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Prowling for Planets  

Mercury, July/August 2000 Table of Contents

Even though we had good reason to believe planets orbit other stars like the Sun, just five years did we uncover the first evidence to support this belief. Now the discovery of such "extrasolar" worlds threatens to become routine.

Debra Fischer

After decades of false starts, the search for extrasolar planets is on firm footing and charging full steam ahead. To those of us in the business of finding planets, it is hard to believe that just five years ago, Michel Mayor and Didier Queloz discovered the first extrasolar planet. The heated debate over the reality of the existence of that planet and the subsequent struggle to interpret such a strange world still echoes in our daily reflections, even as we begin to publish our new planet detections in bunches of six. The novelty of finding an extrasolar planet may have worn off. But now, the real science begins!

Why did it take so long to find planets?

Planets don't emit their own light. Only stars with their tremendous gravitational pressure have the power to ignite the thermonuclear reactions that give off energy as light. In principle, planets could be seen by reflected starlight. However, even a high-albedo, Jupiter-like planet is down by a factor of more than a million in brightness relative to its host star. With our largest telescopes and best detectors, planets are absolutely invisible next to the bright spotlights of the stars they orbit. Invisible, but not undetectable.

Long ago, astronomers reasoned that the presence of extrasolar planets could be inferred by the gravitational tug they exert on the star: as a planet orbits a star, it drags the star around a common center of gravity. Even though the planets are invisible, astronomers knew there should be a smoking gun. The star should wobble with a telltale motion—a signature of the unseen planet(s).

In the first attempts to look for these wobbles, astronomers measured the positions of the stars with painstaking care, using the most distant stars as their unmoving reference points (a technique called astrometry). Confident that if they worked hard enough they could beat this problem down with lots of data, these first planet hunters spent fifty years on their quest, but ultimately failed. Given the technology of the time (photographic plates, primitive computers, and the inability to correct for atmospheric blurring), this technique for measuring tiny wobbles in the position of the stars was just too hard.

At the same time, another technique was being developed to study stars: spectroscopy. Instead of measuring the changing positions of stars, spectroscopy allows us to measure the speed of a star. Starlight is sent through a spectrograph where it spreads out like a rainbow. The absorption lines in the rainbow, or spectrum, of the star, tell us what the star is made out of: typically, lots of hydrogen and helium sprinkled with other elements from the periodic table. And the shift of these absorption lines, due to the Doppler effect, tells us how fast the star is moving along our line of sight (i.e., the radial velocity).

All stars exhibit some Doppler shift because all stars are moving in the gravitational field of our Galaxy. But the stars that move toward us, then away from us, then toward us again—wobbling with periodic motion—are gravitationally bound to another object. The magnitude of this changing velocity, together with the period of the motion, reveals the mass of an invisible companion to a visible star. Our Sun moves with a speed of about 12 meters/sec due to the most massive planet in our Solar System, Jupiter. The periodicity of the Sun's motion is the same as the orbital period of Jupiter: one cycle takes about twelve years.

In the early 1980s, astronomers were measuring velocities with a precision of about 1 km/sec. Then, high-resolution spectrometers, CCD detectors, faster computers, and astronomers with the single-minded goal of pushing this new technology as far as it could go, ushered in a new era in high-precision Doppler measurements. The barometer of velocity precision began to plummet: 500 meters/sec... 300 meters/sec... 100 meters/sec... 50 meters/sec... down to a mere 20 meters/sec in 1995 when the first extrasolar planet was discovered. It is no accident that the planet around 51 Pegasi (with a velocity amplitude of 50 meters/sec) was discovered exactly when the Doppler technique was finally cold enough and quiet enough to permit the measurement of a minuscule periodic shift of the star's absorption lines.

For Five Years

That first extrasolar planet was a surprise in every way. In an orbit twenty times closer to its star than Earth is to the Sun, this planet zipped around the star 51 Peg in just five days, and weighed in with about half the mass of Jupiter. There was a brief moment of stunned silence in the astronomical community. Then, the questions started flying. The temperature would be sizzling-hot so close to a star.

  • If this were a gas-giant planet like our Jupiter, wouldn't the atmosphere boil away?
  • How could such a planet form so close to a star?
  • If it didn't form there, how did it migrate into its present position?
  • If it migrated inward, how did the migration stop to park the planet in its present orbit?

Mass Distribution of Known Extrasolar Planets

The answers came more slowly. The gravity of a massive planet would be sufficient to hold on to its atmosphere, even in such a hot environment. The planet probably didn't form in its present position; more likely, it migrated inward when its primordial orbit was destabilized by gravitational interactions with other planetesimals or with the material in the circumstellar disk. The theorists, who had been constructing planet formation models, based on our Solar System, struggled to interpret this new piece of evidence. If we learned anything from 51 Peg, it was to expect the unexpected.

By the beginning of 1999, the velocity precision had dropped to a few meters per second and almost twenty extrasolar planets had been discovered with the Doppler technique. All of these planets had Jupiter-like masses and all were relatively close to their host stars, presumably having migrated in from a more distant origin. Just when discoveries started to seem ho-hum in the planet-hunting business, there were three big breakthroughs: the discovery of a system with three planets, an observation of a planet transit, and the detection of two, Saturn-mass planets.

Breakthrough: A Multiple-Planet System

A planet, similar to that discovered orbiting 51 Peg, had already been found around the star Upsilon Andromedae back in 1996. At that time, planet-hunters Geoff Marcy (University of California, Berkeley) and Paul Butler (Carnegie Institution of Washington), who reported this discovery, realized that there was more to this system than just one planet. In addition to a four-day period derived from the star's radial velocity data, they observed a longer-period trend that suggested the presence of an additional planet. From Kepler's laws of planetary motion, the farther a planet is from its host star, the longer the planet's orbital period. So, a waiting game ensued as everyone watched to see when the putative second planet would complete one, full, orbital period.

By March 1999, the trend in the velocities for Upsilon And showed that the second planet had executed more than one, full, 3.5-year orbit. However, when the mathematical model was constructed to describe these two planets, a third planet's orbital signature was discovered in the data. Snuggled between the known inner planet and the anticipated outer planet was an unexpected interloper—a planet with an orbital period of 242 days.

The discovery of a triple-planet system was an amazing technical feat, but the truly stunning breakthrough came when theorists demonstrated that this system was both dynamically stable and dynamically full. If another planet were magically dropped into this system, the orbits of the detected planets would become chaotic and at least one planet would be lost—scattered by the gravitational tugs of the other planets. Indeed, this is the same situation in the Solar System. Again, the questions came. Do planets form as single, precious jewels in a protoplanetary disk, or are they born with many siblings? How does nature create dynamically full but stable planetary systems?

The answers are still being worked out on computers throughout the world, but one hypothesis is that enormous numbers of planetesimals form in the protoplanetary disk. As the fledgling planets accrete disk material and grow, a gravitational tug-of-war develops. In the end, only the planets in gravitationally stable niches survive. If correct, this suggests that stars commonly form with several planets. Astronomers are now engaged in another waiting game, watching a few more stars with planets in known short-period orbits that, like Upsilon And, appear to have one or more additional planets in longer-period orbits.

A planetary transit
A planetary transit. Here is an artist's conception of an extrasolar planet passing in front of its parent star. Such a transit of the star HD 209458 was observed by two research groups last year. Illustration courtesy of NASA.

Breakthrough: A Planet Observed in Transit

Insatiable curiosity is the nature of the scientist. Even with the discovery of so many extrasolar planets, astronomers yearned for more information. The Doppler technique only provides the mass of the planet. In fact, because the inclination of a planet's orbit about a star is hardly ever known well, only a lower limit for the planet's mass may be determined. In a statistical sense, the true mass is expected to be usually within a factor of two of the Doppler-determined mass. But, it was unclear whether these extrasolar planets were megalithic Earths or gas giants like Jupiter. The gas-giant scenario was generally favored because astronomers reasoned that as soon as a rocky core formed in a protoplanetary disk, it would gravitationally sweep up an enormous gaseous atmosphere.

It was only a matter of time until the next breakthrough settled this question. Astronomers had anticipated that as the number of extrasolar planets increased, one would eventually be found in an edge-on orbit. As a planet in an edge-on orbit passes in front of the host star, it dims the starlight by an amount that is proportional to the cross-sectional area of the planet. With this measured diminution, it is then straightforward to estimate the density of the planet and determine whether the planet is primarily gaseous or rocky. The Doppler technique predicted exactly when the planet would pass in front of the star but could not determine whether the orbit was edge-on.

Astronomers made careful measurements of the brightnesses of the stars with very close planets (where the probability of catching a transit was highest) before, during, and after the predicted transit times. In the autumn of 1999, two groups independently observed a 2% dimming in the brightness of the star HD 209458 at precisely the predicted transit time. Their observations determined an absolute mass for the planet and verified that this is, indeed, a gas-giant planet, silencing the few remaining skeptics who had argued that the extrasolar planets might be brown dwarfs or even low-mass stars with extreme orbital inclinations.

Planet Orbiting Star HD 46375
Finding a Saturn-mass world. From the Doppler-shifted absorption lines in the spectrum of star HD 46375, astronomers were able to calculate the star's radial velocity as a function of time...

Breakthrough: Saturn-mass Planets

The biggest planets are the easiest to find because they exert a strong tug on the host star. But, despite this ease of detection, only a few planets have been found that are more than five times the mass of Jupiter. None are more than ten times the mass of Jupiter. This observed upper limit in the mass of the detected planets tells us that there is a real, physical limit to how big planets can grow in the planetary nursery.

There is also a lower limit to the Doppler-detectable planet mass. It is difficult to detect planets with less than one half the mass of Jupiter. Recently, this limit has been pushed down somewhat, but it is not likely that Earth-mass planets will be detected with the Doppler technique. This is because lower-mass planets exert smaller tugs on the star with correspondingly smaller Doppler shifts that are harder to measure.

In March 2000, using one of the giant Keck telescopes in Hawai'i, the low-mass threshold, stubbornly held since 1995, was broken. Two planets with masses less than Saturn were found orbiting the stars HD 46375 and HD 16141. It is possible that the Doppler technique will eventually find Neptune-mass planets if they exist in close orbits. The technical feat involved in detecting such low-mass companions is impressive. But as usual, it is the broader interpretation that seems most exciting. Simply put, there are more little planets than big ones. Even though the Doppler technique may never enable us to detect planets with Earth-like masses, there is a clear trend that suggests that low-mass planets may be predominant in nature. This is a theme that rings commonly in nature: there are more low-mass stars than high-mass stars, more grains of sand on the beach than boulders.

orbit of planet of HD 46375 comparison
...And with this information they esitmate HD 46375's planetary companion to have a mass of about 80 percent that of Saturn. Both illustrations this page courtesy of Geoff Marcy (Univ. of CA - Berkeley)

Other Trends

There appears to be a strong correlation between the metal content—to astronomers, every element heavier than helium is considered a metal—of the host star and the presence of a gas-giant planet. This has some interesting implications.

The first stars in the Galaxy were composed only of hydrogen and helium. It took a few stellar generations to enrich the interstellar medium with heavier elements that could condense into grains and form the cores of planets. If the first generation of stars could not form planets, when was the interstellar enrichment sufficient for planet formation to turn on? If the most ancient stars did not form planets, would this reduce the likelihood of ancient civilizations in the Galaxy? It remains unclear whether this correlation with metallicity will extend to terrestrial-mass planets.

As of May 2000, 42 extrasolar planets had been discovered by Doppler surveys. These alien worlds seem different from the planets orbiting the Sun. For example, in the Solar System, most of the planets travel in circular orbits, which provide a reasonably stable temperature on the surface of the planet. This was probably an important factor in the evolution of life on Earth. However, all of the extrasolar planets at distances beyond two-tenths of an Earth-Sun distance from their host stars reside in elongated, elliptical orbits. These planets spend most of the time in the cold, distant reaches of their orbits and then quickly plunge in close around their stars. Such huge swings in the ambient temperature must provide an extra challenge for biological evolution.

The orbital periods of the extrasolar planets range from just three days to a few years. A true analog of our Solar System would have a Jupiter in a twelve-year orbit. It has been suggested that Jupiter plays a protective role for biological evolution on Earth. A big planet like Jupiter may serve as a gravitational sink, swallowing up many of the incoming comets (à la Comet Shoemaker/Levy 9) that might otherwise pummel Earth and cause mass extinctions. To find a Jupiter in a twelve-year orbit, Doppler surveys need a twelve-year baseline of high precision data. Most programs are about half-way there.

The Undiscovered Country

Precision Doppler surveys are only the first reconnaissance for planet hunting. Many clever ideas are now on the table that will build on current discoveries to search for lower-mass, Earth-like planets. Most of these ideas require the stillness of space to achieve the highest possible precision.

  • Above Earth's atmosphere, transit telescopes will stare at crowded star fields, running a time series of high-precision, photometric observations and analyses to look for a slight decrease in the brightness of an individual star as a planet passes in front of it. This work is already being tested on the ground, and space-born, transit telescopes, which will provide the higher precision needed to find terrestrial planets, are likely to launch as early as 2002.
  • Microlensing is a fascinating technique that is already being tested to find planets. This technique relies on a background star to provide a source of light. Then, foreground stars act as gravitational lenses when they serendipitously pass in front of the source star and cause the background starlight to brighten and bend through an annular region called the Einstein radius. If there is a planet near the Einstein radius, it acts as a secondary lens and a bright, narrow peak will be superimposed on the first light curve. Like transit observations, microlensing will achieve the higher precision needed to detect Earth-mass planets from space. As the most populous stars in the Galaxy, low-mass, M-class dwarfs will be common lensing stars, so microlensing observations may provide important statistical information on the rate of occurrence of planets around these faint stars.
  • The Space Interferometry Mission (see "Interferometry in Space") is being designed to find planets with masses that are just a few times that of Earth around nearby stars. With a probable launch date of 2006, SIM is also being designed to serve as a testbed for the technique of nulling interferometry that will be used by the Terrestrial Planet Finder, planned for launch in about 2011. Nulling interferometry with TPF will allow direct imaging and spectroscopy of Earth-like planets around nearby stars.

We've only seen the tip of the iceberg in the discoveries of extrasolar planets. The next generation of planet hunting should be the most productive. Whatever we find, we should expect the unexpected.

DEBRA FISCHER is a postdoctoral fellow at the University of California, Berkeley, working on the Marcy-Butler planet-search project for the past two years. She monitors the radial velocities of some 300 stars from Lick Observatory. She also has a special interest in education and public outreach and is a volunteer with ASP's Project ASTRO. She can be reached via email at fischer@astro.berkeley.edu.

The atmosphere of Earth hampers ground-based searches for planets around Sun-like stars. While Doppler searches have been successful in finding gas-giant planets similar to Saturn or Jupiter, the undisputed prize, rocky Earth-like planets, will be easiest to find from space. NASA is expected to launch the Space Interferometry Mission (SIM) within the next decade to find smaller planets, more like the one we call home.

SIM will consist of two light collectors separated by a ten-meter baseline and, as the name implies, will employ interferometry to measure tiny wobbles in the positions of nearby stars. This high-tech version of good, old-fashioned astrometry demands that the baseline between the light collectors be measured with a precision of about 50 picometers (5´ 10-11 meter), which is about the diameter of a hydrogen atom. Such an engineering feat will undoubtedly have spin-off applications in other fields.

SIM spacecraft
An artist's rendering of NASA's planned SIM spacecraft. Illustration courtesy of JPL.

With an eye to the future, NASA is also considering a design for a post-SIM mission, the Terrestrial Planet Finder (TPF), which will use an array of light collectors to carry out nulling interferometry—a technique that is expected to provide the first real glimpse of planets orbiting nearby stars. Normally in interferometry, it is desirable to fine-tune the pathlength of the light from the collectors so that the re-combined light constructively interferes and the star appears bright. With nulling interferometry, however, the goal is to arrange the optical elements so that the combined light destructively interferes, canceling out the light from the central star and allowing us to see the much fainter planets orbiting the stars. For the first time, we may have a picture of a pale blue dot.

SIM and TPF are engineering marvels that will not be as cheap as some other planned missions that look for changes in brightness from transiting planets or for microlensing events. But they uniquely offer us the undeniable appeal of spying on those stars that are our closest neighbors.

 
 

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