• orbital period = 4.2 days
• observed orbital amplitude = 53 +/- 2.5 m/s
• semi-major axis = 0.05 au
• probable mass = 0.5 Jupiter Masses

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 temperature at a distance of 0.05 AU (1/6 the distance between Mercury and the Sun) from a solar type star is very high (2-3000 K) and one wouldn't expect a low density (gaseous) planet to form there by any accretion process as the low temperature volatiles would be boiled away. Hence, this object, if its always been at this distance from the star (it would also have to withstand a rather vigorous T-Tauri wind) most likely formed by condensation out of some gaseous nebula (the same way that stars form). The ability for this Jupiter-mass like object to retain its atmosphere depends upon the atmospheric temperature and the escape velocity, which in turn depends on its density. In addition, dynamical heating by solar wind particles may be quite important. The ability for this object to retain a hydrogen atmosphere is considered here .
  

  Below is an artists conception (courtesy of Robert Casey) of what this hypothetical new planet might look like:

  

  In this hypothetical case, the atmosphere has been evaporated exposing the surface which is probably rocky. If you went to the link considering the retention of the atmosphere, you should realize that its unlikely that the atmosphere has evaporated. On the other hand, perhaps this object is mostly rocky in which case the question becomes, how come such a large mass (150 earth masses) of rock could form there into an object so close to the host star.

• With a suitable, but rare value for sin i, the mass estimate could be increased up to about 10 Jupiter masses. This raises the possibility that this system is actually a G-dwarf brown-dwarf binary. This would make perhaps a little more physical sense in terms of the small separation but the required parameter space for sin i is pretty small.

• Its also possible that the object has been captured (an extremely rare event) into this tight of orbit.

Things to do next:

• Obtain more precise radial velocity measurements to search for higher order terms, indicative of other planets (there are claims that these exist now)
• Monitor the primary for photometric variability as a function of orbital cycle. The precision of these experiments has to be at the 0.5% level (e.g. .005 magnitudes). In general, this will be hard. The primary is v=5.4 which is very bright. Short shutter speeds for CCDs are bad because the shutter is certainly not reliable to .005 seconds if your exposure time is 1 second. Hence, one must have a system that allows for exposure times around 10 seconds or greater. A nearby star can be used as a reference and differential photometry can be done - but I don't want to give away any more technique hints as we will try the experiment locally.
• Diffraction limited imaging at 10 microns might actually detect the companion directly - though this will be quite difficult

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:

P=4.2293(+-0.0011) days
e=0 (fixed)
K=0.059(+-0.003) km/s
a1sini= 34(+-2)E+6 meter
f(m)=0.91(+-0.15)E-10 solar mass

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?

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.

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:

1. In the temperature scaling arugment, a temperature of 300K was used. This is just an approximation. For instance, the average temperature at 30 degrees latitude is 285K and averaged over the earth this temperature is 276--278 K. This has the effect of lowering the quoted temperature from 1200K to 1100K.

2. For rotating planets, a given latitude zone will heat up but then reradiate that heat over its full circumference. If this planet is tidally locked, as suggested, then this mechanism is in operative and the temperature is appriximately 1/pi (about 30%) higher. Hence the 1100-1200 K range would be 1500-1600 K.

3. The calculation on the solar wind density at 0.05 au is incorrect (believe it or not, I actually knew this at the time) but is okay to an order of magnitude. A better way to state the scaling problem is not in terms of particle density but rather particle flux. This particle flux is given by:

NVR^2

where N is particle density, V is solar wind velocity (which is mostly independent of distance from the Sun) and R equals distance from the sun. In the case where N and V are constant, the scaling should go as R^2 (factor of 400) and not R^3 (factor of 8000) as the document originally stated. In any case, the several hundred fold increase in solar wind flux on this "planets" atmosphere would dominate any atmospheric heating.

Can This object retain its Atmosphere? 51Pegasus










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Thanks to Geoff Marcy, Robert Casey and Amy Hulse for there contributions.


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