Wednesday, November 07, 2007

55 Cancri Offers a Habitable Zone Planet



However, be forewarned. This is a planet 45 times the mass of Earth with a probable surface gravity of 3.5 Gs (based on the assumption that it's a rocky planet like ours with similar density). It's not quite "earth-like" just yet, but perhaps one of it's possible moons could be. Of note is that this planet has an orbital period of 260 days and was found by monitoring the wobble of the system's G-class star. This is the same method that a few years ago could only find massive planets orbiting extremely close to red dwarfs.

The method of detecting the star wobble is to detect the doppler shift in the star's observed light. As a planet is orbiting a star, it ever so slightly drags the star towards it. When the orbit of the planet is such that it's moving towards the earth, the star also moves towards the earth, shifting it's spectral output ever so slightly towards the blue side of the spectrum. Later on, as the planet is moving away from the earth, the star is dragged away from earth, shifting it's spectral output towards the red side of the spectrum. The star's movement is tiny, even more so is the spectral shift.

It becomes apparent that two things impact the movement and spectral shift of the central star and thus make detection easier or harder. The smaller the relative mass of the star versus the planet determines how much the star will be dragged about by the planet. The lower that ratio, the more dragging occurs. This is great stuff, but the orbital period also comes into play. For the same given mass ratio, a rapidly orbiting planet will drag the star back and forth faster than a slower orbit, and since doppler shift is actually measuring the speed of the star's movement, the rapid orbit creates a greater spectral shift and is more easily detected. This is why red dwarfs with massive planets with orbits of a few days have stood out in the planetary detection business.

G-class stars such as 55 Cancri and our sun, due to their much greater mass (and therefore assumed greater mass ratios), make planetary detection much more difficult, so too do the necessarily longer orbital periods of planets in their habitable zones (generally speaking, orbital periods of about a year for G-class stars). What this news really tells us is that the science of planetary detection is maturing quite rapidly. Who knows what will be detected next year.

By the way, the planet was detected by a team led by Debra Fischer of San Francisco State University and Geoffrey Marcy of UC Berkeley. A related article can be found here and a video can be found here.

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