By Alan Boss, Carnegie Institution for Science
Even Carl Sagan would be astonished by what has transpired in the 20 years since the first reproducible evidence for a giant planet in orbit around a sun-like star was announced in October 1995. The announcement of the discovery of a giant planet in orbit around the near-solar twin 51 Pegasus by Michel Mayor and Didier Queloz, followed by its confirmation a few weeks later by Geoff Marcy and Paul Butler, was completely unexpected, not because 51 Peg b has a mass of about half that of Jupiter, or a circular orbit, but because 51 Peg b orbits its star at a distance just 1/100 that of Jupiter, twenty times closer to 51 Peg than the Earth is to the Sun. Theorists such as myself could not imagine forming a presumably gas giant planet that close to a star, a confined space lacking in the raw materials necessary for forming any giant planet. We also feared that if a giant planet formed at a more reasonable distance, similar to Jupiter’s present orbit, subsequent gravitational interactions between the giant planet and the residual planet-forming disk of gas and dust might result in unchecked inward orbital migration of the giant planet toward the growing central protostar that could only result in the planet being swallowed by the voracious youngster. 51 Peg b proved planet formation theorists to be wrong, and we have been playing catch-up ever since.
Two months after the announcement of 51 Peg b, Carl Sagan sent letters to George Wetherill and me regarding his claim to have predicted theoretically the formation of a planet similar to 51 Peg b. Sagan had published a paper with a colleague in 1977 that used a simple model of the planet formation process to predict that if a protoplanetary disk happened to have all of its mass concentrated close to the protostar, then a single, massive planet orbiting at 10 times the distance of 51 Peg b might form. Their 1977 paper concluded, however, that such a formation mechanism was “highly questionable”. With the discovery of 51 Peg b, Sagan was ready to drop the “highly questionable” qualifier, and take credit for the first theoretical prediction of an extrasolar planet. Wetherill and I discussed Sagan’s claim, but had several objections of our own: first, whether the initial conditions assumed for the disk by Sagan were at all feasible, and, second, whether the simple model used was up to the task. Detailed computational models of planet formation were Wetherill’s specialty, building on the firm analytical foundation built by Victor Safronov and his colleagues, and Wetherill considered the simple model used in the 1977 paper to be closer to numerology than to proper physics. We politely refrained from supporting Sagan’s claim to theoretical ownership of 51 Peg b.
One year later, Carl Sagan died at the untimely age of 62 of a rare bone marrow disease, a shock to all of us who knew him as the prophet of the search for life beyond Earth. Just as I remember my seventh-grade classroom where I first heard about the assassination of President Kennedy in 1963, I remember the traffic light I was stopped at when a radio news show reported the death of Carl. By the time of his death, the roster of exoplanets discovered by Doppler spectroscopy (see http://home.dtm.ciw.edu/users/boss/planets.html/) had grown from one to seven, five of which were discovered by Butler and Marcy. The list of exoplanet candidates was now growing at the rate of a planet every month. Carl was a visionary prophet who lived long enough to catch a glimpse of the Promised Land beyond Earth, but not long enough to fully comprehend the prevalence of extrasolar planets.
51 Peg b was not in any way the first claimed discovery of an exoplanet. The most famous of these was the gas giant planet thought to orbit around Barnard’s Star, a red dwarf star similar to Proxima Centauri that is our nearest neighbour after the Alpha Centauri AB/Proxima Centauri triple system. Peter van de Kamp announced in 1963 the discovery of this planet, 60% more massive than Jupiter, and with an orbital period twice that of Jupiter’s twelve years. This planet made a lot more sense to the theorists than 51 Peg b, and it was accepted as a real detection. Van de Kamp used the astrometric method to search for the wobbles of the central star caused by an unseen planet, where multiple images are taken over a decade or longer. Ten years later, in 1973 George Gatewood published an independent set of astronomical plates that showed that the wobbles that van de Kamp thought were caused by a planet around Barnard’s star were caused instead by changes in the 24-inch refractor used by van de Kamp and in the photographic emulsions used for the exposures. As of 1973, there were no good examples of planets outside our solar system, leaving theorists to continue to concentrate solely on the puzzles associated with the formation of the our own collection of rocky planets, gas giants, and ice giants.
There were other claims for exoplanet discoveries in the two decades between 1973 and 1995. Gordon Walker and Bruce Campbell started one of the first Doppler spectroscopy searches in 1983, and after twelve years of observing, published their final paper in early 1995, concluding that they had found no firm evidence of planets with masses greater than that of Jupiter. In 1988, they thought they had found evidence for a Jupiter in orbit around Gamma Cephei, but after taking more data, in 1992 they published a retraction of the claim. The case for an exoplanet around Gamma Cephei is still debated (see http://exoplanet.eu/catalog/gamma_cephei_b/).
In 1988 another Doppler detection appeared, that of an object orbiting the star HD114762, discovered by David Latham and Michel Mayor. This object, however, had a minimum mass of about 11 Jupiter masses, perilously close to the critical value of 13.5 Jupiter masses, which separates Brown dwarfs from Jupiters. Brown dwarfs are massive enough to burn deuterium during their early evolution, whereas planets are forbidden to enjoy the energy generated by hydrogen fusion reactions (see http://home.dtm.ciw.edu/users/boss/definition.html/). Alexander Wolszczan and Dale Frail used the most exotic method of all to discover planetary-mass objects: in 1992 they published evidence from precise timing of the radio wave pulses emitted by the pulsar PSR1257+12 of the presence of not one, but two planets with masses of several times that of the Earth. The fact that these objects orbited in the deadly radiation field of a neutron star that presumably resulted from a supernova explosion made for a fascinating discovery, but one that held little interest for those of us who were fixated on searching for potentially habitable Earth-mass planets around solar-type stars.
In 2004, Butler and his colleagues announced the discovery of the first example of a new class of exoplanets: super-Earths. They showed that the M dwarf star Gliese 436 was orbited by a planet with a mass as small as 21 times that of the Earth, a mass that suggested a composition lacking in gas but rich in rock and ice. Doppler spectroscopy surveys have found hundreds of exoplanets and super-Earths in the intervening years, enough so that by 2009, the prediction could be made that roughly 1/3 of all M dwarf stars were orbited by super-Earths. M dwarfs are at most about 1/2 the mass of the Sun, with much lower luminosities, leading to their having habitable zones much closer to their stars than Earth is to the Sun, but this remarkably high estimate of M dwarf exoplanets was a strong encouragement that the same high abundances would turn out to be the case for G dwarf stars like the Sun.
Proving this point would fall to NASA’s first space telescope designed specifically for exoplanet detection, the Kepler Space Telescope (see http://kepler.nasa.gov/). Kepler was the brainchild of William Borucki, who struggled for decades to convince his colleagues (and NASA) of the incredible power of a space telescope for discovering exoplanets by the transit photometry technique. Launched in March 2009, Kepler has more than repaid the America taxpayers who funded its development and operations, having discovered nearly 5,000 exoplanet candidates (at a cost of roughly $100K each) and over 1,000 confirmed planets. Kepler has proven that exoplanets are everywhere, even around G dwarf stars, in startling abundances. Estimates range as high as there being one habitable Earth-like planet for every star in our galaxy.
As someone who has lived through the ups and downs of the history of the field of planet formation and detection, this revelation never fails to amaze me, and often chokes me up when giving public lectures. I cannot imagine that Carl Sagan would feel otherwise were he to have survived long enough to survey the entirety of this Promised Land. We now dream not just of pale blue dots, but of pale green dots indicative of chlorophyll worlds, of not-too-distant future space telescopes capable of the direct imaging of nearby habitable worlds, telescopes powerful enough to sample the compositions of the atmospheres of these worlds in search of molecules associated with habitable and even inhabited planets. Proxima Centauri is a sterling example of such a nearby star that we will continue to scrutinize in the coming years.
Carl Sagan lived at a time when the optimists among us hoped that maybe one out of a hundred stars might have a planet of some sort in orbit around it. His famous reference to the Earth as a pale blue dot hinted at the likely fragility of life in the Milky Way galaxy, life quite possibly confined to a single refuge in the immense void of an otherwise uncaring and oblivious universe. We now know that nearly every star we can see in the night sky has at least one planet, and that a goodly fraction of those are likely to be rocky worlds orbiting close enough to their suns to be warm and perhaps inhabitable. The search for a habitable world around Proxima Centauri is the natural outgrowth of the explosion in knowledge about exoplanets that human beings have achieved in just the last two decades of the million-odd years of our existence as a unique species on Earth. If Pale Red Dots are in orbit around Proxima, we are confident we will find them, whether they are habitable or not.
About the author. Dr. Alan Boss is a Research Scientist at the Carnegie Institution for Science’s Department of Terrestrial Magnetism. He is an internationally recognized theoretical astrophysicist, whose research interests include the study of star formation, evolution of the solar nebula and other protoplanetary disks, and the formation and search for extrasolar planets. Dr. Boss has served on manifold NASA review panels, and has led both NASA and community working groups on extrasolar planet studies, including Chair of the NASA Astrophysics Subcommittee, Chair of NASA Planetary Systems Science Working Group, Chair of NASA Origins of Solar Systems MOWG, Chair of the IAU Working Group on Extrasolar Planets, President of IAU Commissions 51 and 53, and Chair of the AAAS Section on Astronomy. He received a NASA Group Achievement award in 2008 for his role in the Astrobiology Roadmap and another in 2010 for his role in the SIM Planet Finding Capability Study Team. He is a member and Fellow of several professional organizations including the American Astronomical Society, AGU, AAAS, Meteoritical Society, and the American Academy of Arts and Sciences. He has received numerous NASA and NSF grants, served on many professional committees, and is a Series Editor of the Cambridge Astrobiology Series. He has published two books about the search for planets outside the Solar System, “Looking for Earths: The Race to Find New Solar Systems” in 1998, and “The Crowded Universe: The Search for Living Planets” in 2009. Boss is currently the Chair of the NASA Exoplanet Exploration Program Analysis Group, as well as Chair of NASA’s Exoplanet Technology Assessment Committee and WFIRST/AFTA Coronagraph and Infrared Detectors Technology Assessment Committees.