers

As many sources mention, users at the website Planet Hunters.ORG have found a planet 5,000 light years away that orbits four Suns. To celebrate the website, it was called PH1.

The observation was made by watching variations in the brightness and it was reported on the arXiv in the decently looking article

Planet Hunters: A Transiting Circumbinary Planet in a Quadruple Star System
The mass of it is about 100+ Earth masses or 0.5 Jupiter masses. The planet's radius exceed the Earth radius by a factor of six. If the four stars had generic distances, velocities, and masses, they couldn't survive for too long because the chaotic motino would soon or later direct the stars too close to each other and they would be damaged and start to merge.

However...

However, the four stars are actually ordered hierarchically. Two of the stars are close to each other; and two remaining stars are close to each other, too. So it's really one binary star orbiting another binary star. The planet is very close to one of the pairs.

In this setup, there is nothing too shocking about the stability of the system. The neighborhood of each binary star pretty much ignores the distant binary star system while the planet views the nearby binary star as a single object, when it comes to its trajectory. So the orbits are still essentially Kepler orbits.

I would like to mention that this source of stability is kind of cute and particle physicists who spend lots of time by thinking about the hierarchy problem(s) should think about it, too. Large hierarchies may look unnatural but they actually have an advantage: they help to make systems such as this quadruplet of stars more long-lived.

Can't the hierarchies in particle physics resemble the distances between the stars? And isn't there some explanation of the hierarchies based on this evolutionary advantage? I mean that the Universes with e.g. generic mass ratios of order one have already "destroyed themselves" in some new way. And if there is an explanation of this sort, does it count as an anthropic one?

#### snail feedback (8) :

They're stable for now, but a lot of hierarchical star systems are unstable on long time scales, that's why we can get blue straggler mergers or type 1a supernovae (according to some).

And as an astronomer, one of the things that scares me from particle physics is the concept of vacuum decay, that the vacuum is stable for now, but not forever. I have no idea what's actually going here, but it scares me that everything could go to something else real fast. I only take solace in the fact that this would be a most painless death, as the transition would propagate with velocity "c".

That sounds like an attempt by Niels Bohr pre 1925 to solve the hierarchy problem :-)

My solution, is that god just got tired at ~1Tev.

I would not consider that anthropic reasoning. More like Darwinian. Don't ask me to defend myself!

Well, the anthropic reasoning *is* Darwinian, kind of. We see what we see because other Universes with wrong parameters weren't viable and their soul - intelligent observers in them - died or were born dead. ;-)

An N body system (N>2) is never stable. As it is chaotic, the phase space orbits diverge exponentially after any perturbation regardless how small it is.

See f.ex for a case of a system binary star + planet http://www.uta.edu/physics/main/faculty/musielak/info/ECM.pdf

The figures show the chaotic transitions as the distance of the planet to its sun increases.
Figure 6 shows the small pseudo stability region. It is only pseudo stability because the paper considers a pure 3 body system where no perturbations exist.

The solar system is much more complex but contains equally pseudo stability regions (if one trusts numerical simulations over vey long periods of time).
Lubos iIt is interesting that you would think about an "evolutionary advantage".
Indeed it is believed that the planets were born from an original dust disk orbiting the nascent Sun.
As the accretion process is fundamentally random, why have the planets the sizes they have and why are their orbits pseudo stable?
Well probbaly because as macroscopic bodies began to appear with different masses and different orbital parameters, only those who happened to be born in the pseudo stability region (see Fig.6) would survive. All others have been ejected or destroyed long ago during the 5 billions of years of the solar system's life.

For instance it has been shown that the inner planets (Mercure, Venus) become dramatically unstable when one removes ONLY the Earth from the solar system. The Earth's mass is negligible yet its presence at the place it is and with the mass it has seems to be necessary for Mercure and Venus stability.

"And if there are explanations of this sort, does it count as an anthropic one?"

Almost an Extremely Promising Tricky question!

Especially coming from someone who has (formerly!!?) appeared to have such a strong - almost 'black&white type' - anti anthropic principled attitude! ;-)

I say it is an AP-supporting explanation because any explanation that 'prunes infinity' narrows down the people's scope for being sensibly anti-AP. ;-)

Dear Tom, good to see you here! Your

"As it is chaotic, the phase space orbits diverge exponentially after any perturbation regardless how small it is."

is right in principle but I think you underestimate how small the things actually are in similar hierarchical situations. The tidal forces that are the first perturbation that disrupts the relative quasicircular orbiting within a binary star go like 1/r^3, which is a huge suppression - e.g. one part per million if the distance ratio is 100 - are extremely tiny relatively to the forces that make the stars orbit each other and most of these tidal forces from the other (binary) star get averaged out for a very long time, anyway. So I find it totally plausible that with these numbers, similar 4 stars may preserve the hierarchy and quasicircular orbits (2 small and 1 large) for billions of years. Only when the distance between the stars becomes comparable, the perturbations grow exponentially. So your comment about the inevitable exponential growth is true about the "asymptotic future" but you underestimate how far the future where this exponential growth is actually seen is.

The dependence of Venus and Mercury on Earth is fun if true. It's hard to believe it but it may be right. ;-)