20 Years of Exoplanets: The Search for Worlds Beyond Our Own
Extrasolar planet at 51 Pegasi
As far back as ancient Greece, humans have wondered whether other solar systems might exist in the universe. In about 300 BCE, Greek philosopher Epicurus, for example, proposed that the universe's boundaries were limitless and that space was full of other worlds. Another Greek astronomer of that era, Aristarchus of Samos, had proposed that the sun was the center of the solar system, and that the stars might be other suns.
Historical models of the solar system
However, those ideas faded away, and the Earth-centered solar system became the common belief in astronomy
for thousands of years. The sun and the stars, it was thought, orbited our planet. In 1543, Polish astronomer Nicolaus Copernicus developed the mathematics of the sun-centered solar system and caused a great shift in scientists' view of the universe. His work led to the recognition that Earth does indeed orbit the sun and to the follow-on speculation that other stars had planets circling them.
The invention of the telescope in the early 1600s put our solar system into even clearer focus, revolutionizing astronomy. Galileo was one of the first to use the telescope to search the heavens. Among his findings was spotting the four largest moons orbiting around Jupiter. Over the next two centuries, bigger and sharper telescopes allowed astronomers to make more solar-system discoveries, including finding the planets Uranus and Neptune orbiting the sun.
The early 1900s saw another large leap in our understanding of Earth's place in the universe. Using the Hooker Telescope — the largest telescope of its day — at Mt. Wilson Observatory in California, astronomer Edwin Hubble found that some nebulae, such as the Andromeda nebula, were separate galaxies like our own Milky Way galaxy. Because each galaxy contains hundreds of billions of stars, the potential for other worlds orbiting other stars vastly increased.
The hunt pays off
Until the 1990s, however, astronomers had not found any planets circling other suns. Uncovering planets outside our solar system, or extrasolar planets, is extremely difficult because they are too small, dim, and far away to be directly imaged by telescopes. To find them, astronomers had to develop other methods, such as detecting their effects on their host star.
In 1995, astronomers using ground-based telescopes finally nabbed an extrasolar planet that was orbiting a star similar to our sun. The planet was roughly half the size of Jupiter and was orbiting dangerously close to its star, called 51 Pegasi. The system is located about 51 light-years from Earth. (NASA's Jet Propulsion Laboratory is commemorating the 20-year anniversary of the first extrasolar planet discovery around a sun-like star with several special events.)
The planet detection around 51 Pegasi began a gush of extrasolar planet discoveries. Astronomers attributed the boost in discoveries partly to improvements in technology. Significant improvements, for example, were made in spectrometers, instruments that separate starlight into its component colors for analysis. Spectrometers can help astronomers detect extrasolar planets and sample the chemical elements in the planet's atmosphere. Faster computers and more sophisticated computer software also helped astronomers readily identify slight changes in light from stars to find planets circling them.
Space missions boost the extrasolar planet count
At the start of the twenty-first century, several dozen planets outside our solar system had been found using ground-based telescopes. In 2006, scientists launched the French CoRoT mission, the first space mission dedicated to the extrasolar planet hunt. The mission helped astronomers discover dozens more extrasolar planets.
Another step forward in the search for extrasolar planets began with the launch in 2009 of the Kepler space observatory, NASA's first extrasolar mission. Kepler's goal was to search hundreds of thousands of stars for signs of planets.
The spacecraft has found a diverse array of planets, including exotic, multi-planet solar systems. More importantly, Kepler has found that smaller planets (Earth-sized rather than Jupiter-sized) are likely to be the most common in the Milky Way galaxy. Kepler uncovered several planets that are about the same size as Earth and orbit within the star's habitable zone, where liquid water, which is necessary for life, can exist. Overall, Kepler has added more than 1,000 confirmed planets to the extrasolar planet count, which now stands at nearly 2,000.
Astronomers also have used NASA's Spitzer Space Telescope, which observes in infrared light, to study extrasolar planets. In 2007, the Spitzer telescope detected water vapor for the first time on another world.
Hubble contributes to the search
NASA's Hubble Space Telescope has made some unique contributions to the planet hunt. Hubble made one of the first visible-light images of an extrasolar planet circling the nearby, bright star Fomalhaut, located 25 light-years from Earth. The planet is orbiting about 10 times farther away from its star than Saturn is from the sun.
Extrasolar planet HD 189733b
Hubble also helped astronomers analyze the atmospheres of extrasolar planets. Hubble made the first measurements of the makeup of the atmospheres of two Jupiter-sized extrasolar planets. The giant planets are orbiting too close to their stars and are therefore too hot for life. But the Hubble observations demonstrate that the basic chemistry for life can be measured on planets orbiting other stars. Even if a planet is rocky, Earth-sized, and just the right distance from its star, it also must have an atmosphere that contains the correct ingredients to foster and sustain life. Venus, for example, is Earth's twin by size, but its dense atmosphere is made up mostly of carbon dioxide, which helps keep the planet sizzling hot.
Venus, therefore, is not hospitable for life.
The distant worlds observed by Hubble and other telescopes are just the tip of the iceberg, according to a Hubble survey of the center of our Milky Way. The study revealed that our galaxy should be brimming with 100 billion planets, at least one planet for every star.
The future looks bright
Within our own solar system, astronomers are still learning new information. NASA's New Horizons mission, for example, was the first spacecraft to visit Pluto in the outer solar system. The spacecraft's images of Pluto show a surprisingly complex world with giant ice mountains that had never been seen before.
The study of worlds outside our solar system will be continued by future spacecraft, such as NASA's Transiting Exoplanet Survey Satellite (TESS) and James Webb Space Telescope. TESS will find planets in the same manner as Kepler, but search stars that are closer to our sun. The Webb telescope will use infrared light to examine in detail the extrasolar planetary systems found by TESS and other telescopes. It will even study the chemical makeup of an extrasolar planet's atmosphere. Who knows what new discoveries these observatories will uncover?
Looking to the far future, it is clear that planets in the universe are really just getting started. A new analysis of data from Hubble and Kepler considers the expected rate of new planet formation over the entire predicted lifetime of our galaxy. Our planet Earth formed relatively early in that context. About 90 percent of the planets that ever will form are yet to be born. With roughly 100 billion planets in our galaxy, 100 billion galaxies in the universe, and trillions of years ahead, planet studies in the universe have vast possibilities across both space and time.