Pale Blue Dot II
May 19-21, 1999
Moffett Training and Conference Center
NASA Ames Research Center
Moffett Field, CA
Formation and Abundance of Planets: What Might Be Out There?
Alan
P. Boss
Department of Terrestrial Magnetism
Carnegie Institution of Washington
5241 Broad Branch Road, NW
Washington, DC 20015
Email: boss@axp1.ciw.edu
A new era in the search for life outside the Solar System was entered in 1995, when the first definitive evidence for the existence of a companion with a mass comparable to that of Jupiter orbiting a solar-type star was presented by Mayor \& Queloz (1995). Since that epochal moment, the pace of discovery has been rapid, so that even newly-published review articles (e.g., Marcy \& Butler 1998) are soon out of date. As of May 1999, there was good evidence for at least 20 planetary-mass companions to solar-type stars, as well as a growing number of objects that appear to be better classified as brown dwarf stars. With the discovery of the first confirmed extrasolar planetary {\it system} around a solar-type star, Upsilon Andromedae with a hot Jupiter and two multiple-Jupiter-mass outer planets (Butler et al. 1999), a second milestone has been reached. The next major milestone may be the discovery of an extrasolar Jupiter orbiting at a large enough distance to permit a terrestrial planet to orbit stably in the habitable zone of a solar-type star.
Achieving this third milestone would imply that the Solar System is not unique, and that the basic theoretical mechanisms developed to date to explain our own Solar System may well be applicable elsewhere. While none of the newly-discovered extrasolar planets strongly resembles Jupiter, the currently successful search technique (radial velocity) favors the detection of short-period, massive planets, so one should not conclude that Solar-System-like configurations are rare -- indeed, such systems could still be quite commonplace. Hence we still look to the Solar System for guidance about what we might find elsewhere.
The terrestrial planets are universally believed to have formed through the collisional accumulation of successively larger solid bodies -- beginning with micron-sized dust grains, through kilometer-sized planetesimals, Moon-sized planetary embryos, and culminating after about 100 Myr in the formation of Earth, Venus, and their retinue of smaller terrestrial planets (Wetherill 1990). Because this process is inherently stochastic, it is not possible to make definitive predictions regarding the outcome of collisional accumulation starting from a given set of initial conditions, but certain basic trends can be elucidated, of which perhaps the most important is the effect of Jupiter-mass planets on terrestrial planet formation (Wetherill 1996). If a Jupiter-mass planet forms prior to the final phases of terrestrial planet formation, as seems necessary to explain the gaseous bulk composition of a Jupiter-like planet, then the location of such a massive gravitational perturber has a controlling influence on the formation of Earth-mass planets in the habitable zone -- a Jupiter closer than 4 AU to its star could forestall the growth of Earth-mass planets at 1 AU. None of the extrasolar Jupiters discovered so far orbits with a period large enough to encourage the formation of a habitable Earth-mass planet.
While a consensus exists regarding at least the basic mechanism for terrestrial planet formation, the situation is quite different for the formation of gas giant planets. The conventional wisdom is that gas giant planets form by the process of core accretion, where a roughly 10 Earth-mass solid core forms first by collisional accumulation, and then accretes disk gas (Pollack et al. 1996). However, even when the surface density of solids is high enough for runaway accretion to assemble the solid core (Lissauer 1987), the timescale for subsequent accretion of the gaseous envelope is sufficiently long (typically a few to 10 Myr) to raise the danger that the disk gas may have been dissipated before the solid cores could accrete enough gas to grow to Jupiter-mass. Instead, "failed cores" similar to Uranus and Neptune might result. Recently, an alternative has been investigated, where gas giant planets form rapidly through a gravitational instability of the gaseous portion of the disk (Boss 1997, 1998). Disk instability can occur within thousands of years, well before the disk dissipates, and if this mechanism can occur, it would presumably out race the core accretion mechanism. The disk instability mechanism seems to lead primarily to multiple-Jupiter-mass planets, and so it may be better suited to explaining the formation of the Upsilon Andromedae system than our own, given that the former system contains about ten times more mass in planets than our Solar System. Perhaps both core accretion and disk instability can form giant planets, depending on the circumstances in different protoplanetary disks.
The situation is even murkier for the outermost, ice giant planets. Here there is very little understanding about how Uranus and Nepture formed. The basic mechanism is believed to be collisional accumulation of icy solids, but because time scales for collisions lengthen greatly with increasing distance and orbital period, the process is even slower than in the gas giant planet region. In order to gain some understanding, theorists have had to artificially increase the collisional cross section of the growing planetary embryos (e.g., Levison et al. 1998). In addition, the bodies that do grow tend to have eccentric orbits that inhibit further growth and do not resemble the orbits of the outer planets. Because of these difficulties, the theory of ice giant planet formation must still be considered to be in an embryonic stage.
References:
Boss, A. P. 1997, Science, 276, 1836
Boss, A. P. 1998, Astrophys. J., 503, 923
Butler, R. P., Marcy, G. W., Fischer, D. A., Brown, T. W., Contos, A. R., Korzennik, S. G., Nisenson, P. \& Noyes, R. W. 1999, Astrophys. J., submitted
Levison, H. F., Lissauer, J. J. \& Duncan, M. J. 1998, Astrophys. J., 116, 1998
Lissauer, J. J. 1987, Icarus, 69, 249
Marcy, G. W. \& Butler, R. P. 1998, Ann. Rev. Astr. Astrophys., 36, 57
Mayor, M. \& Queloz, D. 1995, Nature, 378, 355
Pollack, J. B., Hubickyj, O., Bodenheimer, P., Lissauer, J. J., Podolak, M. \& Greenzweig, Y. 1996, Icarus, 124, 62
Wetherill, G. W. 1990, Ann. Rev. Earth Planet. Sci., 18, 205
Wetherill, G. W. 1996, Icarus, 119, 219