Extra-solar planetary systems

The discovery of extra-solar planets has captured the imagination and interest of the public and scientific communities alike, and for the same reasons: we are all want to know the answers to questions such as “Where do we come from?” and “Are we alone?” Throughout this century, popular culture has presumed the existence of other worlds and extra-terrestrial intelligence. As a result, the annals of popular culture are filled with thoughts on what extra-solar planets and their inhabitants are like. And now toward the end of the century, astronomers have managed to confirm at least one aspect of this speculative search for understanding in finding convincing evidence of planets beyond the solar system.

The discovery of extra-solar planets has brought with it a number of surprises. To put things in context, the planet Jupiter in our solar system has been a benchmark in planet searches because it is the most massive planet in our solar system and, from that relatively naive point of view, it is the object that we are most likely to detect in other systems. Even so, this is a challenging task. All the known extra-solar planets have been discovered through high-resolution stellar spectroscopy, which measures the line-of-sight reflex motion of the star in response to the gravitational pull of the planet. In our solar system, Jupiter induces in the Sun a reflex motion of only about 12 m/s, which is challenging to measure, given that the typical spectral resolution employed is approximately several km/s. Fully aware of this difficulty, planet-searching groups have worked hard to achieve this velocity resolution by reducing the systematic effects in their experimental method. As one example, prior to detection, the stellar light is passed through an iodine gas-filled absorption cell to imprint a velocity reference on the stellar spectrum.

However, after honing search techniques in this way for years to detect “Jupiter” in other solar systems, a surprising result has emerged: a much greater diversity of planetary systems than was expected! Searches have revealed planets with a wide range of masses, including planets much more massive than Jupiter; planets with a wide range of orbital distances, including planets much closer to their suns than Jupiter is to our sun; and planets with a wide range of eccentricities, including some with much more eccentric orbits than those of the planets in our solar system (Table 1 and Fig. 1). These results were essentially unanticipated by theory; they reveal the diversity of possible outcomes of the planet-formation process, an important fact that was not apparent from the single example of our own solar system.

Table 1.

Properties of extra-solar planet candidates

Parent star	M sin i	Period, days	a, AU	e	K, m⋅s−1
HD 187123	0.52	3.097	0.042	0.	72.
τ Bootis	3.64	3.3126	0.042	0.	469.
HD 75289	0.42	3.5097	0.046	0.	54.
51 Peg	0.44	4.2308	0.051	0.01	56.
υ And b	0.71	4.617	0.059	0.034	73.0
HD 217107	1.28	7.11	0.07	0.14	140.
Gliese 86	3.6	15.83	0.11	0.042	379.
ρ1 Cancri	0.85	14.656	0.12	0.03	75.8
HD 195019	3.43	18.3	0.14	0.05	268.
Gliese 876	2.1	60.9	0.21	0.27	239.
ρ CrB	1.1	39.6	0.23	0.05	67.
HD 168443	5.04	57.9	0.277	0.54	330.
HD 114762	11.02	84.0	0.351	0.334	618.
70 Vir	6.84	116.7	0.47	0.40	316.8
υ And c	2.11	241.2	0.83	0.18	58.0
HD 210277	1.36	437	1.15	0.45	41.5
16 Cyg B	1.74	802.8	1.7	0.68	52.2
47 Uma	2.42	1093	2.08	0.09	47.2
υ And d	4.61	1266.6	2.50	0.41	72.9
14 Her	3.3	1650	2.5	0.326	73.

M sin i, mass of the companion times the sine of the inclination of the system; a, semimajor axis; AU, astronomical unit (ca. 150 million km or 93 million miles); e, eccentricity; K, reflex motion. 

Figure 1.

Extra-solar planetary candidates span a wide range in mass and orbital separation. M_J, mass of Jupiter. (Figure courtesy of Geoff Marcy.)

This diversity is believed to result from the intricate interplay among the many physical processes that govern the formation and evolution of planetary systems, processes such as grain sticking and planetesimal accumulation (e.g., see ref. 1), runaway gas accretion (e.g., see ref. 2), gap formation (e.g., see ref. 3), disk-driven eccentricity changes (e.g., see ref. 4), orbital migration (e.g., see refs. 5 and 6), and dynamical scattering with other planets, companion stars, or passing stars (e.g., see refs. 7 and 8). What is interesting about our understanding of planet formation following the discovery of extra-solar planets is that thus far, what has changed is not so much our understanding of the relevant physical processes, but rather how these processes fit together, i.e., our outlook on their relative importance and role in the eventual outcome of the planet formation process.

The changing role of one of these processes, orbital migration, illustrates this point as well as the limitations inherent in trying to reconstruct the entire planet formation process from observations of a single system (i.e., our solar system), and the consequent importance of extra-solar planets for an improved understanding of the formation and evolutionary history of planetary systems, including our own. For example, the solar system is believed to have formed from the gravitational collapse of a cloud of cold gas similar to those that we now observe in the Milky Way. Because of the finite angular momentum of the cloud, all of the collapsing material could not fall directly onto the star; some fraction of the gas formed instead a rotating circumstellar disk. The disk was a reservoir of matter that might eventually accrete onto the star and also was the raw material for formation of the planets. In such a system, as the disk accretes onto the star, it can sweep inward any planets that have formed, resulting in inward orbital migration of the planets.

So how has the role of orbital migration changed? A decade ago, several well-known solar system theorists who were working on the formation of Jupiter used to say that their best model was one in which Jupiter formed about where it is now, and through orbital migration, migrated inward and was incorporated into the Sun. So, Jupiter does not exist, or at least it is not typical. At the same time, planet searches were already underway but were not producing detections, a result that was also attributed to inward orbital migration by at least one well-known planet hunter. For it was imagined that planets formed in those systems as they must have in our solar system, but then migrated inward and were similarly absorbed. The tentative conclusion then was, “Maybe we are alone!” Of course, astronomers eventually went on to discover many planets, but the point here is that with only one example of a planetary system, it is easier to regard the system as a fluke if it does not fit the theory. The situation is of course quite different today, where we have numerous examples of Jupiter-like planets spanning a wide range of radii. But even in this situation, orbital migration again plays a central role, this time not as a way of destroying planets, but as a way of moving them from their place of formation to where we see them today (9, 10).

The discovery of extra-solar planets and their diversity has essentially highlighted old questions and reopened many of the questions that we had plausible explanations for when we had only the solar system to explain. In this sense, this discovery has reinspired astronomers to obtain more definitive answers to basic questions about the nature of planet formation, such questions as: Where do planets form? How do planets get to where we now find them? (That is, How do planetary systems evolve?) When and how frequently do planets form? In addition to these questions, there remain basic questions regarding planet formation processes, questions such as how do submillimeter-sized grains accumulate into kilometer-sized rocks, the building blocks of planets? Future observations of extra-solar planetary systems, those in the process of forming as well as those in mature systems similar to our own, when combined with the theoretical insight that they will inspire, will bring us closer to answering these questions.