# GJ 876 e - Laplace Resonance!

## GJ 876 e - Laplace Resonance!

The Lick-Carnegie Exoplanet Survey: A Uranus-mass Fourth Planet for GJ 876 in an Extrasolar Laplace Configuration
http://arxiv.org/abs/1006.4244

Abstract wrote:(Abreviated) Continued radial velocity monitoring of the nearby M4V red dwarf star GJ~876 with Keck/HIRES has revealed the presence of a Uranus-mass fourth planetary companion in the system. The new planet has a mean period of $P_e=126.6$ days (over the 12.6-year baseline of the radial velocity observations), and a minimum mass of $m_e\sin{i_e}=12.9\pm 1.7\,M_{\oplus}$. Self-consistent, N-body fits to the radial velocity data set show that the four-planet system has an invariable plane with an inclination relative to the plane of the sky of $i=59.5^{\circ}$. The fit is not significantly improved by the introduction of a mutual inclination between the planets b'' and c,'' but the new data do confirm a non-zero eccentricity, $e_d=0.207\pm0.055$ for the innermost planet, d.'' In our best-fit coplanar model, the mass of the new component is $m_e=14.6\pm1.7\,M_{\oplus}$. Our best-fitting model places the new planet in a 3-body resonance with the previously known giant planets (which have mean periods of $P_c=30.4$ and $P_b=61.1$ days). The critical argument, $\varphi_{\rm Laplace}=\lambda_c-3\lambda_b+2\lambda_e$, for the Laplace resonance librates with an amplitude of $\Delta\varphi_{\rm Laplace}=40\pm13^{\circ}$ about $\varphi_{\rm Laplace}=0^{\circ}$. Numerical integration indicates that the four-planet system is stable for at least a billion years (at least for the coplanar cases). This resonant configuration of three giant planets orbiting an M-dwarf primary differs from the well-known Laplace configuration of the three inner Galilean satellites of Jupiter, which are executing very small librations about $\varphi_{\rm Laplace}=180^{\circ}$, and which never experience triple conjunctions. The GJ~876 system, by contrast, comes close to a triple conjunction between the outer three planets once per every orbit of the outer planet, e.''

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Sirius_Alpha

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## Re: GJ 876 e - Laplace Resonance!

I was expecting it were an inner planet, instead it is an outer one as inferred with Systemic console. Intriguing

Edasich
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## Re: GJ 876 e - Laplace Resonance!

You think this planet has clouds of water or ammonia?

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Stalker
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## Re: GJ 876 e - Laplace Resonance!

The planet is beyond the habitable zone of the star. Given the new value for the metallicity of the star (245% solar), i would expect this planet to be a "cold super-Pluto" with a moderately thick atmosphere and a very high level of geologic and seismic activity in spite of being billions of years old. The planet would recieve stellar insolation comparable to Ceres.

AVBursch
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## Re: GJ 876 e - Laplace Resonance!

Why not an ice giant?

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Stalker
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## Re: GJ 876 e - Laplace Resonance!

Orbital separation is 0.33 AUs, HZ=0.12 AUs. It would be as cold as at 2.8 AUs from Sun. Rather a lilliputian Jupiter, I think.

Even an ammonia oceanic planet.

Edasich
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## Re: GJ 876 e - Laplace Resonance!

Well clearly I have to redo my Gliese 876 dynamics add-on for Celestia.

This is pretty neat, was wondering when a Laplace resonance would be confirmed in an extrasolar system (HR 8799 got pretty close to being this). Also notable is the fact the system is near-coplanar, which I guess suggests migration rather than scattering. So probably planet "e" formed external to the gas giants, so a Uranus-type composition (ice giant) is probably fairly reasonable, though perhaps the metallicity may have some effect on what ices form the core of the planet.

Lazarus
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## Re: GJ 876 e - Laplace Resonance!

Moreover it lies in orbital region much "warmer" than Uranian one, receiving as much radiation as if it were 3 AUs away from a Solar-like star.

Edasich
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## Re: GJ 876 e - Laplace Resonance!

From 2008...

Unfortunately I don't have access to this, but the abstract is suggestive:
The dwarf star GJ876 may host an extra, Neptune-mass, planet which is in resonance with two giant planets in this system.

Lazarus
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## Re: GJ 876 e - Laplace Resonance!

Lazarus wrote:From 2008...

Unfortunately I don't have access to this, but the abstract is suggestive:
The dwarf star GJ876 may host an extra, Neptune-mass, planet which is in resonance with two giant planets in this system.

Sounds like a confirmation.

Edasich
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## Re: GJ 876 e - Laplace Resonance!

Edasich wrote:Rather a lilliputian Jupiter, I think.

What's a lilliputian Jupiter?

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Sirius_Alpha

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## Re: GJ 876 e - Laplace Resonance!

HZ around gliese 876 is 0.1-0.28 AU so the planet is cold temp around -80,-90 but we have only minimum mass of the planet so no doubts this is neptune type of planet not super pluto

tommi59
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## Re: GJ 876 e - Laplace Resonance!

Still I have to understand what a "Super-Pluto" is...

Maybe they refer to a massive frozen and low-mass planet, but the nickname sounds quite tricky after the new definition for planet.

According to me it is surely a dwarf gas giant alike Neptune, but laying at much closer orbital separation than the former, roughly within the equivalent of Jupiter's orbital zone.

A mini-Jupiter, voilà!

Edasich
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## Re: GJ 876 e - Laplace Resonance!

Interesting there can be another planet with period 15 days so another resonance 4 planets??b,c,e, and hipotetical f

tommi59
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## Re: GJ 876 e - Laplace Resonance!

This paper considers such a planet. They find a region right at about 15 days to be stable, but note that if a planet were there with a mass > 2 Me, it would have been detected by now. They find invoking such a planet useful to explain the eccentricity of d.

(note the paper was written before e was announced, so they refer to the 15-day planet as a "fourth" planet).

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Sirius_Alpha

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## Re: GJ 876 e - Laplace Resonance!

Sirius_Alpha wrote:This paper considers such a planet. They find a region right at about 15 days to be stable, but note that if a planet were there with a mass > 2 Me, it would have been detected by now. They find invoking such a planet useful to explain the eccentricity of d.

(note the paper was written before e was announced, so they refer to the 15-day planet as a "fourth" planet).

As putative one, though. As for Mu Arae's case where "c" was not clear if being the Jovian outer planet or the inner super-Earth.

This system is surprising, glad if such "fifth" planet were really detected (becoming Gliese 876 f). Even more challenging to search for its transit or multi-transits by each planet.

Edasich
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