A Planet Returns?
Considerable discussion has occurred over the subject of the proposed planet in orbit around the sun-like star 51 Pegasi. In 1995, Mayor & Queloz presented their observations showing the periodic variations in the radial velocity of this star, and their planet interpretation of these variations (Nature 378, 355, 1995). The periodic variation in radial velocity was confirmed by Marcy et al. (Astrophysical Journal 481, 926). In due course, I assembled my scattered 1989-1996 observations of this star for analysis and found, using the 4.23-day period from the radial-velocity data, a variation in the shape of the spectral line of neutral iron at 6252.57Å. This result was published in Nature 385, 795, 1997. A much more detailed analysis and modeling with non-radial oscillations was done with the help of A. Hatzes (U. Texas) and published in the Astrophysical Journal (490, 412, 1997).
The weakness of my 1989-96 observations was that they were spread widely in time. To properly study a period as short as 4.23 days, one should have observations much more closely spaced in time. Nevertheless, we used what we had, and we showed that the probability of a chance match of the profile-variation period to the radial-velocity period was less than one in 300. The non-radial oscillations we suggested in place of the planet was successful in reproducing both the radial velocity variations and the profile shape variations. How such oscillations could be excited and other more subtle questions remained unanswered, but then, that is not unusual in a scientific investigation.
The 1997 observing season for 51 Peg is now past, and several of us have worked very hard to collect new (more closely spaced) data. I know of the work of T. Brown and associates (Brown et al. Astrophysical Journal & ApJ Supplement 1998, submitted) and of Hatzes & Cochran (Nature 1998, January 8th issue). My new results appear in this same issue of Nature. None of us sees the profile variations shown by my older data. The figure below is adapted from this last paper in Nature, and shows the lack of signal at the expected frequency.
I can think of only three possibilities to explain the situation: a) the star stopped pulsating, b) my new 1997 observations are too noisy and the signal is masked by the noise, or c) the signal is not there. Since I have not heard otherwise, I presume the star still shows radial velocity variations, and therefore we can rule out possibility a). Since none of Brown et al., Hatzes & Cochran, or I see any sign of the signal, possibility b) seems unlikely. Therefore, the best conclusion is that the profiles do not vary, and presumably the signal seen in my earlier observations was indeed noise no matter how small the calculated probability of occurrence.
This means two things. First, the interesting non-radial oscillations of 51 Pegasi are no longer available with all their potential for revealing the physics of the star. A pity. And certainly a disappointment for many of us. Second, the planet hypothesis is now the front runner. People interested in extra-solar-system planets will be pleased.
