HD 45364: 2 planets in 3:2 resonance.

HD45364, a pair of planets in a 3:2 mean motion resonance
http://arxiv.org/abs/0902.0597

Abstract wrote:

Precise radial-velocity measurements with the HARPS spectrograph reveal the presence of two planets orbiting the solar-type star HD45364. The companion masses are 0.187 Mjup and 0.658 Mjup, with semi-major axes of 0.681 AU and 0.897 AU, and eccentricities of 0.168 and 0.097, respectively. A dynamical analysis of the system further shows a 3:2 mean motion resonance between the two planets, which prevents close encounters and ensures the stability of the system over 5 Gyr. This is the first time that such a resonant configuration has been observed for extra-solar planets, although there is an analogue in our Solar System formed by Neptune and Pluto. This singular planetary system may provide important constraints on planetary formation and migration scenarios.

Moreover both lay in the system's habitable zone.

L_HD45364=0.57 L_Sun HZ=0.75 AUs


Just do the math...

Well the configuration is sort of an opposite to Neptune and Pluto: the inner planet is the least massive one. Give it a decade or two for the system to evolve and there might be a chance of figuring out the true masses. Nice.

This system may be very good news for poor Pluto. From Ken Croswell.com:

Quote :

A newfound pair of extrasolar planets may boost the planetary prospects of beleaguered Pluto. The pair has the same orbital resonance as Neptune and Pluto, and the "inner" planet may sometimes lie beyond the "outer" planet--the same situation that prevails for Neptune and Pluto.

I would have to disagree about the HD 45364 system having implications for the status of Pluto.

Where as Neptune is 8,000 times the mass of Pluto, the planets at HD 45364 are comparably rather even as far as mass goes. Resonant configurations happen a lot in asteroids, typically with Jupiter for asteroids in the main belt or with Neptune for objects in the Kuiper Belt.

The problem with arguments for Pluto's planethood is that there are lots of objects in the 2:3 resonance with Neptune apart from Pluto (e.g. Orcus with an estimated mass almost 1/10 of Pluto's mass)... the system becomes an argument for Pluto if we find lots of Neptune-sized objects in similar orbits with the outer planet. I somehow doubt this will turn out to be the case...