Extrasolar Planets

I

Introduction

Extrasolar Planets, also exoplanets, planets orbiting stars other than the Sun. Astronomers have found more than 200 such planets. Billions of planets likely exist in our galaxy. Finding and studying extrasolar planets helps astronomers learn more about the formation of our solar system. It also contributes to the study of possible life in the universe, because life is more likely to develop on planets than in the extremes found on other bodies in space. See also Astronomy; Planetary Science; Star; Exobiology.

II

Occurrence and Formation of Extrasolar Planets

Astronomers did not develop reliable techniques to find extrasolar planets until the 1990s, but more than ten new solar systems were discovered within the first few years of searching. Many astronomers believe that almost every Sun-like star has a solar system at some point in its development.

Astronomers have found that most young stars are surrounded by disks composed of dust and gas. Some of these disks show evidence of comet-like objects. Some, such as the disk around the star Fomalhaut, show an empty area around the star, or a stripe of empty space in the disk. Astronomers believe that dust in this area could be in the process of condensing into a planet.

By studying our solar system, astronomers developed a theory of how solar systems form known as the core accretion theory. However, many of the extrasolar planets and disks of dust around other stars do not seem to conform to this theory. The theory states that a disk of dust and gas collects around a star as the star forms. Bits of dust in the disk collide and stick together, forming larger and larger chunks of rock and ice. Farther out from the star (where the temperature is cooler), the gases in the disk freeze, adding to the mass available to form these chunks.

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The pieces of rock continue to collide, forming large objects called protoplanets. The protoplanets far from the star are far larger than those closer to the star because of the increased amount of frozen gas material available. Sometimes protoplanets crash together, breaking apart and starting the process of formation all over again. At some point during the last part of the planet formation process, the star goes through a stage in its own evolution in which it blasts away the free gas that remains in the inner solar system.

If the protoplanets in the outer solar system are large enough, their gravitational pull grabs this gas and pulls it in toward the protoplanet. These outer protoplanets then become gas-giant planets, with deep layers of dense gas covering their cores. The smaller inner planets lose any gas that surrounds them. Small planets, such as Earth, that have atmospheres develop them later, when volcanic activity releases gases from within the planet. The star settles into a long quiet period, and the protoplanets grow into planets and develop regular orbits.

Many of the solar systems that astronomers have discovered contain very large planets very close to their host star. Systems like these are probably easier to detect with current methods than systems that resemble our own, so they may seem more common than they really are. Still, the core accretion theory does not explain such systems. Several popular possible revisions to the theory exist. It may be common for very large planets to form far from the star and then be drawn in closer by the gravitational pull of the star. Another possibility is that some situations allow a very large planet to form very close to a star. However, the reported discovery in 2006 of a planet smaller than Neptune orbiting a star at a distance of about 2 astronomical units (AUs) provides support for the core accretion theory and suggests that the very large planets orbiting very close to their stars are anomalous rather than common.

III

Locating Extrasolar Planets

Astronomers need sophisticated techniques to locate extrasolar planets. Planets reflect the light of their stars, but a star is millions or billions of times brighter than its planet. The distance between a star and planet is usually so relatively small that the star’s light obscures the planet from view. The most powerful optical telescopes cannot pick out a planet against the glow of its parent star. Sensitive brightness-measuring instruments called photometers, however, can sometimes detect the dimming of a star as its planet passes in front of it. Some warmer planets emit low levels of infrared radiation (light with longer wavelengths than visible light), and astronomers have recently begun detecting planets directly with telescopes sensitive to infrared radiation. See also Photometry; Infrared Astronomy.

Planets are so difficult to observe directly that astronomers usually have to find them indirectly, by observing the behavior of the host star. When planets orbit stars, the gravitational attraction between the star and the planet keeps the planet circling the star. This gravitational attraction also has an effect on the star. Stars are much more massive than planets, though, so the effect the pull has on the star is much smaller than the effect it has on the planet. The pull between the planet and the star is just strong enough to make the star wobble slightly. See also Orbit; Gravitation.

Astronomers detect the telltale wobble either by watching the star very carefully, or by analyzing the star’s light to see how it changes as the star moves slightly toward and away from Earth. The first technique works if the gravitational pull on the star is very strong and the star is relatively close to Earth. If it is, powerful telescopes can directly detect the back-and-forth movement of the star. Even for the largest planets, however, the movement of the star is tiny and difficult to detect.

The second technique—analyzing the star’s light—is much more powerful and successful. This technique uses the Doppler effect, a change in the appearance of a star’s light caused by the star’s movement. When the gravitational pull between a planet and star pulls the star around in a tiny circle, the star moves alternately away from and toward Earth. When the star moves away from Earth, each wave of light leaves the star from slightly farther away than the wave of light before it, making the distance between waves (called the wavelength) longer. When the star is moving toward Earth, each wave of light leaves from slightly closer to Earth than the one before it did, making the wavelength shorter. This change in wavelength, and consequently, in the frequency and color of the light, is called the Doppler effect.

Astronomers detect Doppler effects in starlight by separating the light of a star into the light’s colors in a process called spectroscopy. The elements present in a star emit light especially strongly in particular colors, creating bright lines on a star’s spectrum, or its range of color. The wavelength of light defines its color. Red light has a longer wavelength than green light, which has a longer wavelength than blue light. The movement of the star shifts the star’s spectrum toward the red end (if the star is moving away from Earth) or toward the blue end (if the star is moving toward Earth). Astronomers watch for the regular changes in a star’s spectrum to show the presence of a planet.

Astronomers can also detect the presence of a planet through a phenomenon known as a gravitational lens. This method, known as a microlensing event, is based on a small brightening effect that occurs when an object passes in front of a star from our line of sight on Earth. The gravitational field of the closer object acts as a lens, momentarily amplifying the light from the more distant star. The lensing object is usually a star, but a companion planet can also cause a smaller brightening effect—a microlensing event—and can be detected over a shorter period of time, such as a day or a few weeks, corresponding to the time it takes for the planet to orbit its companion star. A team of astronomers has created a project known as the Optical Gravitational Lensing Experiment (OGLE) to detect extrasolar planets using this method.

When a planet passes in a front of the star it orbits—an event called a transit—it causes a small dip in the brightness of the star. Measuring the slight change in the brightness can be used not only to directly detect a planet, but to determine its size and orbit. However, the planet needs to orbit in a plane that lies in a telescope’s line of sight on the star. Despite long odds, Earth-based telescopes have detected and studied a few exoplanets using this method.

The first space telescope designed to search for extrasolar planets also uses this transit method. Called COROT (COnvection, ROtation and planetary Transits), the mission was developed by the French space agency with the ESA and a group of countries including Brazil. COROT was launched in 2006 and may detect planets the size of Earth or larger that orbit close in to a star.

NASA’s Kepler space telescope looks for planetary transits, as well. Planned for launch in 2008, Kepler has a larger telescopic mirror than COROT. Kepler could find extrasolar planets in orbit at Earth’s distance from the Sun. It may also be able to see light reflected off planets. Kepler is designed to detect planets the size of Earth and smaller.

More sophisticated space telescopes are being planned that could selectively block out the light from a particular star, a method called occultation. This approach could allow extrasolar planets in any orbital plane to be seen directly without the glare of the star they orbit.

IV

Studying Extrasolar Planets

After astronomers determine that a star has a planet, they can find out more about the system by looking more closely at the star’s spectrum. In one successful technique, astronomers send the light of a star through a sample of iodine before separating the light into its component colors. The iodine absorbs specific wavelengths of light, leaving dark lines on the star’s spectrum. These dark lines act as references, enabling astronomers to measure exactly how far the wavelength of a star’s light is shifted toward the red or blue. By comparing the star’s light at its farthest from Earth to the star’s light at its closest to Earth, astronomers can tell exactly how the gravitational pull between the planet and the star affects the star. The size and speed of the star’s wobble gives astronomers an estimate of the planet’s mass and how far from the star it orbits.

Astronomers can glean even more information about extrasolar planets that, as seen from Earth, happen to pass directly in front of their parent stars. Some light from the stars passes through the planets’ atmospheres. Analyzing the light can reveal the composition of these atmospheres.

All of the extrasolar planets that astronomers had found by the end of 1998 are very large—many times the size of Earth. Some are several times the mass of Jupiter, the largest planet in our solar system. Most astronomers believe that smaller, more Earth-like planets probably also orbit some of these stars, and may be detected with improved equipment and techniques.

Astronomers find solar systems in the process of formation by looking for radiation emitted by disks of dust and gas around stars. The hot gas and dust emit radio waves of specific wavelengths, and astronomers can locate and map the disks with radio telescopes. Watching the disks over a period of weeks or months, astronomers see large clouds of gas evaporate. Many astronomers believe that these features are comets releasing their frozen gases as they near the star. See also Radio Astronomy.

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