From the Academy

Extra-solar planetary systems

Joan Najita*†, Willy Benz‡, and Artie Hatzes§

*National Optical Astronomy Observatories, 950 North Cherry Avenue, Tucson, AZ 85719; ‡Physikalisches Institut, Universita¨t Bern, Sidlerstrasse 5, Ch-3012, Bern, Switzerland; and §McDonald Observatory, University of Texas, Austin, TX 78712 he discovery of extra-solar planets has captured the imagi- Table 1. Properties of extra-solar planet candidates Tnation and interest of the public and scientific communities K, alike, and for the same reasons: we are all want to know the Parent M sin i Period, days a,AU e m⅐sϪ1 answers to questions such as ‘‘Where do we come from?’’ and ‘‘Are we alone?’’ Throughout this century, popular culture has HD 187123 0.52 3.097 0.042 0. 72. presumed the existence of other worlds and extra-terrestrial ␶ Bootis 3.64 3.3126 0.042 0. 469. intelligence. As a result, the annals of popular culture are filled HD 75289 0.42 3.5097 0.046 0. 54. with thoughts on what extra-solar planets and their inhabitants 51 Peg 0.44 4.2308 0.051 0.01 56. are like. And now toward the end of the century, astronomers ␷ And b 0.71 4.617 0.059 0.034 73.0 have managed to confirm at least one aspect of this speculative HD 217107 1.28 7.11 0.07 0.14 140. search for understanding in finding convincing evidence of Gliese 86 3.6 15.83 0.11 0.042 379. planets beyond the . ␳1 Cancri 0.85 14.656 0.12 0.03 75.8 The discovery of extra-solar planets has brought with it a HD 195019 3.43 18.3 0.14 0.05 268. number of surprises. To put things in context, the planet 2.1 60.9 0.21 0.27 239. in our solar system has been a benchmark in planet searches ␳ CrB 1.1 39.6 0.23 0.05 67. because it is the most massive planet in our solar system and, HD 168443 5.04 57.9 0.277 0.54 330. from that relatively naive point of view, it is the object that we HD 114762 11.02 84.0 0.351 0.334 618. are most likely to detect in other systems. Even so, this is a 70 Vir 6.84 116.7 0.47 0.40 316.8 challenging task. All the known extra-solar planets have been ␷ And c 2.11 241.2 0.83 0.18 58.0 discovered through high-resolution stellar spectroscopy, which HD 210277 1.36 437 1.15 0.45 41.5 measures the line-of-sight reflex motion of the star in response 16 Cyg B 1.74 802.8 1.7 0.68 52.2 to the gravitational pull of the planet. In our solar system, Jupiter 47 Uma 2.42 1093 2.08 0.09 47.2 induces in the a reflex motion of only about 12 m͞s, which ␷ And d 4.61 1266.6 2.50 0.41 72.9 is challenging to measure, given that the typical spectral reso- 14 Her 3.3 1650 2.5 0.326 73. ͞ lution employed is approximately several km s. Fully aware of M sin i, of the companion times the sine of the inclination of the this difficulty, planet-searching groups have worked hard to system; a, semimajor axis; AU, (ca. 150 million km or 93 achieve this velocity resolution by reducing the systematic effects million miles); e, eccentricity; K, reflex motion. in their experimental method. As one example, prior to detec- tion, the stellar light is passed through an iodine gas-filled absorption cell to imprint a velocity reference on the stellar relevant physical processes, but rather how these processes fit ACADEMY spectrum. together, i.e., our outlook on their relative importance and role FROM THE However, after honing search techniques in this way for in the eventual outcome of the planet formation process. to detect ‘‘Jupiter’’ in other solar systems, a surprising result has The changing role of one of these processes, orbital migra- emerged: a much greater diversity of planetary systems than was tion, illustrates this point as well as the limitations inherent in expected! Searches have revealed planets with a wide range of trying to reconstruct the entire planet formation process from , including planets much more massive than Jupiter; observations of a single system (i.e., our solar system), and the planets with a wide range of orbital distances, including planets consequent importance of extra-solar planets for an improved much closer to their than Jupiter is to our sun; and planets understanding of the formation and evolutionary history of with a wide range of eccentricities, including some with much planetary systems, including our own. For example, the solar more eccentric than those of the planets in our solar system is believed to have formed from the gravitational system (Table 1 and Fig. 1). These results were essentially collapse of a cloud of cold gas similar to those that we now unanticipated by theory; they reveal the diversity of possible observe in the Milky Way. Because of the finite angular outcomes of the planet-formation process, an important fact that momentum of the cloud, all of the collapsing material could was not apparent from the single example of our own solar not fall directly onto the star; some fraction of the gas formed system. instead a rotating circumstellar disk. The disk was a reservoir This diversity is believed to result from the intricate interplay of matter that might eventually accrete onto the star and also among the many physical processes that govern the formation was the raw material for formation of the planets. In such a and evolution of planetary systems, processes such as grain system, as the disk accretes onto the star, it can sweep inward sticking and planetesimal accumulation (e.g., see ref. 1), runaway any planets that have formed, resulting in inward orbital gas accretion (e.g., see ref. 2), gap formation (e.g., see ref. 3), migration of the planets. disk-driven eccentricity changes (e.g., see ref. 4), orbital migra- tion (e.g., see refs. 5 and 6), and dynamical scattering with other planets, companion , or passing stars (e.g., see refs. 7 and 8). This paper is a summary of a session presented at the fifth annual German-American What is interesting about our understanding of planet formation Frontiers of Science symposium, held June 10–13, 1999, at the Alexander von Humboldt following the discovery of extra-solar planets is that thus far, Foundation in Potsdam, Germany. what has changed is not so much our understanding of the †To whom correspondence should be addressed. E-mail: [email protected].

PNAS ͉ December 7, 1999 ͉ vol. 96 ͉ no. 25 ͉ 14197–14198 Downloaded by guest on October 1, 2021 Fig. 1. Extra-solar planetary candidates span a wide range in mass and orbital separation. MJ, mass of Jupiter. (Figure courtesy of Geoff Marcy.)

So how has the role of orbital migration changed? A decade not as a way of destroying planets, but as a way of moving them ago, several well-known solar system theorists who were from their place of formation to where we see them today working on the formation of Jupiter used to say that their best (9, 10). model was one in which Jupiter formed about where it is now, The discovery of extra-solar planets and their diversity has and through orbital migration, migrated inward and was essentially highlighted old questions and reopened many of the incorporated into the Sun. So, Jupiter does not exist, or at least questions that we had plausible explanations for when we had it is not typical. At the same time, planet searches were already only the solar system to explain. In this sense, this discovery has underway but were not producing detections, a result that was reinspired astronomers to obtain more definitive answers to also attributed to inward orbital migration by at least one basic questions about the nature of planet formation, such well-known planet hunter. For it was imagined that planets questions as: Where do planets form? How do planets get to formed in those systems as they must have in our solar system, where we now find them? (That is, How do planetary systems but then migrated inward and were similarly absorbed. The evolve?) When and how frequently do planets form? In addition tentative conclusion then was, ‘‘Maybe we are alone!’’ Of to these questions, there remain basic questions regarding planet course, astronomers eventually went on to discover many formation processes, questions such as how do submillimeter- planets, but the point here is that with only one example of a sized grains accumulate into kilometer-sized rocks, the building , it is easier to regard the system as a fluke if blocks of planets? Future observations of extra-solar planetary it does not fit the theory. The situation is of course quite systems, those in the process of forming as well as those in different today, where we have numerous examples of Jupiter- mature systems similar to our own, when combined with the like planets spanning a wide range of radii. But even in this theoretical insight that they will inspire, will bring us closer to situation, orbital migration again plays a central role, this time answering these questions.

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14198 ͉ www.pnas.org Najita et al. Downloaded by guest on October 1, 2021