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

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 sin i, can give a mass estimate pending ones estimation of sin i. For the perturbation indicated 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:

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. Hence, 51Peg was observed as part of their general program. It has an amplitude of 53 m/s and is thus easily detected (see above data). Had it been classified as a luminosity class V star, Geoff Marcy would have discovered the companion years ago.

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 perturbations will be detected.

Finding chart for 51peg:

More on 51 Peg:

The discovery of a Jupiter--mass planet in orbit around the solar--type star 51Peg has been reported by Michel Mayor and Didier Queloz (Geneva Observatory) on October 6th at the 9th Cambridge workshop on "cool stars, stellar systems, and the sun" held at Florence (Italy). The claim is based on 18 months of precise Doppler measurements done with the ELODIE spectrograph of the Observatoire de Haute-Provence (France). The parameters of the orbital motion are:

The minimum mass of the companion is 0.47(+-0.02) Jupiter--mass. Alternative explanations for the radial velocity variation (pulsation or spot rotation) seem to be rule out.

Following this annoncement, confirmations of the 4.2d period radial velocity variation have been obtained at mid-October by a team at Lick Observatory, as well as by a joint team from the High Altitude Observatory and the Harvard-Smithsonian Center for Astrophysics.

Intensive photmetric monitoring has also be done at ESO (LA Silla Obs.) by G. Burki, M. Burnet and M. Kuenzli. They report that no evidence of eclipses can be seen. The r.m.s. of the magnitude (on 17 nights) is 0.037-Vmag, not different from two comparison stars. A 4.2d-period photometric variability larger than 0.002 can be ruled out.

M. Mayor, D. Queloz (Geneva Obs)
G. Marcy, P. Butler (SF State Univ., UC Berkeley)
R. Noyes, S.Horner (Harvard, Penn St.)
G. Burki, M. Burnet, M Kuenzli (Geneva Obs, Lausanne University)

Planet or a Binary Star System?

Recall that the measurements of radial velocity only yield v sin i which in turn gives us M sin i. For most values of i, sin i will be greater than 0.5 and this gives a mass estimate of 0.5 Jupiter masses. Since sin i varies between 0 and 1, its expectation value in a randomly distributed sample is 0.5 which corresponds to i = 30 degrees. If i, were in fact, one degree the sin i = 0.017 which is substantially different than 0.5. In this case, M can be large. However, this is not very probable (in essence the probability that i = 1 degree is 1.7%) but this is only one system.

Measurements at ESO claim to constraint the photometric variablity of the host star, 51 pegasi, to be 0.002 mag or less. That means there is essentially no eclipse. The amplitude of any eclipse depends on the radius of the object which in turns depends on its density. As Jupiter has a radius very nearly 10% of the Sun's one might expect a maximum photometric variability of R^2 = 1%. This seems to be ruled out implying that if sin i is closer to 1, then the object must be smaller, and hence denser than Jupiter. Now it gets tricky of course because as you lower sin i you increase the mass and presumably the surface area while reducing the eclipse path. At the moment, I don't know how much of parameter space survives this but am just saying that the lack of evidence for eclipses argues against a high value of i.

On the other hand:

There are two independent arguments as to why we are probably seeing the system near to equator-on, so that sin i is close to 1. First, a rotation period has not been measured directly for 51 Peg, but from its activity level (which is a little less than the Sun's) we can estimate it must be about 30 days, which is to say essentially the same as the Sun's. There is plenty left to do in understanding how rotation and activity relate to one another in solar-type stars, but the relationship between activity and rotation is not merely a tendency, but a certainty. In other words, we can be very, very confident that 51 Peg is *not* rotating with a 4.2 day period, leading to its imitating the presence of a planet (perhaps through spots), because at 1 solar mass a 4.2 day period corresponds to a star that's just reached the ZAMS, and they are *very* active.

At the same time, high-dispersion spectra of 51 Peg have been analyzed by several people to derive vsini from line profiles. They all agree to within the errors, yielding vsini = 2.2 km/s or so, plus or minus about 0.7. That again is essentially the same as the Sun's equatorial rotational velocity. Combine that with the period and you get sin i = 1. Mayor went a little further and derived probabilities of, say, sini < 0.5. But you get the idea. It may be argued that at such low rotation rates we're not really detecting vsini at all, but the signature of rotation in line profiles can be distinguished from other broadening mechanisms, and so I believe the vsini is a real detection. Even in the unlikely case that it isn't, there's a second argument that puts an upper limit on M_2, namely the fact that 51 Peg does not rotate in synchronism with the orbit (again, if its rotation period were 4.2 days, the level of activity would be *very* obvious, even if we are looking at the star pole-on). A companion of stellar mass, even a small star, would exert enough tidal force on 51 Peg A to force synchronism: we see lots of such systems as BY Draconis and RS CVn binaries.

So there you have it - ambiguity. Probability suggests sin i > 0.5 and a Jupiter-mass object which is must be smaller and denser than Jupiter. The small separation is difficult to reconcile with what we think we know about planetary formation leading some to suggests that sin i is low and that M_2 is large. Yet M_2 can't be all that large or else we would see a 4.2 rotational period for 51 Pegasi.

The spectral type of 51 pegasi:


N. Houk, University of Michigan, has determined a spectral type of G2-3 V for 51 Peg from a 1-min IIa-O 10-deg objective-prism plate taken on 1981 Aug. 6 with the Burrell Schmidt telescope at Kitt Peak. The high-quality spectrum (resolution about 0.1 nm; dispersion 10.8 nm/mm, i.e., normal classification dispersion), together with the use of several standard spectra, revealed no duplicity or chemical peculiarities. The spectrum is definitely not as luminous as IV (quoted in the 1982 Bright Star Catalogue from Keenan and Pitts 1980, Ap.J. Suppl. 42, 561), or even 'IV-V'. It is also earlier than the temperature type of G5 given in several older publications.

Confirmation of Velocity Variations of 51 Peg

Geoff Marcy & Paul Butler
U.C. Berkeley
San Francisco State Univ.

On Oct. 6, Michel Mayor and Didier Queloz at Geneva Observatory announced that 51 Pegasus has a planet orbiting it. If confirmed, this would be the first detected planetary system around a normal star. 51 Peg is a Solar-Like star, 40 light years away.

We have obtained 27 independent spectra of 51 Peg using the 3-meter telescope at Lick Observatory. The observations span 4 days. The goal was to detect the wobble of 51 Peg in response to the gravitational pull of the supposed planet. Velocities of stars are derived from the Doppler effect, as a part of our long-term search for planetary systems. Our Doppler technique yields a velocity precision of 3 m/s --- human walking speed.

We find that the velocities of 51 Peg during 4 nights are sinusoidal, with an amplitude of 106 m/s, peak to peak (53 m/s semiamplitude). The RMS residual (scatter) is 2.5 m/s from the sine wave, consistent with our errors. We find a period of 4.2 days which is presumably the orbital period of the unseen companion as it gravitationally pulls its host star around in a circle.

We therefore confirm the sinusoidal velocity variations reported by Michel Mayor and Didier Queloz of Geneva Observatory. The orbital period of 4.2 days is exactly what they reported. Our velocity amplitude of 53 m/s compares favorably with their measured amplitude of 59 m/s, within their errors.

The best, and straightforward, interpretation is that 51 Peg is orbited by an unseen companion having a mass of about 0.5 Jupiter masses, and orbiting the star at a distance of 1/20 the Earth-Sun distance.

The unknown inclination of the orbital plane leaves some uncertainty in the mass of the planet; it could be somewhat larger than 0.5 Jupiter Masses.

Many question remain unanswered. How did a Jupiter-like planet form so close to its host star? Can a planet really survive so close, since its surface temperature would be about 1000 Centigrade? Are there other planets orbiting 51 Peg?

This page contains some corrections to the factual material presented here which have been sent in my informed readers. Thank you: