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Extrasolar Planets: First Reconnaissance

by Dr. Paul Butler - Keck Observatory
Appearance Sponsored by American Astronomical Society

Abstract: Within the last six years, planets that have been discovered have been revealed by the gravitational wobble they impose on their host stars. Our group has found two-thirds of these planets, including all four published systems of multiple planets orbiting Sun-like stars, the only known planet observed to transit its star, and the first two sub-Saturn mass planets.

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 non-circular, eccentric, orbits. Only the systems orbiting 47 Ursae Majoris and HD 27442, with Jupiter-mass companions in circular orbits, reminds us of 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. Toward this goal our group is surveying the 1000 nearest and brightest Sun-like stars in the northern hemisphere using the Lick 3-m (California) and Keck 10-m (Hawaii) Telescopes. In addition we are surveying the 200 brightest southern hemisphere stars from the 3.9-m Anglo-Australian Telescope. Next year we will begin surveying the remaining nearby southern hemisphere stars from the 6.5-m Magellan Telescope in Chile.

These surveys are the only active program capable of detecting "Solar System-like" planets. By 2010 these surveys will provide a first planetary census of nearby stars, allowing us to estimate the ubiquity of planetary systems and of "Solar System" analogs.

Within the last six years, planets that have been discovered have been revealed by the gravitational wobble they impose on their host stars. Our group has found two-thirds of these planets, including all four published systems of multiple planets orbiting Sun-like stars, the only known planet observed to transit its star, and the first two sub-Saturn mass planets.

The search for other worlds started centuries ago, when Earth was, according to Aristotle, the center of the universe. Giordano Bruno, in Italy, in 1588, was the first to state that the sun was a star, and the Earth was a planet. For his insight, he was burned at the stake. Tycho Brahe was the last and greatest of the naked eye astronomers, and was the greatest of astronomers in his day to make precision instruments for measuring stellar and planetary positions. He was the first to map the position of Mars as seen fro Earth. Upon his death in 1603, Kepler inherited all of Brahe’s observational data, and was the first to propose the laws of elliptical motion for planetary orbits. In modern times, the discovery of an extra-solar planet, Upsilon Andromedae, was a news worthy accomplishment.

The theory of solar system formation states that planets, which rotate around the sun in the same direction as the sun itself and in the same orbital plane, must have been formed from a solar disk. Recent observations from the Hubble Space Telescope confirm that all young stars have such disks. But as recently as 1995, no extra-solar planets had been discovered.

As a planet revolves around a star, it causes the star to trace a counter-orbit, or wobble. This wobble can be detected by a Doppler shift in the stellar spectrum. The first observatories to detect such a wobble were the Lick Observatory, the Keck Observatory at Mauna Kea in 1995, and the Anglo-American telescope in 1998. Starting in November 2002, the ESO Telescope at Las Campanas, Chile, will become involved in searching for extra-solar planets.

To study the Doppler effect, it is necessary to know that when a planet approaches the observer, the light is shifted towards the blue end of the spectrum; when it recedes, it is red shifted. Thus, we have the capability to analyze the orbits of extra-solar planets. A high precision Eschelle spectrograph is used to measure the small Doppler shift of lined in a stellar spectrum allowing the astronomer to measure the speed of the star in space. Such measurements are very difficult to make, and are prone to error. A CCD detector can record 90 per cent of the light that strikes it, and is important in this research because of the small pixel sizes available.

47 Ursae Majoris was one of the first planets discovered via the Doppler analysis method. The orbital period of 47 UM appeared to be about 3 years, when made it easy to find the distance of the new planet from the star using Kepler’s Laws, and based on the fact that the planet seemed to have a circular orbit. 47 UM turned out to have a planet similar to Jupiter, but two times as large.

Planets with circular orbits are similar to planets in our own solar system. Planetary orbits were once all thought to be circular. Such is not the case. In fact, circular orbits are unusual. For example, 16 Cygni B has an extra-solar planet with an extremely eccentric orbit. Planets with a highly eccentric orbit have the effect of cleaning out a stellar planetary system, thus reducing the possibility of life on other stars. Most extra-solar planets seem to be in eccentric orbits that move at one point with the orbit size of Earth.

Planet-planet interactions seem to be the cause of high eccentricity orbits. If, for example, Saturn were much larger, it would have the same effect in our own solar system. It is the circularity of Jupiter’s orbit in our own solar system that protects us, and allows for life on earth.

51 Pegasi was the first extra-solar planet discovered. This discovery was made by a team of Swiss astronomers. 12 Pegasi has a very short orbit of four days, and is thus close to its parent star.

Tau Bootes has an even shorter period of only 3.312 days in a highly circular orbit. Because it is so close to its parent star, tides on the planet tend to circularize the orbit.

HD 209458 was the first extra-solar planet to be directly observed, due to the transit of the planet across the face of its parent star. Normally, there is on a 10 per cent chance of discovering such a situation. But in 1999, HD 209458 became the first such a planet to be detected. By knowing how much light is being blocked, astronomers are able to deduce the size of the planet, and in this case, they were able to do so, learning the orbit, mass, and size of the planet.

Here on Earth, we live in a system of multiple planets. But up to now, whenever an extra-solar planet has been discovered, it has turned out to be a single Jupiter sized or larger. The search method is weighted in this direction. However, in depth analysis allowed the discovery of two outer planets in Upsilon Andromedae, both with Jupiter size masses. Thus, Upsilon Andromedae became an example of three large planets stuffed into small orbits in an extraordinarily stable system.

HD 168443 was the second star to be found to have two planets. So it appears that stars with multiple planets may be common. Furthermore, it seems that stars with more iron and heavy metals than the sun ten to have more plants than stars with less iron and heavy metals than the sun. Two thirds of all stars are multiple star systems, and multiple-stars systems frequently have planets.

On June 15, 2002, the 15th newly discovered extra-solar planet was announced. Up to now, the planetary systems discovered have not been really similarly to our solar system. However, the planet orbiting 79 Ceti is closer to Earth size than usual, being about half the size of Saturn.

Up to now, the planets that have been discovered were found using Doppler analysis. We are now planning to apply the techniques of astrometry to detect new planets directly. A giant telescope in space could be used to make the actual detection of extra-solar planets. Such a space telescopes, with a 6-meter objective, would be used to detect movement and the combination of our of phase images to actually show planets in one of the null-out bands. This would be a direct image of a new 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 non-circular, eccentric, orbits. Only the systems orbiting 47 Ursae Majoris and HD 27442, with Jupiter-mass companions in circular orbits, remind us of 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. The Drake Equation is the second most famous equation of the 20^th century, after Einstein’s equation of relativity. However, there is still no data to answer the Drake Equation. The goal of our project is to determine the numbers of stars that have planets and the number of planets capable of sustaining life.

Toward this goal, our group is surveying the 1000 nearest and brightest Sun-like stars in the northern hemisphere using the Lick 3-m (California) and Keck 10-m (Hawaii) Telescopes. In addition, we are surveying the 200 brightest southern hemisphere stars from the 3.9-m Anglo-Australian Telescope. Next year we will begin surveying the remaining nearby southern hemisphere stars from the 6.5-m Magellan Telescope in Chile.

These surveys are the only active program capable of detecting "Solar System-like" planets. By 2010, these surveys will provide a first planetary census of nearby stars, allowing us to estimate the ubiquity of planetary systems and of "Solar System" analogs.

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