CFHT and a Jupiter-sized planet around Epsilon Eridani


The discovery of a Jupiter-mass  planet orbiting a star, Epsilon Eridani, a K2V star at a distance of only  3.22 pc, was announced by an international team of astronomers led by  Artie Hatzes  and William Cochran of the MacDonald Observatory. The  McDonald Observatory Press release is available here.

Geoff Marcy has recently been quoted as saying:  "We're now at a stage where  we are finding planets faster than we can investigate them and write up the  results, "It's wonderful. Planet-hunting has morphed from the marvelous to  the mundane."

This discovery is far from mundane and is of special interest to the CFHT community. The planet has an orbital period just under 7 years and a  semi-major axis  of 3,3 AU. Hence the planet stands off from the primary at an average angular distance of about 1 arc-second.  The orbital eccentricity appears to be  0.6 so that the separation at apastron is as much as 1.6 arc-sec. This planet in Epsilon Eridani may well be the first extra-solar  planet to be directly imaged.

Epsilon Eridani was included among the sample of stars studied by Bruce Campbell, Gordon Walker and Stevenson Yang for some 12 years, beginning in 1980, using the CFHT f/4 Coude spectrograph. This ground-breaking program used  a hydrogen fluoride absorption cell to provide a stable reference system against which the very small Doppler shifts caused by orbital motion could be precisely measured.  This work was seminal in that it demonstrated conclusively that radial velocity measurements could be made to an accuracy sufficient to detect Jupiter-sized planetary companions around the nearby stars. The results of this long survey are discussed in Walker et al 1995.

The Campbell-Walker survey was based on the premise that extra-solar planetary systems were most likely to be similar to our own. This kind of assumption is often made in the absence of any evidence to the contrary. In our solar  system, the gas giants are found at several AU and have orbital periods of  several years. As a consequence, the CFHT survey was designed to measure the radial velocities of the program  stars at intervals of a few months over many years. After more than a decade of painstaking effort the program was halted in 1992 without a conclusive detection of a planetary system.

As so often happens in Astronomy, the discovery of the first extra-solar  planet by Michel Mayor and Didier Queloz in 1995 appeared to turn conventional wisdom on its head. They found that the planet around 51 Pegasi  was only 0.05 AU from its primary and the orbital period was 4.2 days - vastly different from the expected yearly variations. Among the 44 planets listed in the compilation here, only 8 have periods exceeding a year.

However, planetary companions are far easier to detect with the Doppler technique when they orbit close to their primary both because the amplitude of the radial velocity variation is higher and because conclusive observations can be made in a shorter period of time. As time goes on, more distant planets with longer periods are increasingly likely to be found.

The pioneering measurements made by the CFHT team were used by the  McDonald team and,indeed, uniquely provide the data that defines the first of the observed  orbital periods of the newly discovered planet. The amplitude  of the radial velocity curve is a mere 19 km/s. The agreement between the CFHT data and the more recent data is remarkably good.

On behalf of the CFHT, I extend congratulations to Bruce Campbell, Gordon  Walker and Stevenson Yang for their role in the discovery of this new planet.

Greg Fahlman - CFHT Executive Director