In October 1995 - ten years ago this month - two Swiss astronomers announced the discovery of the first planet around a Sun-like star outside of our solar system. A few weeks later the American team of Geoff Marcy and Paul Butler confirmed the discovery, and a few months after that they added two more "extrasolar planets." These landmark events were only the beginning of a deluge of new planets. Some 155 are now known in addition to the 9 (or 8 or 10 depending on how 'planet' is defined) in our own solar system. Hardly a week goes by without the discovery of more. In a way, each discoverer is a new Columbus, unveiling a new planet rather than a new continent. Although these planets are gas giants, Earth-sized planets are not far behind. A thousand years from now our descendants may explore them in person.
Although NASA played only a small role in the early planet discoveries, its interest in extrasolar planets dates back several decades. Already in the 1970s, as part of its fledgling Search for Extraterrestrial Intelligence (SETI) program, NASA sponsored a series of workshops on planet detection. If intelligence was common in the universe, there needed to be planets, unless one adopted an exotic view of intelligence as found in Fred Hoyle's famous novel Black Cloud. These workshops, and others that followed in the 1970s, discussed a variety of planet detection techniques, backed up by only minimal funding. The discovery of new planets, if they even existed, was still considered a long shot.
A turning point came in the 1980s. As spacecraft had been successfully dispatched one-by-one to the planets of our solar system during the 1970s and 1980s, NASA now sought more worlds to conquer. Both through its own committees and the advisory capacity of the National Academy of Sciences, it sought to extend the realm of the planetary sciences from our solar system to other planetary systems. It did so following an array of astonishing and unexpected discoveries.
In 1983 the Infrared Astronomical Satellite (IRAS) found that the bright star Vega was shining ten to twenty times brighter than it should have at long infrared wavelengths, a phenomenon known as "infrared excess". Additional observations showed that the source of the infrared excess was a ring of dusty material surrounding Vega, possibly the first evidence for a solar system in formation. The discovery was trumpeted on the front page of The Washington Post and newspapers around the world. Nor was this by any means a unique phenomenon; by mid-1984 some 40 "circumstellar disks," or "protoplanetary systems" had been found, depending on the interpretation given to the infrared excess. By late 1984 one of the IRAS objects, Beta Pictoris, had been photographed by a ground-based optical telescope.
During the 1990s, attention to the problem of planetary systems reached new heights. Researchers realized that technology was ripe to open a new field. Studies in increasingly greater detail were undertaken demonstrating how planets could be observed from Earth and from space, using a variety of technologies. NASA continued to contribute to the field by funding researchers, and with the Hubble Space Telescope’s observations in 1993 of possible protoplanetary disks around 56 of 110 young stars in the Orion Nebula. And then came the discoveries of real planets using the pioneering ground-based techniques of the Swiss team and Marcy and Butler.
Image left: Hubble Space Telescope image of the Orion nebula, a star-forming region 1500 light years distant. Some of the small knots of matter are believed to be protoplanetary disks, or 'proplyds' that might evolve into planets. Credit: NASA and C.R. O'Dell/Rice University. Full caption and high-res versions of this image are available here.
In the wake of these discoveries in 1996 the search for planetary systems became an important part of a bold new overarching program at NASA known as Origins. Under the banner of Origins, planetary systems science was assured continued attention and funding. By 1997 a detailed "Origins Roadmap" was published. The Roadmap described three ambitious scientific goals for the Origins theme, dealing with galaxies, planets, and life, all keyed to the question of "where did we come from?" These goals were to understand how galaxies formed in the early universe and their role in the appearance of planetary systems and life; how stars and planetary systems form and whether life-sustaining planets exist around other stars; and how life originated on Earth and whether it exists elsewhere.
NASA now has several spacecraft in production or on the drawing boards that will contribute to our knowledge of planetary systems. First in line for launch in 2007 is Kepler, which will monitor 100,000 stars over four years to see if their light dims as a planet crosses in front of its parent star. Its instruments are precise enough to detect Earth-sized planets, in contrast to the gas giants detected so far with ground-based techniques. It could discover thousands of such planets during its lifetime. An innovative spacecraft known as the Space Interferometry Mission (SIM) will target specific stars in the search for planets after its launch in 2009. The James Webb Space Telescope, successor to the Hubble Space Telescope scheduled for launch in 2011, will extend the search for circumstellar matter and extrasolar planets at infrared wavelengths, along with its mission to observe the earliest stars and galaxies in the universe. Further down the line still is the Terrestrial Planet Finder, which may be able to characterize planetary atmospheres.
All these spacecraft will extend the discoveries of the Earth-bound pioneers - discoveries that began ten years ago this month. The goal of looking for Earths and unveiling our origins is of tremendous public interest. Planetary systems are an integral part of cosmic evolution, and thus an essential step in the search for life – and our place in the universe. In short, discovering new worlds is in the best tradition of exploration. And it is only the first step, until someday in a future perhaps 40 generations away, humans will follow, sailing the ocean of stars in the sea of space.
Steven J. Dick
NASA Chief Historian
Boss, Alan, Looking for Earths: The Race to Find New Solar Systems (New York, 1998).
Crosswell, Ken, Planet Quest: The Epic Discovery of Alien Solar Systems (New York, 1997).
Dick, Steven J. and James E. Strick, The Living Universe: NASA and the Development of Astrobiology (Rutgers University Press, 2004).
Dorminey, Bruce. Distant Wanderers: The Search for Planets Beyond the Solar System (New York, 2002).
Goldsmith, Donald. Worlds Unnumbered: The Search for Extrasolar Planets (Sausalito, CA, 1997).
Morrison, Philip, John Billingham and John Wolfe, The Search for Extraterrestrial Intelligence (NASA, 1977; Dover reprint, 1979).
Steven J. Dick