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Summer 1999

Extra-Solar Planet-Hunting on the Keck
May Lecture by Steven Vogt

by Rod Norden and Kris Houser

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Sunset at 14,000 feet (Lick Observatory photo)

On May 15, 1999, MIRA was treated to a wonderful and humorous talk by Dr. Steven Vogt of UCSC on the search for planets beyond the solar system. As a professional designer of sophisticated astronomical instrumentation, he is part of a planetary search team in addition to his other work at UCSC. In his talk for MIRA, Dr. Vogt described current efforts to locate planets beyond our solar system. He treated the audience to first-hand stories about searching for extrasolar planets using the 10-meter Keck telescope on Mauna Kea, for which he designed a very special spectrograph.

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MIRA Board Chairman Gordon Jones greeted
the audience of over 200 (photo by Laura Cohan).

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Dr. Steven Vogt of UCSC gestures during
his lecture (photo by Laura Cohan).

Are We Alone?
Ever since humans first looked up into the night sky, one of our first questions was "Are we alone in the universe?" For most of recorded time, answering this question was only speculation. But in the last four years, we have amassed a significant amount of data that indicate the presence of planets around a number of stars. Now we know at least there are other stars like the sun with large planets orbiting them. Nevertheless, none of these discoveries seems suitable for the formation of life as we know it.

Two things are required for life to form and evolve: 1) the planet must have water in liquid form and 2) the star must remain stable over a long period of time. While water is not rare in the universe, liquid water is! The temperature must be in the extremely narrow range of 32 degrees to 212 degrees Fahrenheit—neither below freezing nor above boiling. This requires a planet to orbit at just the right distance from a star in an orbit that remains stable over a very long period of time (billions of years).

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The twin 10-meter Keck telescopes on
Mauna Kea, Hawaii (Lick Observatory photo).

The Problem of Locating Other Planetary Systems
It would seem to the beginner that we could simply point the world’s most powerful telescope (Hubble Space Telescope) at some of the nearest stars and directly image planets orbiting around them. It is not that simple. While the Hubble has succeeded in imaging several protoplantary disks (solar systems in the process of condensing out of gas and dust) in the Orion nebula, there is much more involved in finding planets after the excess material has dissipated.

Planets shine only by reflected light, not by generating any light of their own. This causes them to appear up to one billion times fainter than the stars they orbit (in visible light).

Dr. Vogt then described the technique actually used by the planet finders. Their technique, which uses the well-known Doppler effect, is made possible by the fact that planets in orbit around stars are actually systems that orbit each other around a mutual center of mass called the barycenter. Thus a star with one or more planets will appear to "wobble" when seen from Earth, even though the planets themselves cannot be seen. The Doppler effect states that when an object moves toward the observer the light waves are shortened (appear bluer) and that when an object moves away, its light waves appear more red. This allows astronomers to measure the star’s "wobble" and thus detect the presence of planets without imaging them directly.

The difficulty in detection has been the size of the mutual orbit and the speed of the star and planet toward and away from the earth. Since a star has much more mass than any planet, it is much closer to the barycenter, and its Doppler shift is very slight. Detecting a 12 meter per second shift in a spectrum of a star (with error of much less) is a daunting task. The measurements have to be on the order of 1 part in 100,000,000, which is very close to the limit of detection in the spectrometer. Since this requires extreme sensitivity from one year to the next, it is also important to have an accurate reference value. Geoff Marcy, a graduate student working with Dr. Vogt at UCSC, determined that a clear bottle containing a drop of iodine heated to a gas (50 degrees C) in the light path of the spectrograph, would provide such a reference value. Iodine produces many, many dark absorption lines against which spectral lines of the star can be measured. This allows determination of the Doppler shift to a high degree of precision.

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To give you an idea of the scale of the Keck telescope,
this photo shows a man kneeling in the center, surrounded
by the 10-meter array of mirrors (photo Lick Observatory).

Summary of Planetary Systems Found So Far
Using this new instrumentation plus sophisticated computer programs, the first planet beyond the solar system was co-discovered in 1995. A team of Swiss astronomers announced the discovery, but the Lick Observatory team had been observing the same star and others for more than year. The star is 51 Pegasi and the planet has the mass of Jupiter, but it orbits just above the atmosphere of the star with a period of 4.3 days. This is very different from any planet in our solar system.

Dr. Vogt described several other planets that have been discovered. One such system is 47 UMa in which the planet has 2.3 times the mass of Jupiter orbiting at the distance of Mars from the star. While there is no chance of life as we know it on giant gaseous planets such as this one, this planet may have moons on which life may form.

Several unusual planets have been found with eccentric (non-circular) orbits, also unlike any planets in our solar system. Among these unusual planets are 70 Vir and HD168443. These planetary systems have radial velocity curves that are not sine waves. Such curves can be due to tilted orbits or eccentric orbits of the planets or both. Detecting the contribution of each is particularly difficult.

The most unusual star discovered thus far is Upsilon Andromedae. This is the first triple planet system. The star was found to have planets early in the search, but the radial velocity didn’t match any orbit of a single planet. Observations had to be accumulated over a long period to determine contributing factors. With sufficient data, planets of 0.6, 2.0 and 4.5 Jupiter masses were found at distances from just beyond the stellar atmosphere to nearly that of Jupiter itself. Prior to this, all systems found had only one planet.

As the search continues using the 10-meter Keck telescopes in Hawaii and the 9-meter in Texas, Dr. Vogt mentioned that a greater variety of planetary systems will be discovered and that some planets yet to be discovered may well meet the criteria for harboring life.

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