51 Pegasi b has the distinction of being the first planet to be discovered orbiting another sun-like star. The discovery sent shock waves through the astronomical community, not only for the fact that it was one of the first known exoplanets, but because of its totally unexpected nature. 51 Pegasi b is a massive Jupiter-like planet orbiting at only 0.05 Aus from its sun, far closer than Mercury. At the time, theories of planet evolution allowed giant planets to form only at distances greater than several AU, at about the distance of Jupiter from the sun. The discovery of 51 Pegasi b forced astronomers to re-examine their theories. It wouldn't be so bad if 51 Pegasi b was a lone freak case. But, the discovery of planets orbiting 55 Cancri, tau Bootes, and other sun-like stars have shown that such "Hot Jupiters" are relatively common. And, attempts to attribute the detections to photospheric anomalies in their host stars rather than actual planets proved untenable. Hot Jupiters were here to stay. But, how could such massive planets exist in such tight orbits? When 51 Pegasi b was first discovered, one theory was that it was a titanic terrestrial world, a rocky planet with the mass of Jupiter. But, it seemed impossible for there to be enough rocky material in a protoplanetary disk to form a super massive planet at such a small distance. Yet, it couldn't possibly be a gas giant, because a jovian atmosphere would surely evaporate into space under the intense stellar radiation. Light was shed on the puzzle of the Hot Jupiters when simulations of protoplanetary disks showed that gas giants like Jupiter could migrate inward towards their stars, either due to drag against disk material or by gravitational perturbations with the disk. Tidal forces and disk clearing near the star would tend to park such migrating planets at orbital distances quite similar to those of the Hot Jupiters. It would appear that 51 Pegasi b and its kin originally formed far from their stars but spiraled inward to their current orbits. But can gas giants survive so close to their parent stars without having their atmospheres stripped away? Calculations of the temperature of 51 Pegasi b's atmosphere showed that, against expectations, the planet could indeed hold on to a Hydrogen/Helium atmosphere for astronomically significant timescales. In face, the calculations showed that the planet would loose only 5% of its atmosphere over the lifetime of its star. Further evidence of the jovian nature of Hot Jupiters came in 1999 when a Hot Jupiter was discovered around HD 209458. What's special about this planet is that its orbit is inclined 90 degrees from our vantage point, meaning the planet regularly passes in front of the disk of its star. Such transits allowed astronomers to determine the radius, and therefore the density, of the planet, confirming that it is a gas giant and not a super massive terrestrial. Further investigations into HD 209458 showed that the planet has an extended atmosphere that trails away from it like the tail of a comet. Here was evidence that the planet is losing material due to stellar heating. 51 Pegasi b has about the same calculated temperature as HD 209458 b, so there is a good chance that its atmosphere is evaporating to some degree as well, although direct observations would be required to verify this.
51 Pegasi b is the archetypical Hot Jupiter. Heated both inside and out to temperatures high enough to vaporize silicates, its atmosphere would be a bottomless inferno. Similar to HD 209458 b, 51 Pegasi b likely has an upper atmosphere of tenuous gas extending thousands of kilometers above its surface clouds. At the upper edges of this extended region, gases may be heated enough by stellar radiation to escape the planet's gravity and fall into a toroidal cloud around the star, similar to the sodium cloud around Jupiter fed by volcanic outgassing from its moon, Io. Or, these gases may form a comet like tail. Either way, the escaped material is doomed to eventually fall into the host star. Below the tenuous extended atmosphere is the thicker main atmosphere. 51 Pegasi b is so hot that its atmosphere is puffed up, giving the planet a larger radius than Jupiter, despite its smaller mass. 51 Pegasi b's atmosphere becomes incandescent at lower levels, giving the planet a red hue. On the planet's limb, we see through the comparatively cooler gases above this incandescent layer, so the edges of the planet appear blue due to Rayleigh Scattering. It's too torrid for normal clouds to form, but the temperature is right for sparse clouds of silicates. Calculations show that 51 Pegasi b became tidally locked soon after it came to rest in its current orbit. This means that the planet always shows the same face to its sun, just like the moon always shows the same face to the Earth. Measurements of the planet's magnetic field would confirm this. Yet, if you were in orbit of 51 Pegasi b, you would still see clouds pass across the planet's surface faster than its four day orbital period. This is due to powerful winds driven by both internal and external heat, that move across the planet faster than it rotates. A similar situation exists for Venus. Our sister planet actually rotates backwards, and very slowly. One day on Venus is 243 Earth days long. Yet, Venus' clouds circle the planet once every four days.
Recent studies indicate that stars with Hot Jupiters, like 51 Pegasi b, might emit superflares every century or so. If this is happening within the 51 Pegasi system, then any as yet undiscovered and more distant icy worlds may have been melted smooth by these titanic stellar eruptions. However, the link between Hot Jupiters and superflares is, so far, only theoretical.
View a VRML model of the system. Please be patient while the file downloads. For a VRML tour of our galaxy's exoplanets, check out Extrasolar VR.
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