Methods of detecting extrasolar planets

The search for extrasolar planets, or exoplanets, beyond our solar system and orbiting other stars has yielded several hundred such objects. Detection methods have been mostly limited to finding large Jupiter-like planets, but as such methods improve, there is special interest in finding small Earth-like exoplanets in habitable zones that could support extraterrestrial life.

Overview

Interest in planets orbiting other stars has a long history, but highly sensitive detection methods are required, and the first extrasolar planets were not confirmed until the 1990s. After Nicolaus Copernicus introduced his heliocentric theory of a sun-centered planetary system in the sixteenth century, astronomers began to realize that space might be endless, with an infinite number of stars. At the end of the sixteenth century, Giordano Bruno proposed that the stars were also suns with their own planets and suggested that there might be an infinite number of other populated worlds.

In 1855, an astronomer at the Madras Observatory of the East India Company claimed that a revolving double-star system in the constellation Ophiuchus (Ophiuchi 70) had orbital anomalies in its eighty-eight-year period that were probably caused by a planet around one of the stars. This claim was repeated in 1896 by American astronomer Thomas See, who calculated that the anomalies were caused by a planet with a thirty-six-year period. These claims were refuted in 1899 by Forest Moulton, who proved that such a three-body system would be highly unstable.

In the 1960s, Peter van de Kamp of the Swarthmore College Sproul Observatory claimed to have discovered possible planets around Barnard’s star, also in Ophiuchus and the second closest star to our Sun. This faint star, which is moving rapidly toward the Sun at about 140 kilometers per second, appeared to have a tiny wobble in its motion consistent with two Jupiter-size planets. However, this apparent wobble was not found by other observers, and it was later shown to be caused by lens adjustments. Since the reflected light of a planet is much dimmer than its parent star, most extrasolar planets have been discovered by indirect detection methods beginning in the 1990s. However, since 2004, astronomers using the European Southern Observatory’s Very Large Telescope Array in Chile have produced direct images of several brown dwarf stars with companions, and in 2005, one of these was confirmed as a planet with a mass several times larger than Jupiter’s. Eleven indirect methods have been used to discover most of the known extrasolar planets, and other detection methods continue to be developed.

Three search methods try to detect the tiny elliptical wobbling of a parent star caused by the gravitational influence of an orbiting planet: astrometry, Pulsar timing, and radial-velocity detection. The oldest method of searching for extrasolar planets is by astrometry, which requires precise measurements of tiny variations in the position of a star. Several astrometric discoveries of exoplanets were claimed in the 1950s and 1960s, but none was confirmed. Such movements are probably too small to observe with ground-based telescopes but were demonstrated with the Hubble Space telescope in 2002. Launched in 2021, the James T. Webb Space Telescope could also prove useful. Future plans to search with the National Aeronautics and Space Administration’s (NASA’s) Space Interferometry Mission may reveal many new planets by astrometry. This method is most sensitive to planets with large orbits and long periods, complementing other methods that are more sensitive to small orbits with short periods.

The first confirmed discovery of an exoplanet used a pulsar timing detection method. Pulsars are rapidly rotating neutron stars that emit rapidly pulsed radio waves at highly regular rates, matching the rotation rate. In 1992, radio astronomers Alexander Wolszczan and Dale Frail detected slight periodic changes in these millisecond pulse rates and recognized that they were caused by wobbling of the star due to three planets. This method is so sensitive that it can detect planets smaller than those detectable by any other method—down to a tenth of Earth’s mass—but is limited by the limited number of known pulsars. Although the existence of such small pulsar planets is of interest, they do not offer the possibility of life as we know it since a neutron star emits radiation deadly to such life.

The radial-velocity or Doppler method detects back-and-forth variations in the wobbling of a star and has accounted for the majority of exoplanet discoveries. These radial motions relative to Earth cause shifting of the star’s spectral lines due to the Doppler effect, which decreases and increases the wavelength of the light as the star moves toward and away from Earth, respectively. Modern spectrometers can detect velocity variations down to about one meter per second, including the High Accuracy Radial Velocity Planet Searcher (HARPS) Spectrometer at the European Space Agency’s 3.6-meter telescope in Chile. This method requires high precision and is limited to nearby stars within about 160 light years. It is most sensitive to large planets with short periods, known as “hot Jupiters” because of their proximity to the Sun, while longer periods require many years of observation. From the period of the planet, the orbital radius can be found. Velocity variations permit an estimate of a planet’s minimum mass, which can be considerably larger if the orbit is highly inclined to the line of sight.

Many new exoplanets have been detected by using developed methods. The transit method measures the tiny dimming of a star when a planet passes in front of it. This method can reveal the size of the planet and can be combined with data from the Doppler method to find its true mass and density. As of 2023, over 4,000 exoplanets were discovered using transit detection.

The gravitational microlensing method is based on observations of a star whose gravitational field functions like a lens, focusing light from a distant star directly behind it in the same line of sight. Anomalies in the lensing light curve can reveal planets orbiting sun-like foreground stars down to the size of Earth. However, the lensing observations cannot be repeated when the chance alignment of two such stars changes. Despite the challenges involved in using this method, scientists have discovered an additional 204 exoplanets as of 2023.

The circumstellar disk method analyzes the infrared radiation emitted by dust disks that surround many stars. Images of dust disks have been obtained by the Hubble, Spitzer, and Webb space telescopes, and some of these have features that imply the presence of planets.

The unmanned Kepler space observatory was launched by NASA in March 2009 with the purpose of locating and recording data on potential Earth-like planets. Kepler detects changes in the level of starlight coming from tens of thousands of target stars and transmits the data back to Earth, where scientists analyze the data to determine whether the star is an Earth-size body that is the right distance from the Sun to potentially contain liquid water. As of 2023, Kepler has found thousands of potential candidates, and scientists have confirmed thousands of extrasolar planets, some of which are hot Jupiters while others reside in the habitable zone and could contain liquid water.

Three detection methods that may be used in the future are of more specialized interest. The eclipsing binary method looks for anomalies in the light variation as one star of a binary system passes in front of its companion, giving evidence of planets in the system. In the orbital phase method, future space telescopes may be able to detect light variations due to the reflected light from planets that produce phases like that of the Moon. The polarimetry method studies the tiny fraction of polarized light from a star if it passes through the atmosphere of an orbiting planet.

Knowledge Gained

Discoveries of extrasolar planets since the first confirmed pulsar planets have increased rapidly and have added much to our knowledge of the varied nature of such planets. The first confirmed planet orbiting a sun-like star (51 Pegasi) was discovered by the radial-velocity method in 1995 by Michel Mayor and Didier Queloz of the University of Geneva in Switzerland. They were surprised to find that its period was only about 4.2 days and that its orbital radius was much less than that of Mercury. These measurements gave a minimum mass of about half that of Jupiter, or at least 150 times Earth’s mass.

The discovery of a planet around 51 Pegasi was confirmed by a California team led by Geoffrey Marcy, who used the radial-velocity method to discover nearly two-thirds of about three hundred possible extrasolar planets found over the next dozen years. These are mostly hot Jupiters whose masses are assumed to be less than the limit of about thirteen Jupiter masses that distinguish a planet from a brown dwarf star. Among their achievements was the discovery of the first multiple-planet system, with three Jupiter-size planets around a sun-like star, and the first transit detection of a planet previously discovered by the radial-velocity method, giving its actual mass and confirming that it was a planet. Over 800 multiple-planet systems have been found, and six pulsar planets are known around two separate pulsars.

Among the thousands of extrasolar planet candidates, the vast majority have Jupiter-size masses. Since most of these were discovered by the radial-velocity method, only their minimum masses are known and the actual masses of some could eventually show that they are brown dwarfs. Extrasolar planets have been confirmed by various methods that have determined their actual masses. Early discoveries were mostly hot Jupiters with large masses situated very close to their parent star and with very short periods. They seem to defy theories of planet formation based on studies of our solar system, which suggest that large gas planets form farther from their parent star than the smaller rocky planets. However, it is now believed that hot Jupiters probably formed farther out and then migrated in due to larger amounts of dust in their circumstellar disks.

Frequent observations of these hot Jupiters appear to be the result of a selection effect, since such planets are easier to detect over shorter time periods because of the larger and faster wobbling of their parent stars. As detection methods have improved, many such large planets have been found with larger orbits comparable to those of Jupiter and Saturn. Since 2004, a few Neptune-sized planets have been discovered with masses in the range of about seven to fourteen Earth masses. Most exoplanets have much more eccentric orbits than those in Earth’s solar system, which is not due to an observational selection effect and is still a major puzzle for astronomers.

Context

The primary interest in extrasolar planets arises from the possibility of finding Earth-like planets in the habitable zone near their parent stars, where liquid (as opposed to ice) water and, therefore, life might exist. The hot Jupiters and eccentric orbits of most of the exoplanets found so far appear to have little possibility of life. However, a few discoveries are beginning to reveal Earth-like qualities and similarities to our solar system. About two hundred exoplanets have now been discovered by gravitational microlensing, including a two-planet system found in February of 2008 similar to the Jupiter-Saturn system, which plays the important role of sweeping up errant comets and asteroids that might otherwise make life on Earth impossible.

In 2008, the Optical Gravitational Lensing Experiment (OGLE) at Princeton University found two rocky “super-Earth” exoplanets with masses of only five and three times the Earth’s mass, the latter orbiting a brown dwarf, suggesting that it might be a planet covered with water. In May of 2008, several dozen possible new super-Earths were announced by an MIT group in Chile using the radial-velocity method with the HARPS spectrometer.

Of the many methods designed to detect exoplanets, NASA’s Kepler mission uses the transit method with a special space photometer that can simultaneously scan one hundred thousand stars and measure their brightness. The first planet confirmations, which were primarily large hot Jupiters, came in January 2010. In April 2013, NASA announced that Kepler had detected the first of the planetary systems that contained small, Earth-size planets in the habitable zone. In 2020, NASA announced that in reviewing previous data supplied by Kepler, an additional Earth-sized habitable zone planet was also discovered.

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