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The Discovery of a Planetary Orbit
Around the Nearby Star 51Pegasi

Initial Discovery by Michel Mayor and Didier Queloz

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Binary Star system or Planet?

Planetary motions around their host stars, in principle, cause small variations in the radial velocity of the star as viewed from the earth. The amplitude of these variations depends upon the gravitational influence of the planetary system about the host star. This in turn depends upon the mass and distance from the star of the individual planets. The above radial velocity curve was obtained by Geoff Marcy at the Lick Observatory . The sinusoidal nature of the curve indicates an orbital variation. The observed amplitude, v, can give a mass estimate pending an estimation of sin i. In principle all that can be known from these observations is the mass function (M1 +M2)*sin i. In this case, M1 is known to be one solar mass so we are left with M2*sin i. For the perturbation shown above, the parameters that are fit to the data are

The following two images show the context of this configuration with respect to the scale of the Jupiter-Sun separation in our own solar system. Jupiter is at 5 AU and this object is at 0.05 AU or 100 times closer. The separation from the center of the sun to the planet (in pixel units) is correct on both scales but the size of the Sun is exaggerated. The planet is close to the star but is not in contact with its atmosphere.

The above configuration is somewhat unusual for many reasons:

In any case, as in the surprising discovery of planets around Pulsars, this data shows that our expectations of the formation of planetary systems and where they are located are pretty naive. Clearly, planetary- size bodies are robust and can exist in a wide range of environments.

Things to do next:

Why was this object missed:?

Herein lies an interesting story of astronomical misclassification. 51Peg appears in the Bright Star Catalog as a luminosity class IV star - a sub-giant. These stars are known to be pulsationally unstable and exhibit sinusoidal radial velocity variations. Hence, these stars are not initially selected for survey. Geoff Marcy's experiment at Lick can achieve a precision of 3 meters/sec. The precision of the Swiss discovery experiment is 15 meters/sec which means they can do good work on pulsational radial velocity variations but are not real sensitive to planetary orbital induced variations. 51Peg was observed as part of their general program but the initial detection of 53 m/s could have been pulsationally driven. The more accurate velocity measurements, however, yielded a precise orbit and eccentricity thereby making the orbital motion interpretation more viable. Had 51 Peg been classified as a luminosity class V star initially, it is likely that this discovery would have been made earlier.

Keep in mind that the perturbation that Jupiter exerts upon the Sun is 12 meters/second (quick back of envelope calc - I think its right) so until very recently, this precision was not reachable. Indeed, in addition to the work of Geoff Marcy, the Canada-France- Hawaii Telescope has been used by Gordon Walker of UBC and his team in a long term monitoring program of a couple of dozen solar type stars. Their resolution is 15 m/s and hence that survey also doesn't detect Sun/Jupiter like systems. If Marcy and others get down to the 1 m/s level of accuracy, I am confident that many new perturbations will be detected.

Thanks to Geoff Marcy, Robert Casey, Amy Hulse, Jeremy Goodman, David Soderblom and the Adler Planetarium for their contributions.

For additional information Go to Encarta

David Hobson would like to name this planet Vulcan.

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