Beyond Our Solar System: Searching for Extrasolar Planets

What if the Universe contains other planets like Earth outside the Solar System? After all, the Sun is only one of 100 billion stars in the Milky Way Galaxy alone. Why shouldn't similar extrasolar planets exist around other stars, perhaps in vast numbers?

Planet Hunting Strategies

For centuries, scientists have pondered the possible existence of extrasolar planets, but only in recent years have such planets actually been discovered. In 1995, University of Geneva astronomer Michel Mayor and his graduate student Didier Queloz detected the first planet orbiting a star other than the Sun. Since then, this exciting new field of astronomy has exploded. As of February 2007 more than 212 extrasolar planets have been discovered. Most of them are very close to their mother star, and are Jupiter-like: massive and gaseous.

Still, scientists have yet to see an extrasolar planet directly or even take a picture of one. Thus far, their presence has only been inferred from either their effect on the orbit of their host stars or by measuring the amount of infrared light they emit. The Spitzer Space Telescope is the only instrument that has successfully used the latter strategy. (To learn more about the Spitzer Space telescope, read "Seeing the Invisible.")

The job of finding and imaging extrasolar planets is extraordinarily difficult. The biggest challenge is the planets' feeble luminosity. A star, fueled by nuclear reactions, is typically a billion times brighter than any orbiting planet, whose light is primarily reflected from the host star. This overwhelming brightness renders nearby extrasolar planets invisible to astronomers. Invisible does not mean undetectable, however.

The Telltale Wobble

One technique scientists use to detect extrasolar planets is to look for a slight wobble in the motion of the host stars. The wobble is caused by the gravitational tug of the planet as it orbits the star. Astronomers predicted this effect decades ago, but the movement is miniscule and tricky to detect.

The key to detecting the wobble is a phenomenon called the Doppler effect. When you hear a police car approaching, the Doppler effect causes the sound of its siren to increase in pitch. Likewise, as the siren recedes, its pitch drops. The change in pitch occurs because the sound waves are being compressed or stretched. The same thing happens with light waves. When a star wobbles slightly in the direction of an observer on Earth, the light it emits appears to shift towards the blue end of the spectrum, which has shorter wavelengths. When the star wobbles away from Earth, the wavelengths stretch slightly and the light appears redder.

The shift in wavelength when a star wobbles is very tiny, so this technique has thus far detected only very massive planets. That's because the more massive the planet, and the closer it is to the host star, the larger and quicker the wobble. These instances produce relatively larger shifts in wavelength. For example, the Sun wobbles at about 12 meters (40 feet) per second in response to the gravity of Jupiter, the most massive planet in our Solar System.

Seeing Secondary Eclipses

The Spitzer Space telescope has pioneered a newer planet-finding technique called the "secondary eclipse method." It has added several more extrasolar planets to the roster since 2005. The method is possible only because Spitzer detects infrared wavelengths of light, not visible wavelengths. In visible light, the glare of a star overwhelms the light reflected by any planetary companions. But planets emit their own infrared radiation. When Spitzer observes a solar system, the star still outshines the planet, but much less than it would in visible light. This makes it easier to detect the planet itself.

Finding extrasolar planets with Spitzer is still hardly as simple as taking a snapshot. Instead, Spitzer relies on the fact that some planets periodically eclipse, or block, the light from their host stars. Imagine Spitzer has its infrared eye trained on a nearby star with a planet in orbit around it. When the planet passes in front of the star, it blocks a portion of the starlight. When the planet continues to circle around the back of the star, it no longer blocks the starlight. Spitzer can measure the amount of infrared light from the star plus planet vs. the star alone, subtracting to determine the amount of infrared light emitted by just the planet.

The secondary eclipse method does not image extrasolar planets directly. But it has allowed astronomers to measure the amount of infrared light emitted from extrasolar planets for the first time. More recently, Spitzer has focused its spectrometer instrument on two such planets, HD 209458b and HD 189733b, using the same secondary eclipse method. Spectrometers measure spectra—more detailed graphs of the specific wavelengths of light the planets emit and absorb. Spectra can be used to identify molecules in the atmospheres of the planets. That's because specific molecules absorb and emit specific wavelengths of infrared radiation as they vibrate. Using this method, scientists found no evidence of water around HD 209458b and HD 189733b. But they did find evidence of clouds of silicates, or tiny sand grains, in HD 209458b's atmosphere.

Several ground- and space-based missions are now in the works to search for extrasolar planets. Driving this quest is a curiosity that a world may be found with a composition similar to that of Earth, or a world with life forms on it. "Is Earth's environment unique?" asks Museum astrophysicist Rebecca Oppenheimer, "Or could others be spread across the Universe?" Like many of her colleagues, she suspects the answer is yes.

Scientists have surveyed, photographed, and even landed spacecraft on the planets in the Solar System. Now, their drive to understand more about the nature of planets is taking them further afield. More than 100 extrasolar planets—planets that orbit stars other than the Sun—have been detected in recent years, but not one has yet been observed directly. That's because the light from a star is typically a billion times brighter than any planet that orbits it, rendering the planet invisible to scientists on Earth.

To overcome this obstacle, astronomers have turned to coronagraphy, an imaging technique invented in the 1920's by the French astronomer Bernard Lyot. A coronagraph is a device that attaches to a telescope. It consists of a series of mirrors that aim light from a star at a small opaque disk. The disk blocks out the brightest light emitted by the star, thereby permitting dimmer objects near the star to be seen. Think of it as an artificial eclipse inside the telescope. Lyot built the first coronagraph to make pictures of the corona, the pale halo of gas that emanates from the Sun. Lyot's coronagraph was basically an opaque disc; it blocked out the Sun, and the luminous "atmosphere" of gas became visible.

The Lyot Project Kicks Off 

Rebecca Oppenheimer, an astrophysicist at the Museum, has built a coronagraph that she hopes will take the first-ever photograph of an extrasolar planet. The Lyot Project Coronagraph, as it is known, was assembled on the sixth floor of the Museum's Rose Center for Earth and Space in a dust-proof "clean room" designed specially for the construction of delicate stargazing instruments. This one, however, looks less like a sophisticated astronomy device than a playground for pint-size robots. Ten mirrors of different sizes sit in aluminum mounts bolted to a large steel table. The contraption weighs nearly a ton and floats on a cushion of air that protects it from vibrations.

In March 2004, the coronagraph was flown to Hawaii and installed in the Maui Space Surveillance System, an observatory at the summit of Maui's Mount Haleakala. Stationed 3,000 meters (10,000 feet) above sea level, the observatory sits above most of the moisture, cloud cover, and atmospheric turbulence that can disrupt a clear view of the night sky. Indeed, before the telescope even feeds an image to the coronagraph, it will refine the image to compensate for atmospheric turbulence, which would otherwise scatter and blur incoming starlight. The technique is called "adaptive optics," and the Maui Space Surveillance System is equipped with the world's most precise adaptive optics system. It employs 941 tiny motors, or actuators, glued to the back of a mirror 23 centimeters in diameter and 2 millimeters thick. Each actuator can push or pull the mirror to shift its surface by a few thousandths of a millimeter. The telescope feeds the adjusted starlight to the coronagraph, which has its own adaptive optics system consisting of several little motors that track the incoming starlight. A computer analyzes the incoming light and decides whether to further adjust any of the motors. "When you're trying to see something that could be a billion times fainter than the star, a slight misalignment could completely impair your ability to see," Oppenheimer says.

Little Room for Error

Creating the Lyot Project Coronagraph was a design challenge, requiring a team of at least ten scientists from the American Museum of Natural History, the University of California at Berkeley, the University of Hawaii, Caltech, and the Space Telescope Science Institute. The concept is straightforward: take an image, subtract the star, then make a new image that will be analyzed for visible signs of an extrasolar planet. Of course, Oppenheimer says, "in practice it's more complicated." The Maui telescope feeds starlight to the coronagraph as a very thin beam of light; the beam then bounces off ten small mirrors before passing through a filter, which blocks the brightest light and saves the rest for analysis. For the device to function properly, each mirror must be aligned to within half a micron (one two-thousandth of a millimeter) of perfection. "We're talking about something much more refined than aligning your eyeglasses," Oppenheimer says with a laugh. The work can be "very mundane," she admits, "but it's fun. In the end you have something that works. Instead of simply reducing data on a computer or looking at equations, as theorists do, you're actually building an instrument. You take it out to the telescope, and you get to see two years of your own work actually do what you wanted it to do. It's just fantastic."

Planet Hunting

Since it was installed on Haleakala, the coronagraph has surveyed about 100 nearby stars, looking for anything faint—disks of debris, or possibly even planets—that might be orbiting them. "To me the most exciting thing about the Lyot Project is that we're exploring a region of space that has never really been explored before," says Oppenheimer. The team took pictures of stars relatively near Earth, including some identified by NASA's Spitzer Space Telescope as having material in orbit around them. The initial survey in Maui has not only studied several planet-forming debris disks around nearby stars, but has also led t o many insights and improvements in the techniques of coronagraphy and extrasolar planet imaging. In the next year and a half, the Lyot Project will move to the Palomar Observatory in Southern California, where a more capable adaptive optics system is under development. There, a longer-term, deeper survey of nearby stars will be conducted with vastly improved sensitivity.

If and when an object is found orbiting a nearby star, scientists will study the light it produces or reflects to determine whether it is a planet or some other celestial object. Closely analyzed, the spectrum of light may reveal telltale signs of certain chemical molecules, which could indicate that the possible planet has an atmosphere. Scientists will also look for signs of distinct minerals to analyze what the planet is made of. "This information tells us about the physics of the object itself, whether it might host life, and what sort of life it might host," Oppenheimer explains. "For example, what would a planet five times larger than Earth look like? Would it be a rocky place with very short-legged animals, because of the intense gravity? And understanding the physics of these things will settle the debate about what a planet is—a subject of considerable debate among astronomers today."

Most everything that scientists know about planets and what to expect from them—what they look like, how and where they form—is based on the study of the planets that orbit the Sun. Oppenheimer's hope is that, in their hunt for extrasolar planets, astronomers will find something wholly unexpected—"something so different from predictions that new theories will have to be formed in order to explain them. The bottom line is that we don't know what we'll find."

NASA's Great Infrared Observatory

On August 25, 2003, NASA's Spitzer Space Telescope was launched from Cape Canaveral aboard one of the largest ever Delta rockets. The muscle was needed because Spitzer is the largest infrared telescope ever launched. Spitzer orbits in a much colder, more distant region of space than does the Hubble Space Telescope: it orbits the Sun rather than Earth and sees much farther out into space. Among its objectives is to observe the vast, dense clouds of gas and dust that fill many regions of space. Scientists believe these interstellar clouds are key to understanding how stars and planets form.

The Old, the Cold, the Dusty, the Dirty

Unlike the Hubble Space Telescope, which views the cosmos in a wide range of spectral wavelengths, Spitzer looks completely in the infrared-light range. Infrared light has longer wavelengths and lower energies than visible light. In astronomy, the infrared range is particularly useful for studying not hot but warmish matter, such as interstellar dust clouds and the dusty disks that surround some stars. Hotter objects, including some stars, emit more energetic light; they can be seen in visible light but are largely invisible in the infrared. As Michelle Thaller, a senior staff scientist at Caltech and manager of the SIRTF education program puts it, "Infrared probes the old, the cold, the dusty, or the dirty."

What's So Special About Infrared?

Light exists in many different wavelengths depending on its energy. Humans perceive only a small segment of that spectrum. Thaller compares viewing the Universe in only optical light—the wavelengths visible to the human eye—to listening to Beethoven's Ninth Symphony and hearing only the three notes around middle C. "We wouldn't be able to understand the structure, the beauty, any of the composition," she says.

Spitzer surveys all kinds of celestial objects that produce little or no visible light, everything from submicron-size interstellar dust grains to small stars too dim to be detected by their visible light to dusty disks that might be planetary systems in the making. Viewed through Spitzer, the visible-light stars appear very dim, while cooler stars or stars surrounded by dust appear brighter and are easier to detect.

Detecting Dusty Disks

Stars are thought to form within giant clouds of gas and dust. Planets originate in gaseous disks around young stars. As these protoplanetary disks cool, grains of dust collide and accrete into rocks, boulders, and eventually planets. Spitzer is detecting protoplanetary disks of dust by measuring their infrared brightness. In effect, the space telescope is taking a census of possible nearby planetary systems in the making.

Finding these nurseries is an important first step toward the larger goal of understanding how planets form. In addition, interstellar dust may hold clues as to how life evolved. The dusty disks are thought to contain organic molecules fundamental to humans and all forms of life. "We're going to answer the 'Where did we come from?' question in a very real way," Thaller says.

Keeping Cool

Because infrared is primarily heat radiation, Spitzer must be cooled to within a few degrees of absolute zero (-459 degrees Fahrenheit or -273 degrees Celsius), the lowest temperature possible, which is characterized by a complete lack of heat. If the telescope is not kept this cold, its own heat will interfere with its ability to observe the faint heat signals coming from space. If you go outside on a sunny day and close your eyes, it is easy to feel the heat from the Sun on your face. "But imagine going out at night and trying to feel the actual heat from the stars," says Thaller. In the past, large tanks of liquid helium kept telescopes cool, functioning rather like a big thermos bottle. These tanks made telescopes very heavy and expensive to launch. In contrast, Spitzer uses only a small amount of liquid helium as a coolant. Reflective paint and a heat shield do the rest of the work of keeping the telescope at the correct low temperature.

Along with the telescope, Spitzer carries an array camera that takes detailed images of the infrared sky and precisely measures the amount of incoming light. Spitzer also has a spectrometer that analyzes light to see what chemical elements are present in it.

Spitzer's instruments have revealed landmark information about protoplanetary disks and extrasolar planets in the telescope's first three years of operation. Spitzer will continue to see the invisible until the helium runs out—perhaps until 2009. To learn about some of the discoveries scientists have made using the Spitzer Space Telescope, click on the Astro Bulletins in the Related Links for this feature.