When Michel Mayor and Didier Queloz of Switzerland discovered a planet orbiting the star 51 Pegasi in 1995, many skeptics probably shook their heads in disbelief. But when the planet’s existence was confirmed by Geoff Marcy and Paul Butler later that year, there was little room for skepticism. Since then, Marcy and Butler have discovered more than 30 planets orbitng other stars. And there are countless others yet to be discovered. What could be so important to make Jack Lissauer call the discovery of extrasolar planets “One of the great scientific advances of the 1990s”? And how are these discoveries made?
The discovery of extrasolar planets is important for many reasons. The first concerns our knowledge of the universe. Before 1995, we knew that planets orbited remnants of large stars after supernova explosions, called pulsars. As far as we knew, there were no extrasolar planets orbiting stars like we orbit our sun. Knowing of the reality of extrasolar planets changes our understanding and our way of looking at the universe. So far we’ve not yet discovered a terrestrial planet. Just as we once couldn’t detect planets orbiting stars, we now cannot detect smaller sized and possible terrestrial planets with the methods available. But because we’ve yet to detect them doesn’t mean they’re not there. Our ability to discover planets outside our own solar system may lead to the discovery of an earth like planet somewhere else in the universe. The potential benefits to that happening are innumerable. Because other planets may help us better understand our own, discovering planetary companionship to stars is an important step for the field of astronomy. The methods by which to discover planets are just as important as the discoveries themselves.
So far planets orbiting stars have been detected by one of two methods. The textbook Universe calls one the astrometric method, involving a measurement of a star’s position in the sky relative to other stars. A star with a potential orbiting planet will change position in the sky, likely in a cyclic pattern. The limits to this method is that it not only requires strict precision, but also requires the observer to trace a star for a long period of time.
More likely, astronomers use the radial velocity method. When a planet orbits a star, it gravitational pull affects the orbit of the star. It’s important to remember that both the planet and star rotate around a center of mass. If a planet is large enough, its pull produces a change in the star’s spectrum, making it seem to wobble. This change in wavelength is a Doppler shift. As a star approaches the earth in the wobble of its orbit, its light is shifted toward the blue end of the color spectrum. As a star recedes, its light is shifted toward the red. By calculating how much a star shifts, we can reconstruct its orbit. By using Newton’s laws, we can determine the planet’s distance to the star, and from that we can find its minimum mass.
While the radial velocity method has helped Marcy and Butler discover 33 extrasolar planets in a mere 6 years, it also has its limitations. For example, it’s unable to help us determine the exact mass of a planet. In order to calculate a planet’s exact mass, we need to know how a planet’s orbit is inclined to our line of sight, and the Doppler shift is little help in this determination. As a result, some objects we may think are planets are really brown dwarfs.
Like the astrometric method, the radial velocity method requires much precision. Shifts are tiny because star’s orbits are slow. The tiniest inaccuracy by an astronomer could result in wrong calculations of planetary mass, orbital size, and speed. In “Hunting Planets Beyond” Marcy and Butler write that the “discovery of multiple planet systems requires a combination of precision, patience, and luck.” By patience, they mean that using the Doppler technique requires a long period of time with which to detect a planet’s changes in wavelengths. For them, observations must span a time covering at least two orbits. This means they’ve not been able to detect any planets with orbital periods longer than six years. As a result, their methods discriminate to finding larger planets with short orbits, particular planets that are Jupiter’s size or more and with orbital periods of three years or less. These are easiest to detect because their tug on stars is greater, while smaller sized planets with weaker tugs and longer orbits are not as easily detected. Robert Naeye, of Astronomy magazine gives us the following analogy. If an alien who lived in another solar system were using our Doppler method of detecting planets in our solar system, only Jupiter and possibly Saturn could be detected. For all the alien knows, he may believe his is the only life bearing planet in the galaxy, unbeknown to him the planets he cannot detect.
The discovery of planetary systems has not been without its surprises. When the first astronomer saw the first wobbling star, planetary companionship must have entered into his mind as a possible explanation. Close behind came an assumption that if other solar systems exist, they must be similar to our own. What we’ve learned so far is that our solar system, while it may not be alone, is quite unique. Or as Marcy and Butler write, “planetary systems with circular orbits may be the exception rather than the norm.” Our circular orbit keeps us stable, and we may very well “owe our existence to it.” If our orbit had been disturbed by a larger planet, chances are it would be too elliptical to support life.
As recent as earlier this year the science world learned that Marcy and Butler were at it again. Measuring Doppler shifts, they discovered two new planetary systems. One orbits the star HD 168443, which is 123 light years from Earth. This star has a planet seven times as big as Jupiter orbiting closer to the star than Mercury orbits our Sun. HD 168443 has even a bigger planet, this one at least seventeen times as big as Jupiter orbiting its star a bit bigger than Mar’s orbit around the Sun. Currently, whether it’s a star or a brown dwarf is up for interpretation, since some insist it’s too large to be a planet. The other planetary system orbits the red dwarf Gliese 876. One planet, discovered in 1998, is nearly twice the size of Jupiter and has a 61 day orbit. The other, about half the size of Jupiter orbits Gliese 876 in about a month. These two planets tug their star in the same direction every sixty days, which gives it a easy to detect large wobble. Understanding the harmony of these two planets may help us understand the diversity of planet formation.
The future of astronomy may unlock the door to understanding in greater details the mysteries of the cosmos. In their article in Physics Today, back in April of 1993, Anneila I. Sargent and Steven V.W. Beckwith write, “owing to the painfully long orbital periods, it will probably be a decade before the existence and properties of nearby planetary systems can be determined with confidence.” Just two years later, the existence of 51 Pegasi’s planet was discovered, and since then Marcy and Butler’s team have discovered 33 others. The current state of discovering planetary systems is ever changing, expanding our technology in order to detect those planets too small or too far away for us to have already discovered. Marcy and Butler see optical and near-infrared interferometry becoming a reality in the next ten years. They see large telescopes like the Keck Telescope and the European Southern Observatory’s VLT measuring the position of stars 10 to 100 times more precisely than current techniques. This is the sort of precision needed to detect Saturn or Neptune mass planets. Between 2006 and 2008 NASA will launch SIM (Space Interferometry Mission), a mission capable of detecting planets as small as ten Earth masses with orbits of five years or less. Although their images are not yet available to us, someday we’ll be able to know what the planets in other solar systems look like. With these advanced techniques, the future holds much promise for more discoveries, opening a door that has always before been locked. The only difference is that now, we have the key.