Extrasolar Planets

Extrasolar Planets

The question of whether or not other planetary systems similar to our own exist has intrigued astronomers and the general public alike for centuries. It was only in the in the 1990s that astronomers began to discover direct evidence for planets outside the solar system.

Method of Detection

The planets of the solar system are visible because of the sunlight that they reflect. Unfortunately, planets that orbit other stars are far too faint relative to their stars for current astronomical telescopes to observe them directly as faint points of light next to their much brighter stars. Instead, astronomers use a variety of techniques to indirectly infer the presence of these extrasolar planets.

The method by which all of the known extrasolar planets have been discovered is the radial velocity (or Doppler) technique. The mass of the orbiting planet causes the central star to be pulled around in an orbit. Astronomers detect the resulting small, periodic shifts in the apparent speed of the star. By measuring the shape of the resulting Doppler curve over time, they are able to deduce a lower limit on the mass of the planet and estimate the separation of the planet from the star. This planet-star distance is typically expressed in astronomical units (AUs); one AU is the average distance from Earth to the Sun.

Extrasolar Discoveries

The first detection of a planet orbiting another Sun-like star was accomplished in 1995 using the radial velocity technique. This planet, orbiting the star 51 Pegasi, was found by two Swiss astronomers, Michel Mayor and Didier Queloz, of the Geneva Observatory. A pair of American astronomers, Geoffrey Marcy and Paul Butler, soon followed with the announcement of several other new planets. Each of these teams has since expanded into large groups that are now surveying thousands of Sun-like stars in search of new worlds.

These radial velocity surveys are able to detect only massive planets, that is, planets which have masses similar to Jupiter.^* Less-massive planets produce a correspondingly smaller tug on their parent star and are thus more difficult to detect. Recently, observers of radial velocity have increased the sensitivity of their measurements, and announced the discovery of several planets with masses similar to that of Saturn (one-third that of Jupiter's mass). Future improvements in the technique should allow for the detection of planets with even lower masses. Unfortunately, stars themselves are somewhat variable, and the flutter that results from their intrinsic variability implies that small, rocky worlds similar to Earth (a mere 1/300th of Jupiter's mass) will not be detectable by this method.

By early 2002, more than seventy extrasolar planets have been discovered. Based on the total number of stars in the current surveys, this implies that at least 7 percent of Sun-like stars have at least one planet. This estimate, however, is only a lower limit: The surveys have not been in operation long enough to see planets at large distances from their stars. Our own Jupiter takes nearly twelve years to circle the Sun. Planetary systems similar to our own—that is, those with massive planets in large, circular orbits far from the central star—would not yet have been detected. One of the chief goals of the radial velocity surveys between 2002 and 2012 is to search for such systems.

After astronomers find a planet orbiting a given star, they continue to monitor that star in the hope of detecting additional planets. In 1999, Butler and colleagues announced the first detection of a multiple-planet system, in orbit about the Sun-like star Upsilon Andromedae. Recently, six other stars have been demonstrated to harbor multiple planets, and many other stars show hints that they too possess multiple planets.

Hot Jupiters

The first distinct subclass of extrasolar planets to emerge from radial-velocity surveys consists of the so-called Hot Jupiters (or 51-Peg-type planets). These planets have masses similar to that of Jupiter, but they are located 100 times closer to their stars than Jupiter is from the Sun. The existence of such planets challenges conventional theories of planet formation. Gas giants such as Jupiter presumably form at large distances from their stars, where the environment is sufficiently cool for a core of ice and rock to coagulate and nucleate the formation of the planet. If this theory is correct, then the Hot Jupiters must have undergone a migration from the site of their formation to their current location. The cause of this migration mechanism and the details of why it did not operate in the solar system are the subjects of intensive theoretical investigation.

Due to the proximity to the parent star, there is a reasonable chance—one in ten—that the orbit of a Hot Jupiter is tilted at just the right angle so that the planet will be observed, with each orbit, to pass in front of the disk of star. The resulting dimming of the light from the star, called a transit, was first observed in 1999 for the Sun-like star HD209458. These observations proved that the radial velocity variations, by which the planet had been initially detected, truly were due to an orbiting planet (and not some form of undiagnosed variability in the star). Moreover, for the first time, astronomers were to estimate both the physical size and mass of a planet and thus calculate its density. Their conclusion was that this planet was indeed a gas giant, similar in mass and size to Jupiter. Later, astronomers further scrutinized this star with the Hubble Space Telescope. By observing how light of different colors is filtered by the outer reaches of the planet, they detected its atmosphere, the first such detection for a planet outside the solar system.

Astronomical Missions

The successes of the techniques described above, and the realization that these ground-based methods will not allow for the detection of small, rocky worlds similar to Earth, have inspired several astronomical satellites.

In 2009 the National Aeronautics and Space Administration (NASA) plans to launch the Space Interferometry Mission (SIM). Using SIM, astronomers will perform very precise astrometry to detect the reflex motion of stars due to orbiting planets. They hope to survey hundreds of stars for large terrestrial planets (greater than 5 Earth masses), orbiting at distances of several AU from their stars.

In the decade after SIM, NASA will launch the Terrestrial Planet Finder, with the objective of enabling astronomers to detect extrasolar planets that are true analogs of Earth and to study the atmospheres of those planets. In particular, NASA plans to search for atmospheric components, such as ozone, which may be attributable to life.

see also Hubble Space Telescope (volume 2); Jupiter (volume 2); Stars (volume 2); Sun (volume 2).

David Charbonneau

Bibliography

Doyle, Laurance R., Hans-Jorg Deeg, and Timothy M. Brown. "Searching for Shadows of Other Earths." Scientific American 283, no 3 (2000):58.

Marcy, Geoffrey, and Paul Butler. "Hunting Planets Beyond." Astronomy 28, no. 3(2000):42.

^*Jupiter is the largest planet in the solar system.