(Discovery of) Extrasolar Planets
R. Paul Butler
Carnegie Institute of Washington
About the Lecture
Over the last six years planets have been discovered around 70 nearby Sun-like stars. All of these planets have been revealed by the gravitational wobble they impose on their host stars. Our group has found about two-thirds of these planets, including the first system of multiple planets orbiting a Sun-like star, the first two sub-saturn mass planets, and the first transit planet.
The planets detected to date have profoundly challenged the theories of planet formation, including Jupiter-mass planets in very small (4 day) orbits, and Jupiter-mass planets in extremely eccentric orbits. Only two of the planetary systems discovered to date are even remotely similar to the Solar System. Now that planets have been detected, we would like to know what fraction of stars have planets, what fraction of planetary systems are similar to the Solar System, and how many other types of planetary systems exist. Our current goal is to survey all 2,000 Sun-like stars out to 150 light-years. We are currently observing 1,200 stars with the Lick 3-m (California), Keck 10-m (Hawaii), and the 3.9-m Anglo-Australian Telescopes. Next year we will expand these surveys with the 6.5-m Magellan Telesccope.
The Keck, Lick, and Anglo-Australian surveys are the only active programs capable of detecting "Solar System-like" planets (i.e. Jupiter and Saturn-mass planets) beyond 4 AU. By 2010 these surveys will provide a first planetary census of nearby stars and allow us to estimate the ubiquity of planetary systems and of "Solar System" analogs.
About the Speaker
R. Paul Butler received a B.A. in Physics (1985) and B.S. in Chemistry (1986) and a M.S. in Physics (1989) from San Francisco State University. He received a Ph.D. in Astronomy (1993) from the University of Maryland. He has previously served as a Research Scientist at San Francisco State University, a Visiting Research Fellow at U.C. Berkeley, and a Staff Astronomer at the Anglo-Australian Observatory. He is presently a Staff Scientist at the Carnegie Institution of Washington Department of Terrestrial Magnetism. Honors include the ABC News Person of the Week, International Astronomical Union Bioastronomy Medal, and the National Academy of Sciences Henry Draper Medal.
President McDiarmid called the 2133rd meeting to order at 8:18 p.m. on September 21, 2001. The Recording Secretary read the minutes of the 2131st meeting and they were approved.
The speaker for the 2133rd meeting was R. Paul Butler. The title of his presentation was “(Discovery of) Extrasolar Planets”.
The existence of other worlds has been an open question for thousands of years. Epicurus said there were an infinite number of worlds. Aristotle said there was only one. Giordano Bruno was burned at the stake in 1600 for the heresy of disagreeing with the religiously approved opinion of Aristotle. Mr. Butler began his project in 1986 to detect extrasolar planets by looking for periodic variations in stellar Doppler shifts. This small acceleration is the gravitational wobble planets impose on the stars they orbit.
The first unambiguous planet around an extrasolar planet around the star 51 Pegasi was reported in October 1995 by the Swiss group Mayor & Queloz.
The Keck I 10-meter telescope in Hawaii, the Lick 3-meter telescope in California, and the Anglo-Australian 3.9-meter telescope are the only active survey programs capable of detecting planets the size of Saturn and larger beyond 4 AU. Next year, the 6.5-meter Magellan telescope will become available for this work.
These telescopes use an Echelle grating spectrograph with CCD detectors designed by Steve Vogt. With these instruments shifts in spectral lines can be detected almost at the atomic level on the CCD, distances of only about 80 atoms. The spectral shift detection limit is now about three meters per second, good enough to see at least Jupiter mass planets around near-by Sun-like stars.
The first extrasolar planet to be detected by the Anglo-Australian search team was reported in January 1996. It orbits 47 Ursae Majoris with a period of 2.99 years, an eccentricity of 0.10, and has 2.52 Jupiter masses. The period is determined directly from the period of the variation in the Doppler shift. The mass is estimated from the magnitude of the Doppler shift change, the estimated mass of the star, and the orbital period. The eccentricity is estimated by analyzing the pattern of the spectral shift; circular orbits with zero eccentricity will be purely sinusoidal while more eccentric orbits will be more saw-toothed. [The earth has an orbital eccentricity of 0.02.] The next extrasolar planet was discovered around the star HD 27442. With 1.34 Jupiter masses it has a period of 1.14 years, and an eccentricity of 0.06. A planet discovered around 16 Cygni B, a star very similar to the sun, has a period 2.18 years, 1.66 Jupiter masses and a large eccentricity of 0.67 evidenced by the saw-toothed pattern of its spectral shift. A planet discovered around 51 Pegasi has 0.45 Jupiter mass, a period of 4.233 days, and an eccentricity of 0.01. A planet around tau Boötis has 4.08 Jupiter masses, a period of 3.312 days, and zero eccentricity, a circular orbit. The smallest planet detected orbits 79 Ceti, with 0.8 Saturn mass and a period of 75.91 days.
The first transit of an extrasolar planet was observed in 1999 around the star HD 209458. The planet has 0.63 Jupiter mass and a period of 3.524 days. Because this gas giant planet orbits its star so closely, it is relatively hot and so, although less massive than Jupiter, its atmosphere expands to be 40 percent larger than Jupiter's producing almost exactly the predicted 1.5 percent drop in light of the star during the transit.
Multiple planet systems can be observed in superimposed sinusoidal wave patterns. The star upsilon Andromedae has one planet with 0.72 Jupiter mass, a period of 4.617 days and an eccentricity of 0.04, a second planet with approximately one Jupiter mass and a period of eight months, and a third planet with about 3 Jupiter masses and a period of 3.5 years. Star HD 168443 has two companions, one with a period of 60 days another with a period of 4.5 years and 17 Jupiter masses.
In the last six years, planets have been discovered around 70 nearby Sun-like stars. The planets detected so far have profoundly challenged the theories of planet formation, including as they do Jupiter-mass planets in very small (4 day) orbits, and Jupiter-mass planets in extremely eccentric orbits. Only two of the planetary systems discovered to date are even remotely similar to the Solar System. There is some bias in the current detection methods for seeing the large effects produced by large planets in small orbits. The smallest detected planet has 0.325 Jupiter mass.
A project has begun to complete by 2010 a planetary census of 2000 Sun-like stars within 50 parsecs. Improved techniques may permit transit observation by interferometry and infrared imaging, and CCD spectral shift detection with a limit of less than three meters per second. This survey should allow us to estimate what fraction of stars have planets, what fraction of planetary systems are similar to the Solar System, and what other types of planetary systems exist. In considering the Drake equation for estimating the probability of observing extraterrestrial civilizations, the good news is that 95% of the surveyed stars do not have gas giant planets in their habitable zones.
Mr. Butler kindly answered questions from the floor. President McDiarmid thanked Mr. Butler for the society, and welcomed him to its membership. The President made the announcements about the next meeting, parking, and refreshments, and adjourned the 2133rd meeting to the social hour at 9:39 p.m. [These minutes were corrected by the Recording Secretary after they were read.]
John S. Garavelli