Observational requirements for planet formation models

Up to the time of the discovery of the companion to 51 Peg (Mayor & Queloz 1995; Marcy & Butler 1995) the major effort of theoretical studies was to explain the nature of Jupiter, Saturn, Uranus, and Neptune. A successful formation theory of giant planets at that time needed to explain the following general characteristics:

(1) The observed bulk composition characteristics of Jupiter, Saturn, Uranus and Neptune (e.g., Pollack & Bodenheimer 1989). Specifically:

• the similarity of the total heavy element contents of the four giants,

• the very massive H2 and He envelopes of Jupiter and Saturn and much less massive (but not negligible) gaseous envelopes of Uranus and Neptune, and,

• the enhancement of metals over solar abundance in the atmospheres of all four giant planets;

(2) Giant planets need to form quickly. Observed dust disks around young stellar objects indicate disk ages of <10 Myr (Strom et al. 1993).

The explanation for the bulk-composition characteristics comes as a natural consequence of the core accretion scenario. The results of the computer simulations of Paper 1

demonstrated that the mass of the solid component of the giant planets agreed with observations of the gas giants in the Solar System, and that the solid component is relatively independent of the position of the planet in the solar nebula. It was also shown that the formation time of their nominal model (within the context of the assumptions made in their calculations) was within the time compatible with the solar nebula dispersal timescale.

In the decade since the discovery of the companion to 51 Peg, the catalog of extrasolar planets has greatly expanded, there are more observations of protoplanetary disks, and improved interior models of Jupiter and Saturn. To be considered successful, the current observational characteristics that need to be explained by a theoretical formation model are:

(1) The observed bulk composition characteristics of Jupiter, Saturn, Uranus, and Neptune, with emphasis on the enhancement of metals over solar abundance in the atmospheres of all four giant planets (e.g., Young 2003). The interior models of Jupiter (Saumon & Guillot 2004) indicate that the total solid mass ranges from 8-39 M®, of which 0-11 M® is concentrated in the core. Saturn models indicate a total heavy element mass of 13-28 M®, with a core mass between 9-22 M®. Uranus and Neptune models indicate heavy element masses ranging from 10-15 M® and a gaseous mass between 2-4 M® (Pollack & Bodenheimer 1989);

(2) The upper limit to the formation timescale is still 10 Myrs, but from observations of dust disks around young stellar objects (Cassen & Woolum 1999; Haisch et al. 2001; Lada 2003; Chen & Kamp 2004; Metchev et al. 2004) indicate disk ages of <10 Myr with a preference for the time to be 3 to 5 Myrs.

(3) The extrasolar planets exhibit a wide range of eccentricities and semi-major axes. In a few cases, there are long-period, low-eccentricity planets whose orbits are comparable to that of Jupiter. In addition, there is the observed correlation of extrasolar planets forming around parent stars with high metallicity. Santos et al. (2004) observed that 25-30% of the stars with [Fe/H] above 0.3 have a planet, whereas less than 5% of the stars with solar metallicity have observed companions.

Overall, it should be noted that at the time of this conference, A Decade of Extrasolar Planets Around Normal Stars, the CAGC model can explain the bulk compositional properties of the gas giants in the Solar System, and computer simulations based on this model can form planets in a timely fashion—namely, on a timescale of less than 10 Myr and even between 3 and 5 Myrs. In addition, the CAGC model is beginning to address the correlation of more frequent planet formation around parent stars with high metallicity (Kornet et al. 2005) and the effect of migration on the formation timescale (Alibert et al. 2005). Further discussion of how the CAGC model addresses these observational constraints is in Section 5.

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