An epistellar Jovian exoplanet is a large planet with mass comparable to that of Jupiter found within 0.1 Astronomical Units (AU) of its star. The orbits followed by these planets are sometimes called "blow torch orbits" due to the high temperatures resulting from the planet's proximity to the sun. In fact, 51 Pegasi b has a surface temperature of 1300°C.
51 Pegasi b was the first such planet to be discovered. First cataloged in 1995, controversy raged about its classification: did the regular flickering and wobbling of the star 51 Pegasi imply a planet, or simply unusual pulsations? However, most scientists have now come to a consensus that the star 51 Pegasi is orbited by a large, supermassive, very close planet. The discoveries of other such planets around 55 Cancri, Tau Böotis, and Upsilon Andromedae has bolstered the view, and epistellar Jovian exoplanets are now considered quite normal.
As the first to be discovered, 51 Pegasi b has been most often researched of all the planets. It was discovered around the star 51 Pegasi, a 42-lightyear-distant star with properties similar to those of our sun. It orbits only 0.05 AU from its star and, as mentioned above, reaches surface temperatures of 1300°C. It could either be a gas giant, truly similar to Jupiter, or an extremely massive rocky planet like Earth. If it is terrestrial, its oceans would be composed of liquid aluminum and its atmosphere would be rock vapor - that is, vaporized magma.
In 1995 when 51 Pegasi b was first discovered, it was theorized that it had formed at a greater distance from its star, then slowly migrated toward the star and lost angular momentum due to interactions with the remaining planetary disk (see planet formation).
However, it has been proposed that there is no direct physical reason for excluding the possibility that such massive planets could form close to the star. Theories put forward include that the planet could form from gravitational disturbances in the planetary disk, that a small rocky core accumulated and then acquired an enormous gaseous atmosphere, and that the planet, like others, simply formed as an accumulation of planetesimals. It is believed that planetary formation at distances of less than 0.05 AU requires only a relatively low planetary-disk temperature, high density, and low angular momentum.
51 Pegasi Planetary System Data
Created by Dan Corbett, Kate Stafford, and Patrick Wright for ThinkQuest.