The detection of planetary mass bodies through Doppler spectroscopy yields no information about these objects except for the orbital semimajor axis and eccentricity and a lower limit to the planetary mass. In fact, because all that is measured is the radial component of the reflex motion of the star, the planet itself is not directly detected (9). We have no information about the size of the planet, hence no way to gauge its bulk density and thus composition. Without other types of data, we must assume that a Jovian-mass object is like Jupiter in size and composition. Because hydrogen and helium are the most abundant elements in the cosmos, this is not an unreasonable assumption, but it is nonetheless an assumption.
A breakthrough that revealed the nature of one Jovian-mass extrasolar planet came in the successful observation of a planetary transit across the disk of a star. The system, HD209458, consists of a star roughly the age of the Sun and just slightly more massive, along with a planetary companion at least 0.7 times the mass of Jupiter, orbiting just 0.047 AU from the parent (15). The transit observation consists of observing the dimming of the light from the star by about 1% as the nonluminous planet crosses the disk. Because the orbital radius of the planetary companion is known, and the star's radius is determined fairly reliably from stellar evolution theory, the dimming can be geometrically related to the physical size of the planet. The decrease in light as the planet blocks part of the stellar disk is best fitted by a planet with radius between 1.25 and 1.55 that of Jupiter, on the basis of data from two ground-based telescopes and a set of Hubble Space Telescope observations of the transit (5, 15).
That transits occur in this system immediately sets a tight constraint on the orbital inclination of the planet-star system as seen from Earth: the orbit plane of the planet must be roughly along the line of sight to Earth. Because very slight departures from coplanarity of the planet's orbit with the Earth line of sight affect which part of the stellar disk is transited, the timing of the transit allows a numerical value to be put on the orbit inclination. For HD209458, it is within 3° of being coplanar with the line to Earth. Thus the minimum mass derived from the radial velocity studies for HD209458 b, 0.7 Jupiter masses, is in fact the physical mass; then combining the mass with the radius, one finds the planet's bulk density to be between 0.3-0.5 g/cm3, half that of Jupiter or Saturn. The derived radius of HD209458 b immediately rules out a rocky planet, which would have a far smaller radius for the determined mass of 0.7 Jupiter masses (16). The planet must be primarily hydrogen, with presumably an admixture of helium and heavier elements. But why is the planet so large compared with Jupiter?
The facile answer, which seems intuitive, is that the proximity to the parent star caused HD209458 b to expand. There is, however, an important subtlety here that is key to understanding the early history of the body. The expansion cannot be a superficial effect of the outer atmosphere. Why? The scale height of the atmospherekT/mg, where k is Boltzmann's constant, T is atmospheric temperature, m is atmospheric molecular mass, and g is gravityis about 400 km (for an effective temperature of about 1,200 K and molecular hydrogen-helium composition). This scale height is less than 1% of the radius of the planet. Hence, even though the scale height is about 20 times that in Jupiter's much colder atmosphere, it is not large enough to be implicated in a swelling of the planet by a factor of 1.4 relative to Jupiter.
What is in fact happening is that the prodigious stellar flux retards the cooling of the planetary interior. Formation of a massive self-gravitating object from the collapse of spatially dispersed gas and dust must, by simple application of the virial theorem, lead to an initially hot distended object, which then cools and contracts as thermal energy is removed from the interior (17). Detailed theoretical models of the cooling and shrinking of giant planets over time provide a satisfactory fit to the details of the giant planets of our own solar system (18). These models show that isolated giant planets (not affected by irradiation from the parent star) cool quickly. It takes less than 1 million years for such an object to drop below 2 Jupiter radii (6).
Strong stellar irradiation, which giant planets on close orbits such as HD209458 b receive, flattens the atmospheric temperature profile. In consequence, the rate at which heat can be transported outward from the deep interior is reduced, and contraction of the planet with time is retarded. Assuming that HD209458 b was born in place at 0.046 AU from the star, detailed models of these effects (6) yield excellent agreement with the planet's radius at its current age of 4-7 billion years (the age being derived from the properties of the star and stellar evolution theory). But more importantly, one cannot arrive at such an expanded radius for the companion if it moved inward to its present orbit later than a few tens of millions of years after formation. It can be shown that it would then take longer than the age of the universe for external heat to diffuse in to the interior and expand the planet to its observed size (6).
It is remarkable that, from basic information about an extrasolar planet derived from transit and radial velocity data, we can constrain aspects of its history. We now know that HD209458 b is a hydrogen-rich gas giant like Jupiter. We know that it either formed in place at 0.046 AU or it moved in to its present orbit within the first tens of millions of years after formation. This migration, in turn, does not preclude terrestrial planets on Earth-like orbits in that system, because HD209458 b could have been in place early enough not to disrupt terrestrial planet formation on reasonable timescales (19).