EXTRA-SOLAR SYSTEM PLANETS AND THE STUDY OF SOLAR SYSTEM EVOLUTION

By Susan S. Carroll

For centuries, astronomers have speculated on the existence of planets outside our own solar system, citing any number of indirect pieces of evidence that planets are sprinkled all over the Milky Way Galaxy. Numerous newborn stars - too young to have begun making planets - have been observed wreathed in thick disks of dust. But after a decade of searching, astronomers found only one example of a young star whose disc showed the characteristic thinning toward the star that indicates the formation of planets. The disk around the star, Beta Pictoris, was first observed in 1984.

On Tuesday, April 21, 1998, two teams of scientists presented infrared images of a disk of glowing dust encircling the star HR 4796A at a NASA press conference. HR 4796A is a young star about 220 light-years from earth. The inner part of the disk emits considerably less radiation than the outer part, and indication that there is less dust closer to the star. One explanation is that one or more unseen planets have swept the area clear of dust. Although the star is only 10 million years old, as opposed to our sun which is about 4.6 billion years old, scientists believe that Jupiter and Saturn had already formed by the time our sun was that age. One team, headed by Michael W. Werner of NASA’s Jet Propulsion Laboratory, made their discovery using the mid-infrared camera on the 10-meter Keck II telescope atop Hawaii’s Mauna Kea. A separate team, which included Ray Jayawardhana and Lee W. Hartmann of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, used a 4-meter telescope at the Cerro Tololo Inter-American Observatory in La Serena, Chile.

These young potential solar systems offer astronomers intriguing possibilities of observing solar system evolution and refining theories on the way in which our own solar system formed. Planets are thought to form from a disk of dust and gas, and ice, swirling around a young star. As material within the disc collides and begins to clump together, larger chunks are formed. These chunks continue to collide until they form first small, than larger bodies orbiting the parent star. Small grains of dust may evolve into rocky planets, such as Earth, while accumulations of gas and dust further out in the disc may create larger bodies, like Jupiter or Saturn. These large planets are the ones that clear out most of the gas and dust in the cloud swirling around the star and leave a "hole" in the inner part of the disk, which then begins to resemble a donut.

Two other stars, still quite young, but about 10 times older than HR 4796A, also have dusty discs associated with them. Using the James Clerk Maxwell submillimeter radio telescope on Mauna Kea, researchers have found that two of the brightest stars in the sky, Fomalhaut and Vega, have dusty disks that may have begun to spawn planets. The discovery was made the same week as the NASA press conference announcing the disk around HR 4796A. Like the disk surrounding HR 4796A, both Vega and Fomalhaut have disks that appear edge-on, from our perspective on Earth.

Fomalhaut, estimated to be about 200 million years old and 26 light-years from Earth, appears to be surrounded by a disk centered on a space that may have been cleared by planet formation. The brightest radio emissions, and thus, the heaviest concentration of dust and gas, appear to be about as far away from Fomalhaut as the Kuiper belt objects beyond Pluto are from our own sun.

Vega, on the other hand, at 350 million years old, has a most puzzling radio image. The brightest emission, and therefore the heaviest concentration of dust, appears to come from a single blob twice as far from Vega as Pluto’s average distance from our sun. One possible explanation is that the blob represents dust orbiting a massive planet at that location. However, the blob might also represent emissions from a distant galaxy located in the same region of the sky.

When researchers found this "blob", they turned their attention back to Beta Pictoris and, to their surprise, found a similar blob in orbit around it. In addition, there is a much fainter blob directly opposite this one. The two blobs could be related, in that material was thrown out in opposite directions from the center of the disk.

These star systems are wonderful examples of the different stages of the formation of solar systems. As the amount of dust and gas decreases around the star, planets are forming from this material. The less dusty the disk around the star, the more likely the dust has coalesced into larger bodies in orbit around the star. However, direct glimpses of these bodies is not yet possible. Planets, unlike stars, shine with the reflected light of their parent suns. And because of their proximity to their suns, in terms of astronomical distances, the light of the star itself overpowers the planets themselves, making direct observation out of the question, with current instrumentation.

The hunt for planets outside our solar system has intrigued many teams of astronomers for decades. With the development of more sophisticated instrumentation, it is now possible to indirectly observe planets in orbit around suns. In theory, solar systems could form around any star in our galaxy. However, whether or not the solar system remains stable depends on the type of star, and the way the planets formed. One theory holds that many more stars than we think have at one time had planets in orbit around them; but the orbits proved to be unstable and the planets spiraled to their death into their sun. Other stars may have had the pre-requisite disk of gas, dust, and ice, but perhaps because of continual bombardment by meteors, the chunks of material were never able to coalesce into a body large enough to be considered a planet; or, the body may have formed, only to be smashed to pieces by the next meteor impact.

Humans on earth have long struggled with the question of whether or not we are alone in the galaxy, or even the entire universe. A planet capable of supporting life as we know it has to meet a number of criteria; there is only one such planet in our own solar system of nine that meets these criteria. Although the question is not likely to be answered in our lifetimes, there have been some intriguing discoveries made during the last decade that have put us closer to the answer than ever before.

In 1991 radio astronomers Alex Wolszczan and Dale Frail discovered planets orbiting a type of collapsed star known as a pulsar. Although the discovery itself was momentous, it still left open the question of whether planets could exist around sun-like stars, a definite pre-requisite to those interested in the search for extra-terrestrial life.

Then on October 6, 1995, Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland announced the discovery of a planet orbiting the solar-type star 51 Pegasi. This discovery was nothing short of miraculous and changed everything. This planet belongs to a family that stunned the astronomical community; a Jupiter mass planet so close to its host star it takes only an astonishing 4.2 days to complete an orbit. This planet is about half the size of Jupiter, and is only 5 million miles from its sun, 1/8 the distance that Mercury is from our Sun. Calculations by Adam Burrows and colleagues at the University of Arizona showed that this planet was massive enough to hold onto its atmosphere, despite its proximity to its sun.

Just three months later, Geoff Marcy of San Francisco State University and colleague Paul Butler of the University of California/Berkeley announced two planets of their own. One of these was a planet with a mass of at least 2.3 Jupiters orbiting the solar-type star 47 Ursae Majoris. This planet orbits at a distance of 2.1 AU, which would put it on the inner edge of the asteroid belt if it were in our solar system. While it may not be Jupiter’s twin, it could be a cousin. Knowing that he and Marcy still had mountains of data to sift through, and that several other research teams were in the hunt as well, Butler predicted that "Within one year, more extrasolar planets will be known than there are in our solar system." This statement, far from boastful, turned out to be prophetic.

In April of 1996, Butler and March found a second example of a 51 Pegasi type planet orbiting the star Rho 1 Cancri. Within 3 months Marcy and Butler found two more orbiting the stars Tau Bootis and Upsilon Andromedae. This evidence indicates that roughly 5% of solar-type stars have these so-called "hot Jupiters". Detection of an eighth planet was reported in April of 1997, when a nine-member team led by Robert W. Noyes of Harvard University detected a planet orbiting the star Rho Coronae Borealis.

Because no one has been able to image or take spectra of these hot Jupiters - they were discovered indirectly through their gravitational pull on their host stars - nobody knows exactly what these planets are like. Because there shouldn’t be enough rocky material close to a star to form a giant planet, most astronomers think these planets are enormous gas bags similar in composition to Jupiter and Saturn.

Here is how the planet detection system in current use works. Because the star in a solar system is so bright, compared with the reflected light of its planets, detection of planets around any given star is extremely difficult. For instance, our own sun outshines its planets by approximately one billion times in the visible light spectrum. This makes direct visible observation nearly impossible with current instrumentation. So an indirect method of observation is used, called the Doppler planet-detection technique. This involves looking at "wobbles" in the star’s motion, caused by gravitational tugs by the planets themselves. The orbiting planet exerts a gravitational force on its sun, tugging the star around in a circular or oval path - a miniature of the planet’s own orbit. Although the wobbling motion of the star is very small from a great distance, the Doppler shift of the starlight itself can be measured. As the star wobbles back and forth relative to Earth, it’s light waves become stretched, then contracted, in a cyclical pattern, shifting back and forth between the red and blue ends of the spectrum. From this pattern, the Doppler shift, astronomers can retrace the path of the star’s wobble and compute the mass, orbit and distance from the host star of the planet.

According to prevailing theory, a planetary system forms from a disk of gas and dust that surrounds a newborn star. A giant planet forms from a core of mostly ice, with some rock thrown in. Once this core attains the mass of several Earths, its gravity snatches up huge quantities of gas from the surrounding protoplanetary disc. However, the young sun’s intense heat and wind prevents ice from condensing in the inner disk. In addition, the heat and wind drive the gas out of the inner disk, preventing gaseous planets from forming there. Gas giants, according to current theory, can only form in the frigid outer portion of the protoplanetary disk, at a distance of at least 5 times the distance between Earth and our Sun. This theory is now open to question, with the discover of these so-called hot Jupiters.

Most planets, both inside and outside our solar system, including these hot Jupiters, have orbits that are near-perfect circles. But three of the newly discovered planets have highly elliptical orbits, and thus belong to the family of "eccentric planets."

The first of these was discovered in 1989 by astronomer David Latham’s group of the Harvard-Smithsonian Center for Astrophysics. The object has a mass of at least 9 Jupiters and orbits the star HD 114762. The planet’s eccentric orbit brings it as close as .22 AU to the star and as far as a Mercury-like .46 AU. However, because this objects is much more massive than Jupiter and has such an eccentric orbit, astronomers are reluctant to classify it as a planet. This is why Latham was not credited with discovering the first planet orbiting another star.

But if this object is not a planet, then what is it? For the time being, astronomers are content to call it a brown dwarf. Brown dwarfs are similar in composition to gas giant planets like Jupiter, but instead of forming from a disk like planets, they form from collapsing gas clouds, like stars. Brown dwarfs don’t shine like stars because they lack the 80 Jupiters’ worth of mass necessary to ignite nuclear reactions in their cores. So in a sense, brown dwarfs are transitional objects that bridge the gap in mass between stars and planets.

For 8 years, the HD 114762 object was the only example of its type known in science. But in January 1996, Marcy and Butler found a near-twin, an object with a minimum mass of 6.5 Jupiters, orbiting the star 70 Virginis. This object’s orbit takes it from .27 AU to .59 AU.

Determining whether these objects are planets or brown dwarfs will be difficult. The mass alone isn’t the deciding factor, because there is no theoretical reason why the heaviest planets couldn’t be more massive than the lightest brown dwarfs.. Even a spectrum, which would reveal the chemical composition of their outer atmospheres, probably wouldn’t be enough because both brown dwarfs and gas giant planets are composed primarily of hydrogen and helium.

The critical difference lies deep down in the core. Because a brown dwarf forms from a collapsing gas cloud, like a star, it maintains a gaseous composition all the way to the very center. A planet, on the other hand, forms from accreting material in a disk, and will have a core made of ice and rock. But because we’re looking at these objects from enormous distances, it will be virtually impossible to determine their interior structures, which may be the key that will unlock the mystery of how they formed.

Some astronomers argue that eccentricity can be the deciding factor. Mayor and Queloz have recently discovered 10 brown dwarfs orbiting solar-type stars and most of them have highly eccentric orbits. Stars in binary systems usually orbit each other in highly eccentric orbits, too. So the high eccentricity of the HD 114762 and 70 Virginis objects might be telling us that they formed in the way stars do, making them brown dwarfs.

Marcy and Butler counter that the least massive of Mayor and Queloz’ 10 new brown dwarfs has a minimum mass of 17 Jupiters, significantly heavier than the HD 114762 and 70 Virginis companions. They contend that these latter two objects are indeed planets, or that they belong to a separate population.

A decade before the hot Jupiters were discovered, planet formation models predicted that under certain conditions, gas giant planets would experience drag forces with their disks that would cause them to spiral inward from their birthplace. But these models have a sad ending; the planets would spiral all the way into the star, thus destroying themselves as separate entities and becoming absorbed into the star’s mass.

The third of these eccentric planets is quite different from the other two. This planet was independently discovered by both Marcy and Butler’s group, and William D. Cochran and Artie P. Hatzes of the University of Texas McDonald Observatory on Mt. Locke in western Texas. Its minimum mass of 1.5 Jupiters is much lower, and it has by far and away the most eccentric orbit of any planet discovered to date. This planet’s orbit comes as close as .6 AU from its sun, 16 Cygni B, and as far away as 2.8 AU, roughly the distance of the asteroid belt in our own solar system. This planet would have extreme seasonal variations. But here, seasonal variation is driven by the differences in the distances from its sun, rather than the way Earth’s seasons are driven, by the tilt of its axis. In addition, the sun this planet orbits is part of a triple-star system; 16 Cygni B and 16 Cygni A (which is slightly larger), leisurely orbit each other in a cigar shaped ellipse that takes at least 125,000 years. The third member of the system, 16 Cygni C, is a red dwarf star roughly half the size of the sun that orbits the main pair at a distance of at least 100,000 AU. No one knows how this planet ended up with such an eccentric orbit, but perhaps the simplest explanation is that the planet formed in a circular orbit, but each time 16 Cygni A comes in for a close approach, its gravity tugs on the planet, gradually yanking the orbit into its current elongated shape.

Perhaps the most intriguing planets of all are the two Jupiter-like planets George Gatewood of the University of Pittsburgh found orbiting Lalande 21185, a red dwarf star only 8.25 light years away - five times closer than the next closest known extrasolar system planet. Although each of the dozen or so planets found to date can be considered somewhat uncertain because all of them have been detected through indirect means, Lalande 21185 is by far the most "iffy". Gatewood uses a different technique than the other planet hunters, one that doesn’t immediately determine a mass and an orbit. He will need to observe the star for several more years before he can pin down specific masses and orbits. All Gatewood can say for now is that Lalande 21185 appears to be orbited by at least two Jupiter-mass planets at distances of about 2.2 AU (again, the inner asteroid belt) and 11 AU (a little beyond Saturn) respectively. There are also hints that a third planet orbits the star much further out.

But before theorists go overboard, one must consider that the searches are most sensitive to massive planets orbiting close to their host stars, which is exactly what astronomers are finding. The planets found, say scientists, are the easiest ones to find, and therefore may not be representative of what is really out there. To understand the whole story of planets and planet formation, astronomers need to identify entire systems, like the one that might be orbiting Lalande 21185.

Marcy and Butler have compelling evidence for second planets orbiting Rho 1 Cancri and Upsilon Andromedae, demonstrating that astronomers are rapidly making progress in this area. With instruments and telescopes improving rapidly and more teams looking at stars, no doubt many more planet-like objects will be found orbiting distant suns. Marcy and Butler have begun a second Doppler survey of 400 stars, using the 10-meter Keck telescope in Hawaii. Mayor and Queloz recently tripled the size of their Northern Hemisphere Doppler survey to about 400 stars, and soon will begin a Southern Hemisphere survey of 500 more stars. Within the next year, Doppler surveys of several hundred additional stars will begin at the nine-meter Hobby-Eberly Telescope at McDonald Observatory.

By the year 2000 both the 10-meter Keck telescopes and a binocular telescope at the University of Arizona will become optical interferometers, precise enough to image extrasolar system planets. NASA has plans to launch at least three spaceborn telescopes to detect planets in infrared light. One of these proposed telescopes will be a space-based interferometer, the Terrestrial Planet Finder, which should be able to obtain pictures of potential candidates for habitable planets orbiting distant suns, starting in about the year 2010.

Interferometry is perhaps the most intriguing, and potentially the most rewarding technique for the location of distant planets. Here’s how it works: In 1974 Ronald Bracewell of Stanford University showed how two small telescopes could together search for large, cool planets similar to Jupiter. Bracewell’s proposed instrument consisted of two one-meter telescopes placed 20 meters apart. With both telescopes pointed at the same star, Bracewell showed that he could invert the light waves from one telescope (turning the peaks of the light waves into valleys, or troughs), and then merge that inverted light with the light from the second telescope. If the images overlapped precisely, the star’s light would be "cancelled out", thus eliminating the glare that hides planets. The planet’s spectrum, coming from a slightly different direction, could thus be detected. This instrument is called an interferometer because it employs the interference of light waves to discern details about a light source.

Bracewell’s envisioned telescope would have the sensitivity to spot Jupiter-sized planets; detection of Earth-sized planets would require far more sensitivity. There is another problem as well. Although many ground based instruments have the sensitivity to detect strong infrared radiation emanating from stars, the telescope’s own heat, plus atmospheric conditions, would make detection of a planet almost impossible. Another problem to overcome arises from background heat radiated from our solar system’s cloud of dust particles, revealed in the so-called "zodiacal glow". Bracewell realized that this glow would nearly overwhelm the signal of a giant planet, let alone that of an Earth-sized one. The obvious solution was to place the telescope in space.

Of course, simply finding an Earth-sized planet in orbit around a star is not enough; the driving impetus is to find such a planet that has the requirements for supporting life. Planets similar to Earth in size and distance from their sun offer the best hope for carbon-based life because of the likelihood of their having liquid bodies of water. Water is the solvent for life’s biochemical reactions. However, the temperature of the planet, and/or its distance from its sun means little if the planet does not have enough gravitational pull to hold on to oceans and an atmosphere.

In 1986, Roger Angel, Neville Woolf, and Andrew Cheng, of the University of Arizona, proposed that mid-infrared wavelengths would be the best place, spectroscopically speaking, to look for the signatures characteristic of life on other planets. This type of radiation has wavelengths 10 - 20 times longer than that of visible light. Furthermore, at these wavelengths, a planet emits about 40% more photons, or light particles, than it does in the visible spectrum, making the planet’s radiation much easier to separate from that of its sun. Although there are many criteria for extraterrestrial life, there are three molecules that must be present if life as we know it will exist. These characteristic molecules are ozone, which is an indication of oxygen lower in the atmosphere, water, and carbon dioxide.

Using our own solar system as a test, a survey of the infrared radiation of each planet was analyzed. Of the nine planets in our solar system, only Earth showed the infrared signature of life. Although Mars and Venus both have atmospheres of carbon dioxide, only Earth showed the signatures of ozone and plentiful water. Ozone itself is a sensitive indication of atmospheric oxygen; it would have appeared on Earth approximately a billion years before oxygen itself would have revealed its infrared spectral signature.

In 1990, Roger Angel showed that the level of precision to locate smaller planets was possible if more than two telescopes were involved. Alain Leger and his research team at the University of Paris showed that if the instrument were placed into orbit at the approximate distance that Jupiter is from the sun, the background thermal radiation from dust within the solar system would be practically negligible because it would be so cold. He also demonstrated that an orbiting interferometer that this distance with telescopes as small as one meter in diameter would be sensitive enough to detect an Earth-sized planet.

In 1995 NASA selected three teams to investigate methods for discovering extrasolar system planets. At the University of Arizona an international team, including Bracewell, Angel, and Leger, was assembled to study the problem. Angel and Woolf hit upon the idea of an interferometer with two pairs of mirrors, all arranged in a straight line. They surmised that since an interferometer of this arrangement could cancel starlight very effectively, it could span roughly 75 meters, a length that they say offers important advantages. It would permit astronomers to reconstruct actual images of planets orbiting a star, as well as to observe stars over a wide range of distances without expanding or contracting the array. Angel and Woolf postulated that this instrument could even identify earth-like planets that would otherwise be elusive, analyzing such planets for carbon dioxide, water and ozone, and perhaps even methane, another indicator of life.

The discovery of another Earth-like planet that perhaps harbors life may arguably be space exploration’s crowning achievement. But even detection of such a planet does not mean that the "life" on that planet will be able to communicate with us; early forms of life on Earth, such as blue-green algae, were not in the habit of sending out radio signals announcing their presence. It is far more likely than not that if such an Earth-like planet is found, the life on it will not be at our stage of development.

And yet, even though the odds of finding life as we know it are slim, the search continues. For who among us has not gazed skyward, and wondered about the possibility of extra-terrestrial life? The search for such life is, after all, one of humankind’s most compelling aspirations.

Extra-Solar System Planets Currently Known

* minimum mass

Selected Bibliography

Marcy, Geoffrey W. and R. Paul Butler. "Giant Planets Orbiting Faraway Stars". Scientific American: Magnificent Cosmos: 9(1):10-15. Spring 1998.

Angel, Roger and Neville J. Woolf. "Searching for Life in Other Solar Systems". Scientific American: Magnificent Cosmos: 9(1):22-25. Spring 1998.

Cowen, R. "Dust Disks Hint at Baby Solar Systems". Science News, 153(7):260. April 25, 1998.

Naeye, Robert. "The Strange New Planetary Zoo". Astronomy 25(4):42-49. April 1997.

Naeye, Robert. "2 New Solar Systems". Astronomy 24(4):50-55. April 1996.

MacRobert, Alan M. and Joshua Roth. "The Planet of 51 Pegasi". Sky & Telescope 91(1):38-40. January 1996.

Copyright 1998 by Pulcherrima Productions. No part of this article may be reproduced without express written consent from either Pulcherrima Productions or the author.

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