Exoplanet

Indirect methods of observation have revealed the existence of an ever-increasing number of planets orbiting stars other than Earth's sun. These extrasolar planets, or exoplanets, have surprised astronomers and led to new theories about planet formation, because they often differ from the planets in Earth's solar system. Discoveries of Earth-like exoplanets have also prompted further speculation into the possible existence of extraterrestrial life.

Overview

The discovery of extrasolar planets orbiting sunlike stars has excited the imaginations of astronomers and laypersons alike. If it can be demonstrated that planetary systems are a common occurrence among the billions of stars within the Milky Way galaxy, the possibility of extraterrestrial life in the universe takes on greater credibility. The idea that intelligent civilizations may exist on other planets could become more compelling.

Early in the twentieth century, spectroscopic evidence from Barnard’s Star, a nearby red dwarf one-seventh the mass of the sun, indicated a slight wobble that seemed to imply gravitational interaction by one or two Jupiter-mass planets with decade-long orbits. However, by 1980, further work showed that the wobble of Barnard’s Star was more likely the result of a companion star too small to observe. The mass of an unseen companion can be estimated from the amount of wobble detected from a visible star.

Double-star systems such Barnard’s tend to rotate around their common center of mass in larger orbits than the tiny wobble of a star with a planetary system. Masses between about ten and eighty Jupiter masses usually qualify as brown dwarfs, defined as objects that formed, like other stars, by gravitational collapse of a dust cloud rather than by accretion from a stellar disk, but are too small to sustain the nuclear fusion processes that energize the cores of most stars.

The first confirmed extrasolar planetary system was discovered in 1991, but it was a far cry from a sunlike solar system that could support life similar to that on Earth. Pennsylvania State University radio astronomer Alex Wolszczan was observing a millisecond pulsar (PSR 1257+12) that he and Dale Frail had just discovered using the 305-meter Arecibo radio telescope in Puerto Rico. This pulsar resulted from the collapse of a massive star about a billion years ago. It is now a neutron star that spins 161 times each second, generating a radio pulse about every 6.2 milliseconds. However, Wolszczan found that these pulses varied periodically from the usual high degree of regularity exhibited by other pulsars.

Analysis revealed two periods in the pulse variations from PSR 1257+12, one lasting sixty-six days and the other ninety-five. Wolszczan and Frail proposed that two Earth-sized planets orbit the pulsar, gravitationally tugging on it and causing its radio pulses to arrive slightly earlier and then later than expected. Calculations showed that one planet, since designated PSR 1257+12 c, had at least 3.4 Earth masses and orbited at a distance of 0.36 astronomical unit, or AU (1 AU is the distance from Earth to the sun). The other, PSR 1257+12 d, was at least 2.8 Earth masses and 0.47 AU from the pulsar. In 1994, additional observations revealed a third planet, PSR 1257+12 b, with a period of about twenty-five days and a mass of about 0.02 Earths, located at a distance of 0.19 AU.

Discoveries of planets orbiting sunlike stars began in 1995, revealing two new and unexpected types of planetary objects: small-orbit, hot-Jupiter-type planets and eccentric-orbit Jupiter-like planets. In October 1995, Swiss astronomers Michel Mayor and Didier Queloz of the Geneva Observatory announced evidence of a companion object orbiting 51 Pegasi (in the constellation Pegasus), which is about forty light-years away. A new generation of optical instruments and computers revealed a periodic Doppler shift of the light coming from the star. This suggested a tiny wobble caused by a planet of at least 0.46 Jupiter’s mass and a period of only 4.2 days in a circular orbit at an orbital distance of 0.05 AU. At this small distance, the planet orbiting 51 Pegasi has a surface temperature of about one thousand kelvins. The planet was officially named 51 Pegasi b, though it is informally referred to as Bellerophon.

In a 1996 survey of 120 nearby sunlike stars, Geoffrey Marcy of San Francisco State University and Paul Butler of the University of California, Berkeley, used a refined form of Mayor and Queloz’s method to discover six new Jupiter-size planets. The existence of the first two planets, announced in January of that year, were discovered from the tiny wobbles of stars in Virgo and Ursa Major, located forty-six and eighty light-years away, respectively. The planet around the star 47 Ursae Majoris, designated 47 Ursa Majoris b, has a minimum mass of 2.5 Jupiter masses, an orbital period of about three years, and an orbital radius of 2.1 AU (less than half of Jupiter’s orbital radius of 5.2 AU). The planet orbiting 70 Virginis, designated 70 Virginis b, has a minimum mass of 6.6 Jupiter masses (revised to 7.4 in 2015) and a highly eccentric orbit (0.40 eccentricity) of just under 117 days at an average orbital radius of 0.48 AU. In 2002, Marcy and Butler, along with Debra Fischer, announced their finding of 47 Ursae Majoris c. The planet has an orbital period of 2,391 days and was initially believed to have roughly the same mass as Jupiter, though later calculations revised that to about 0.54 Jupiter's mass. (A third planet in the system, 47 Ursae Majoris d, was discovered in 2010; about 1.64 times Jupiter's mass, it has a period of about thirty-eight years and an orbital radius of 11.6 AU.)

The four other planets included three hot-Jupiter planets similar to 51 Pegasi with nearly circular orbits. At forty-six light-years away, 55 Cancri has a planet, 55 Cancri b, with a mass at least 0.8 times that of Jupiter, an orbital period of 14.6 days, and an orbital radius of 0.11 AU. At a distance of sixty light-years, Tau Bootis has a planet with a minimum mass of 3.87 Jupiter masses, a period of 3.3 days, and an orbital radius of only 0.046 AU. Located fifty-five light-years away from Earth, Upsilon Andromedae has a planet with mass at least 0.68 that of Jupiter, an orbital period of 4.61 days, and an orbital radius of 0.06 AU. Eleven years later, in 2007, astronomers confirmed the existence of two other planets in the Upsilon Andromedae system. The middle planet has an approximate orbit of 242 days and is at least twice the mass of Jupiter. The outermost planet, at 2.5 AU, is at least four times as massive as Jupiter, with an orbital period between 3.5 and 4 years.

Marcy and Butler also announced a possible second planet orbiting 55 Cancri with a minimum mass of about five Jupiters, an orbital period of about twenty years, and an orbital radius of five to ten AU. In 2002, 55 Cancri d was found orbiting the star at a distance of between five and six AU, with a about four times that of Jupiter. At that time, a third planet—55 Cancri c, with a mass roughly that of Saturn’s and a highly eccentric orbit—was also speculated to exist. In 2004, 55 Cancri e was discovered. Dubbed a "super-Earth"—an extrasolar planet with a mass greater than Earth's but significantly less than that of Uranus or Neptune—55 Cancri e is about twice the diameter of Earth and about eight times its mass, with an orbital period of about eighteen hours. Its composition is uncertain, but it is believed to be a large terrestrial planet. The observations that led to its discovery also confirmed the existence of 55 Cancri c, measuring its mass at 0.217 Jupiter (later revised to 0.165) and its orbit at about forty-four days. Another planet in the system, 55 Cancri f, was discovered and presented before the American Astronomical Society in 2005, but it was not officially presented in a peer-reviewed journal until 2007. With an orbital period of 260 days and a mass between forty-five and fifty-five times that of Earth, 55 Cancri f was the first extrasolar planet discovered to be orbiting in the habitable zone of its star, as well as the first one to receive an "f" designation. It is not believed to contain life, as it is most likely composed of gas, but hypothetically any satellites it may have could contain at least microbial life-forms.

Evidence for a potential nearby planetary system was also announced in 1996 by George Gatewood of the University of Pittsburgh. He collected photometric data on many of the nearest stars with the thirty-inch refractor telescope at Allegheny Observatory. The dim red dwarf star Lalande 21185—the ninth-nearest star to the sun, at 8.3 light-years away—appears to have two Jupiter-size planets in orbits similar to those of the gas giants in Earth's solar system. Gatewood analyzed data from fifty years of photographic observations and eight years of photoelectric measurements to reveal tiny accelerations of the star that suggest one planet of about 0.9 Jupiter mass with a period of about 5.8 years in a circular orbit with an orbital radius of about 2.2 AU (similar to the asteroid belt) and a second planet of about 1.1 Jupiter mass with a period of about 30 years in a circular orbit with a radius of about 11 AU (similar to Saturn). A third planet may orbit beyond these two Jupiter-like planets. While the existence of Lalande 21185's planets remains unconfirmed, the star's relative proximity suggests the possibility of eventually capturing an image any planets that may exist with the Hubble Space Telescope.

Two more eccentric planets were announced in 1997. A group of Harvard astronomers led by David Latham had, in 1988, discovered an object with a mass of at least 9 Jupiters orbiting the star HD 114762 in an eighty-four-day eccentric orbit that varies from 0.22 AU to 0.46 AU (0.35 eccentricity). For eight years, this object was classified as the smallest known brown dwarf, but after Marcy and Butler announced the 70 Virginis planet with a very similar eccentric orbit varying from 0.27 to 0.59 AU and a minimum of 6.5 Jupiter masses, the companion of HD 114762 appeared to qualify as a possible planet. Designated HD 114762 b, the planet's existence was later confirmed, and its mass was measured at around 11 Jupiter masses.

A third eccentric planet has by far the greatest eccentricity (0.67) of any known planet. Discovered by Marcy and Butler, it was also independently discovered by William Cochran and Artie Hartzes of the University of Texas at Austin. The planet orbits the star 16 Cygni B, a near solar twin that belongs to a triple star system one hundred light-years away. Designated 16 Cygni B b, it has a mass of 1.68 Jupiters and an eight-hundred-day orbit that varies in radius between 0.6 and 2.8 AU, giving it wild seasonal variations.

Another hot-Jupiter planet orbiting the star Rho Coronae Borealis appears to fill a gap between the very close 51 Pegasi–like planets (less than 0.11 AU) and the 47 Ursae Majoris planet (2.2 AU). Discovered in 1997 by a Harvard University team of astronomers led by Robert Noyes, it has a nearly circular orbital radius of 0.22 AU, a period of 39.8 days, and a mass of about 1.04 Jupiter masses. Given the existence of giant planets with orbits ranging from 0.049 AU (Tau Bootis) to 2.2 AU in a relatively continuous distribution, planet formation theories faced dramatic challenges, especially since existing theories predicted that Jupiter-size planets could not form within 5 AU of their host stars.

In 2005, HD 209458 b, informally known as Osiris, an exoplanet of 0.7 Jupiter mass orbiting at about 0.047 AU around its parent star, was directly detected using the Spitzer Space Telescope. The telescope was able to observe infrared light coming from the planet itself. Astronomers noted differences in the light being produced when the planet was transiting in front of the star and also when the planet was blocked by the star. By factoring out the star’s constant light, scientists were able to isolate the planet. From this, they were able to estimate the temperature of Osiris to be at least 1,023 kelvins.

HD 189733 b was discovered in October 2005, at a distance of about sixty-three light-years from Earth. The planet is considered a hot Jupiter, with a mass of about 1.14 times Jupiter’s and an orbital period of just 2.2 days. In 2007, using the Spitzer Space Telescope, astronomers in Switzerland detected water vapor within the atmosphere of HD 189733 b.

The first super-Earth planets were found orbiting a pulsar in 1991. The two planets were only measured at four times the mass of the Earth, too small to be considered gas giants. The general definition of a super-Earth is a terrestrial exoplanet that has a mass between one and ten times that of Earth. Other scientists say a super-Earth must be five to ten times the mass of Earth. In 2007, Stéphane Udry and his team announced the finding of two super-Earths orbiting around Gliese 581. Both planets are within the so-called habitable zone (the area where liquid water could potentially exist) of their star. Gliese 581 b orbits the star at a distance of 0.041 AU and has a mass of about 15.2 times that of Earth, while Gliese 581 c orbits at 0.074 AU and has about 5.6 times the mass of Earth.

In 2007, the Mount John University Observatory in New Zealand discovered the smallest extrasolar planet known to that point. The planet, MOA-2007-BLG-192L b (or MOA-192 b for short), is only 3.3 times as massive as Earth and orbits a brown dwarf at a distance of 0.62 AU. The first group of super-Earths within the same planetary system was found orbiting HD 40307 in 2008. The three planets—HD 40307 c, HD 40307 d, and HD 40307 f—all orbit the star at a distance less than that of Mercury from the sun, with nearly circular orbits.

The launch of the Kepler telescope in 2009 ushered in a new age of rapid discovery of many more exoplanets. While it was previously believed that such planets were rare, with Earth's solar system representing a possibly unique arrangement, it soon came to be accepted that many—perhaps most—stars have orbiting planets. Most estimates placed the number of exoplanets in the Milky Way galaxy alone at one hundred billion or more. By the time Kepler was “retired” in 2018, the telescope had spotted about 5,000 exoplanets and confirmed more than 2,600.

Among these discoveries have been exoplanets that are potentially habitable according to known standards of life. The first roughly Earth-sized exoplanet to be discovered in the habitable zone of its star was Kepler-186 f, announced in 2014. On January 6, 2015, Harvard scientists announced the discovery of the most Earth-like exoplanet discovered up to that point: Kepler-438 b, in orbit around the red dwarf Kepler-438. Radiation emitted by solar flares from the red dwarf, however, could have done significant damage to the atmosphere of Kepler-438 b and rendered it uninhabitable. In December 2015, scientists at the University of New South Wales in Australia discovered a potentially habitable planet close to Earth's solar system. The planet, Wolf 1061 c, is in the habitable zone in orbit around a red dwarf called Wolf 1061. Proxima Centauri b, an Earth-sized exoplanet (1.27 Earth masses), was discovered in the habitable zone of the star Proxima Centauri—the closest star to Earth's solar system—in 2016.

In early 2017, the discovery of seven rocky, Earth-like exoplanets orbiting the ultracool (less than 2,700 kelvins) red dwarf star TRAPPIST-1 was announced, marking perhaps the best potential for habitable conditions to date. November 2017 saw the announced detection of the exoplanet Ross 128 b, the second-closest Earth-sized (1.4 Earth masses) exoplanet known to orbit in its star's habitable zone, after Proxima Centauri b. By the end of 2017, thirty small, rocky planets in the habitable zone had been confirmed by the Kepler mission.

Further research and discoveries continued to change scientists' conceptions of exoplanets. A major development in astronomy was the detection of water vapor in the atmospheres of sever-al exoplanets. On Earth, water is necessary for the evolution and existence of life, leading to speculation water-based exoplanets could be home to exterritorial life. In 2022, astronomers found that the planets Kepler-138 c and Kepler-138 d, which orbited a star 218 light years away, could be composed of large amounts of liquid water. If the observations proves true, it would make the first time exoplanets have been discovered with so much liquid water.

Methods of Study

Detecting extrasolar planets from Earth is extremely difficult, requiring a new generation of computers and optical instruments. Planets are about a billion times fainter than their host star, making them virtually undetectable by direct methods. An indirect method involves searching for a tiny wobble in the motion of a star as it and any companions orbit about their common center of mass. Although the gravitational interaction between a star and a planet-sized object is too small to observe directly, the radial velocity (back and forth along the line of sight) alternately increases and decreases the wavelength of light from the star, causing an alternating Doppler shift toward first the red and then the blue end of its spectrum.

The velocity of a star can be determined from the magnitude of its Doppler shift. The shift in wavelength due to a Jupiter-size planet is only one part in ten million. An absorption cell, consisting of a bottle of iodine vapor placed near the focus of the telescope, absorbs certain known wavelengths of light, producing dark lines in the spectrum that act as a reference for measuring the Doppler shift accurate to within one part in a hundred million. These shifts are recorded by sending light from a star into complex spectrometers consisting of prisms, mirrors, and gratings costing several million dollars.

Periodic variations in the Doppler shift reveal the period of a planet’s orbital motion. The velocity of the star and the period of its motion can be analyzed to determine the radius of the orbit (using Kepler’s laws) and the minimum mass of the planet (based on Newton’s laws). However, the unknown inclination of its orbit allows for a larger wobble than its apparent radial motion and thus a larger possible mass by a factor of about two. The periodic variation in Doppler shift also reveals the shape of the orbit, since a circular orbit produces a perfect sine wave, while an eccentric orbit produces an irregular variation that can be analyzed by computer to determine the orbital shape.

Using these methods, Marcy and Butler detected radial motions accurate to within plus or minus 3 meters per second, compared to at least 10 meters per second required to detect a planet. Since Jupiter, which contains most of the mass of the solar system at 318 times the mass of Earth, causes the sun to move at a speed of up to 12.5 meters per second, Jupiter-size planets can be readily detected. Most of the new planet discoveries have been based on stars wobbling at speeds between about 10 and 300 meters per second. Planets much smaller than Jupiter cannot be detected accurately with this method, and those with periods of several years require that data be collected over a long enough time span to determine their periodic variations.

Marcy and Butler began collecting Doppler-shift data in 1987 for their survey of 120 sunlike stars, using Lick Observatory’s three-meter telescope, but it was the computer methods used by the Swiss in their discovery of 51 Pegasi b that finally yielded results. Their first discoveries resulted from running six computers day and night at the University of California, Berkeley, to analyze data from sixty stars. These methods revealed a variety of planets that shocked astronomers because their orbits were so unexpected. Hot Jupiters and eccentric orbits have initiated a new generation of theories about planetary formation and the uniqueness of Earth's solar system.

The Spitzer Space Telescope (SST) was launched in 2003. It consists of three main instruments: the Infrared Array Camera (IRAC), an infrared camera that operates simultaneously on four different wavelengths; the Infrared Spectrograph (IRS), a spectrometer able to observe at four wavelengths; and the Multiband Imaging Photometer for Spitzer (MIPS), which is made up of three different far-infrared detector arrays. In 2005, the SST was the first telescope to detect light from exoplanets HD 209458 b and TrES-1 b. However, the light was not turned into actual images.

In 2004, a group of astrophysicists in France captured the first photograph of an extrasolar planet orbiting a brown dwarf. The planet appeared only as a small red dot. It is speculated to have a mass two to five times that of Jupiter, but it orbits the star at a distance greater than Pluto’s average distance from the sun. This “exoplanet” did not form from an accretion event the way scientists currently believe planetary formation occurs. Also, a brown dwarf would not have enough material to form a Jupiter-sized planet, especially at such distances. Because of these objections, many astronomers do not consider the photograph to be of a real planet.

The next challenge was to capture a photograph of an actual exoplanet. Two of the programs dedicated to imaging a planet are located in Chile. In 2007, the Gemini South Observatory installed the first optics system specially designed to photograph exoplanets. In 2008, it started a two-year program to conduct a survey of young stars using its Near-Infrared Coronagraphic Imager (NICI). The NICI consists of a coronagraph and two cameras that can simultaneously photograph the star and its surroundings at two different infrared wavelengths. The two photographs would then be subtracted, leaving behind an image of the planet. This method will also help eliminate false planets that are actually background stars or stray starlight.

A possible first photograph of extrasolar planet 1RXS J160929.1-210524 b was announced in September 2008. The image was taken using the Gemini North Telescope on Mauna Kea in Hawaii. The extrasolar planetary system is about five hundred light-years away from the Earth. Planet 1RXS J160929.1-210524 b has a mass about eight times that of Jupiter and orbits its star at a distance of 330 AU. Some scientists are skeptical of its status as an extrasolar planet because of its great distance from its star (Neptune, by comparison, is only 30 AU from the sun).

In May 2016, the National Aeronautics and Space Administration (NASA) announced that the Kepler telescope had confirmed the largest number of exoplanets at one time to date, doubling the number of exoplanets that it had previously confirmed. Using a method of statistical analysis, a researcher from Princeton University determined that of the more than 4,000 potential planets identified by Kepler as of July 2015, 1,284 of them had a more than 99 percent probability of being a planet. This conclusion meant that of the almost 5,000 total planet candidates that had been reported up to that point from all sources, more than 3,200 had been verified. Based on their size, NASA estimated that 550 of the newly verified exoplanets could be rocky planets similar to Earth.

In December 2017 NASA reported the discovery of an eighth exoplanet in the Kepler-90 system, making it the first known planetary system with the same number of planets as in Earth's solar system. Perhaps even more significant, however, was the method by which the exoplanet was detected. Scientists made a breakthrough in using artificial intelligence to sift through the backlog of data from the Kepler telescope and pick out less-obvious signs of planets. The incorporation of machine learning and neural networks in the hunt for exoplanets promised to even further increase the number of known objects throughout the galaxy.

Soon after, in February 2018, NASA announced the discovery of ninety-five new exoplanets using the Kepler telescope. According to NASA, these exoplanets "range in size from mostly rocky super-Earths and fluffy mini-Neptunes to Jupiter-like giants" and "include a new planet orbiting a very bright star"—HD 212657, "the brightest star ever discovered by Kepler to have a transiting planet."

The search for exoplanets received a significant boost in 2021 with the launch of NASA’s James Webb Space Telescope, the most powerful space observatory evert placed in orbit. Within months, the telescope began sending back highly detailed images of distant stars and galaxies. In January 2023, James Webb found its first exoplanet, a rocky world about the same size as Earth designated LHS 475 b. As of early 2023, NASA has confirmed more than 5,200 exoplanets and is continuing to study evidence on more than 9,000 exoplanet candidates.

Context

The discovery of extrasolar planets may offer new hope for the existence of planetary systems like Earth’s solar system that could support extraterrestrial life. However, the unexpected nature of these planets has raised new challenges for planet formation theories and even new doubts about the possibility that any of them might harbor life. Pulsar planets were probably formed from the remnants of a companion star during a supernova explosion that produced a spinning neutron star, and they are bathed with high-energy radiation that would make life impossible. Other newly discovered planets orbit more sunlike stars but have either extremely small or highly eccentric orbits that also make them unlikely candidates for life.

Still, the growing number of known exoplanets indicates that these bodies are quite common, which alone statistically improves the chances of life existing somewhere. Additionally, many scientists increasingly reject the narrow constraints of the habitable zone as too biased toward familiar forms of life. The existence of extremophile life on Earth, and the potential for forms of life wholly different from those known to humans, stand as arguments favoring the existence of some kind of organism on one of the billions of exoplanets. While Earth's solar system must still be considered unique in harboring water-based life-forms, exoplanet discoveries show that it is not as unusual a system as once believed.

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