As we look up at the rich fields of stars on a clear night sky, the age-old question seems to ask itself: Are we alone?
For most of humanity's sojourn on Earth we have been unable to answer this question. Science fiction writers have spent decades joyfully speculating on the subject, but it is only in the past ten years that we have taken a significant step toward the answer. There is already some evidence that some planets and moons of the solar system may harbor conditions that could support life. But what about worlds outside our cosmic backyard?
Up until the early 1990s, the question of extrasolar planets was still a speculative one. Last month, however, the Anglo-Australian Planet Search Team reported the discovery of the 102nd extrasolar planet. It is a gas giant about the size of Jupiter, in a nearly circular orbit around a sun-like star in the constellation of the Crane, in the Southern hemisphere. In the last decade nearly 90 solar systems have been found, not counting our own. Of these, 11 are multiple-planet systems.
Considering how long humankind has been star-gazing, why has it taken until now to find planets orbiting other suns?
Finding an extrasolar planet is a staggeringly difficult task. A planet does not have any innate luminosity, and if it is as small as our Earth, it will be about a billion times fainter (in visible wavelengths) than a sun-like star. Against its star's glare it is practically invisible. Even today no telescope either on Earth or in space can directly observe an extrasolar planet.
So how have astronomers and astrophysicists achieved the impossible?
Looking for a Needle in the Cosmic Haystack
Planets betray their presence in subtle ways.
Most stars are so distant from Earth that they appear fixed in our night sky (their apparent movement across the sky is only due to the Earth's motion). It takes hundreds or thousands of years for the stars' relative positions to change. If a star appears to move slightly against this relatively unchanging canvas, there is a chance that this motion might be caused by an unseen planet.
This slight movement, or wobble, is the effect of the small gravitational pull of the planet. The to-and-fro movement of the star appears as a Doppler shift in the star's spectrum that alternates between the blue and red ends. (The Doppler effect is the change in frequency of a wave due to the motion of its source relative to us -- in the case of sound waves, it is familiar to any one of us who has heard the whistle of an approaching train drop in pitch as it moves away from us.) The wobble can also be detected via astrometry -- careful measurements of the star's position over time. Until very recently astronomers were handicapped by the fact that the Earth's atmosphere prevents such precise measurements of tiny motions -- it is like trying to see through a shifting veil. Two recent developments now enable us to see the night sky with startling clarity. One is, of course, the Hubble Space Telescope, and the other is the advent of adaptive optics, an ingenious way of compensating for the effects of the atmosphere on ground-based telescopes by using a special mirror that flexes a hundred times a second.
But because the star's wobble is so small, astronomers expect the Doppler method to work best for very massive Jupiter-like planets that are relatively close to the parent star. Similarly, the transit method -- the occultation of the star by a planet, resulting in a change in brightness of the star as the planet passes briefly between it and us -- can best detect large planets. Earth-sized planets, therefore, are still beyond our detection capabilities, except for one very bizarre class of star called a pulsar. A pulsar is the remnant of a supernova explosion. It is a star only about 20 km across and is typically about one and a half times the mass of our sun. Matter is so compressed in such a star that a pin-head-sized speck of material would weigh as much as a battleship. The magnetic field of a pulsar is about a million million times that of Earth. Pulsars also rotate rapidly. As a result, they can emit radio waves, X-rays or gamma-rays, which sweep out towards Earth rather like a lighthouse beam, at extremely precise intervals. A disturbance in the timing of the radiation from the pulsar can indicate a planetary companion.
But the extreme conditions on a pulsar's planets make it unlikely that life as we know it could exist there. What prospects are there for detecting Earth-sized planets of less violent stars? One method that has great promise for detecting Earth-sized planets is called gravitational microlensing. According to Einstein's theory of General Relativity, gravitation bends light rather as an optical lens does. If we have two stars roughly in line with each other as seen from Earth, the closer star can act as a gravitational lens, causing multiple images of the farther star. When the lensing object is much lighter, like a planet, it will not bend light enough to form two or more separate images. Instead it will cause a momentary increase in brightness of the distant star's image.
Other methods being developed include coronography and nulling interferometry, both designed to subtract the light of the parent star so that any planets may become visible. The first attempts to block out the star's glare used a physical mask called the coronograph in which all but the star's outer layer, the corona, is concealed. The second relies on the fact that light is a wave, and that two beams of light that are out of phase (the crests of one aligned with the troughs of the other) will interfere destructively, effectively canceling each other out. Once divested of the star's overwhelming brightness, any planetary companion should leap into view.
Another method of looking for extra-solar planets is to look at star nurseries. Stars typically form when a cosmic dust cloud begins to condense due to gravitational forces. If conditions are right, the star will ignite at the center, while planetary material will begin to condense into planets further from the star. Once the star has shed its embryonic veil of gas and dust, the still-glowing proto-planets may be clearly visible.
A Brief Overview of Planet-hunting
The dream of other worlds is a compelling one indeed. Way back in 1916, astronomer E. E. Barnard discovered a star about 6 light-years away that had a very large proper motion (the motion against the backdrop of fixed stars). This led to speculation as to whether Barnard's star had planetary companions. Astronomer Peter van de Kamp spent most of his life studying this star, looking at over 2000 photographic plates to see if there was a wobble in the star's motion. After 40 years of devoted study he concluded that Barnard's star did indeed harbor a Jupiter-like planet. However, subsequent studies, including observations through the Hubble Space telescope, have failed to confirm van de Kamp's findings. If Barnard's star does indeed have planets, they are likely to be small ones.
The first confirmed discovery of extra-solar planets occurred in 1992. Astronomers Wolszczan and Frail, studying timing signals from a pulsar named PSR 1257+12 in the constellation Virgo, deduced the existence of two planets. Subsequent observations raised the number of planets to four, at least two of which are about 3 times the mass of the Earth, and one is far smaller. These planets are bombarded by intense radiation and as such are thought to be unlikely candidates for life-bearing worlds.
Impression of One of Three Planets Orbiting PSR 1257+12
Copyright © Lynette R. Cook, All Rights Reserved, Used with Permission.
Then, in 1995, the year Peter van de Kamp died, Mayor and Queloz of Geneva Observatory detected a wobble in the star 51 Pegasi. It is now believed that a Jupiter-like planet circles this star at a distance about one-eighth that of Mercury from our sun. The surface temperature of the planet is estimated to be a blistering 1300 degrees Celsius (2372 degrees Fahrenheit). This was the first of the so-called Hot Jupiters to be discovered -- a gas giant close enough to its sun to be scorched!
After 1995 there was a flurry of new discoveries, some of which had to be retracted after mistakes in the data and their interpretation were found. Most of the 100-odd real candidates have been found via the wobble method -- these are massive giants about 1 to 13 Jupiters in mass. A multi-university team led by astronomers Geoffrey Marcy and R. Paul Butler has discovered the largest numbers of planets.
The planetary status of some of these giants is still in question. It is possible that some of these alleged planets -- particularly the heavier ones -- are actually brown dwarfs, which are essentially failed stars. Brown dwarfs are typically about 10-80 times the mass of Jupiter but shine only briefly by burning deuterium. So far the largest number of extrasolar planets has been found using the Doppler method, which has a shortcoming in that it relies only on one component of the star's wobble -- the component along our viewing axis. If the orbital plane of the planetary system is aligned such that the wobble has a significant component along another direction, we may be seriously underestimating the wobble, and hence the mass of the planet. Researchers such as David C. Black of the Lunar and Planetary Institute in Houston charged in October 2001 that many so-called extrosolar planets may in fact be dim stars. Others point out that were this the case, many more planets with a minimum mass close to a brown dwarf's mass would have been found -- which is not so. Only more accurate measurements with different methods can clearly resolve the issue.
The next significant discovery in the planetary pageant was that of a multi-planet system, the first one found orbiting a normal star (as opposed to a pulsar). In 1999, Marcy and Butler's team found three planets circulating a sun-like star called Upsilon Andromedae about 44 light-years away from us. The planet masses are 0.71, 2.11 and 4.61 times that of Jupiter, and the farthest is about 2.5 AU (1 AU is the distance between the Earth and our sun). This was an important discovery as it allowed us to compare our own solar system with another one for the first time.
Impression of the Upsilon Andromedae System
Copyright © Lynette R. Cook, All Rights Reserved, Used with Permission.
After the first rush of discoveries of Hot Jupiters, astronomers have begun to discover planets at more "normal" distances from the parent star. An example is the outermost planet orbiting the visible sun-like star 47 Ursae Majoris in the Big Dipper, some 51 light-years away from us. The planet lies 3.73 A.U. from the star, comparable to Jupiter at 5 A.U. from our sun. Another planetary system around the star 55 Cancri, 41 light-years away, contains a planet between 3 and 5 times the mass of Jupiter which completes its orbit in 13 years (Jupiter completes its orbit in 12 years).
Impression of Planet Orbiting 55 Cancri
Copyright © Lynette R. Cook, All Rights Reserved, Used with Permission.
Recently more and more planets with circular orbits are being discovered. The 102nd planet, for instance, describes a nearly circular orbit around its sun. Our solar system, too, has nearly circular orbits. Before the discovery of extra-solar planets, we had no way of knowing if this was the norm or whether we were some kind of fortunate exception. So far, most of the planets found have highly eccentric orbits (that is, orbits far from being circular -- oval or elliptical). This also makes it less likely that there is life (as we know it) on these worlds. A circular orbit means that the planet has a more or less stable temperature, a condition conducive to life. In multi-planet systems, highly eccentric orbits might mean that some planets get too close to each other, which can cause disturbances ranging from massive tidal forces and disruption of the planet's orbit to destruction of the planet itself.
Presently there is not enough data to speculate as to the relative abundance of planetary systems with circular orbits. Still, the more such planets discovered, the greater the chances that we will find life outside our own solar system.
In November 2001, another important milestone was reached. A Jupiter-sized planet orbiting a yellow, sun-like star called HD 209458 lying 150 light-years away in the constellation Pegasus, passed in front of its sun. As the star's light was filtered through the atmosphere of the planet, astronomers David Charbonneau and Timothy Brown and their team were able to discern from spectrographic analysis of the light that the atmosphere contained sodium. The planet is mainly gaseous; at a mere 4 million miles from the star its atmosphere is 1093 degrees Celsius (2000 degrees Fahrenheit)!
While we do not expect such a planet to harbor life, it is encouraging to speculate that the spectroscopic analysis of such transit events will, in time, reveal the chemical signatures of life-bearing planets.
One possible candidate for an extrasolar planet actually appears as an image captured on a photographic plate -- a picture of a newly formed star and its possible planetary companion. The still-aglow companion may in fact be a distant celestial object, admits Ray Jayawardhana, the young Sri Lankan astronomer who discovered it in 1999 -- but time will tell. If the object appears to tag the star, it is very likely to be a planet about twice the mass of Jupiter, orbiting the star at a distance about thrice that between Pluto and our sun. The star is part of a small nursery of a few dozen stars called TW Hydrae. There are 9 other star nurseries near us that are being studied.
An interesting, if bizarre, addition to the lexicon of extrasolar planetology is the so-called free-floating planet. In 2000, astronomers led by Philip Lucas and Patrick Roche found twenty drifting bodies in the Orion nebula. By looking at the spectra of these objects they found hints of the presence of water vapor, which indicates that these bodies are rather light. Whether they are planets is a subject of controversy and also a matter of definition. Detractors believe that they are simply lightweight objects formed by the fragmentation of gas clouds. Meanwhile, in 2001, a team led by Kailash Sahu of the Space Telescope Science Institute detected, through the Hubble Space Telescope, six freely floating objects in the star cluster M22. These were observed through gravitational microlensing. These objects are estimated to be only about 80 times the mass of our Earth, and if confirmed, may be among the smallest objects found outside the solar system.
Each discovery of an extrasolar planet has expanded our knowledge of our universe, and at the same time has left us with deeper mysteries. For instance, standard theories of planetary formation hold that gas giants like Jupiter typically form far from the star, as is the case in our solar system. However, many extra-solar planets appear to be uncomfortably close to their sun, less than 1 A.U.! How can this be possible? Could they have been knocked closer to the sun because of gravitational interaction with another planet? And what about the many planets with highly eccentric orbits? Then there is the question of how some planets could have ended up circling pulsars. Theorists are busy refining and reconstructing their ideas.
To answer these and other questions, astronomers and institutions from around the world have embarked on several ambitious projects. NASA's Kepler mission is named after Johannes Kepler, the 16th century astronomer whose Laws of Planetary motion are a standard part of the college physics curriculum. Scheduled for launch in 2006, it is essentially a photometer, designed to measure the brightness of some 100,000 stars in the neighborhood of our solar system. It will detect extrasolar planets using the transit method and is expected to be sensitive enough to detect the tiny changes in brightness of the parent star produced by Earth-like planets. Kepler will be particularly interested in terrestrial planets in the habitable zone, which is the region around the parent star where water can exist as a liquid. The time of transit will reveal the planet's orbit and the extent of dimming will give us an idea of its size. Another NASA mission in the planning stage is the Terrestrial Planet Finder, which will use either coronography or nulling interferometry to find Earth-like planets. Meanwhile the European Space Agency and the European Southern Observatory are designing, through their ground-based Genie project, the largest ever test of nulling interferometry. In the next decade their ambitious Darwin project will use space telescopes to perform nulling interferometry.
It is clear that our old view of the universe, obtained through the shifting veil of our planet's atmosphere, must now give way to new insights. Although we are still far from answering the question about life on other worlds, chances are that Earth and our solar system are not as unique as we once thought. Perhaps it will be no more than a few decades before we know for certain.
Vandana Singh has a Ph.D. in theoretical particle physics and presently devotes her time to writing science fact, science fiction, and fantasy. Her first short story recently appeared in the critically acclaimed anthology Polyphony. For more about her, see her Web site.
An earlier version of this article appeared in the webzine of the South Asian Women's Forum.
This article is based to a large extent on the following excellent reports and websites and the references therein. I claim full responsibility, however, for any errors or omissions.
The graphics on this page are paintings and not real pictures of extrasolar planets. For a breathtaking array of such paintings by master artist Lynette Cook, who has produced imaginative perspectives of the planets discovered by several groups, including the Marcy and Butler team, visit her Web site.
Reports by Ron Cowen in the weekly magazine Science News, October 21, 1995, October 28, 2000, May 19, 2001, May 26, 2001, August 18, 2001, November 10, 2001, January 26, 2002, June 15, 2002. Some of these can be found on their Web site.
The site of Marcy and Butler's team.
A very thorough catalog and historical report from Arizona State University.
The site of the Observatoire de Paris, containing the most up-to-date information and a catalog of extra-solar planets.
Pulsar pages of the Royal Observatory at Greenwich.