Extrasolar Planets

Extrasolar planets, or exoplanets, are planets outside of our Solar System, so they do not orbit our Sun. Exoplanets that orbit other stars are difficult to detect directly because they are literally lost in the glare of their stars.

Image at right: Artist concept of an extrasolar planet (Courtesy of NASA).

Scientists study the process of star formation throughout the Milky Way. They also try to understand what types of star-planet systems may exist.

Significant amounts of gas and dust can be leftover from the "protostellar" disks that form new stars. Similar to the formation of the planets in our Solar System, it is likely that solid particles of dust within the protostellar disks combine and stick to each other in collisions, slowly building a core around which an extrasolar planet grows. Since most stars form from protostellar disks, and these disks should form planets, astronomers think that it is common for planets to form around other stars.

Features

: Our solar system compared with the solar system of 55 Cancri (Courtesy of NASA)

As of December 2011, more than 700 extrasolar planets have been detected. The majority of them have large masses (because massive planets are easier to detect than smaller planets). Many are considerably more massive than Jupiter, the largest planet in our Solar System. Most are gaseous like Jupiter, but they are much hotter.

During the 1990s, research showed that many giant extrasolar planets orbit much closer to their parent stars than the Earth and other planets in our Solar System. This was quite surprising to many astronomers who thought that giant planets could not form close to a star. During the last decade, many smaller planets have been detected. Some of these planets are part of planetary systems which might be more similar to our Solar System. We still have much to learn about how different planetary systems form.

Learn more about extrasolar planets on the Extrasolar Planets Encyclopaedia website.

Missions

: Kepler Spacecraft (Courtesy of NASA)

Most extrasolar planets have been found using ground-based telescopes. The Hubble Space Telescope and Spitzer Space Telescope are providing useful information as well. COROT, launched by the European Space Agency in December 2006, is a current space mission dedicated to the search for extrasolar planets.

Launched in 2009, NASA's Kepler Mission has begun to survey our area of the Milky Way galaxy to detect Earth-size and smaller planets in or near the habitable zone (where liquid water can exist on a planet's surface).

Read the latest news about the Kepler Mission on NASA's website.

Learn more about missions involving extrasolar planets on NASA's website.

Myths, Stories, and More

: Habitable zone according to the size of the star (Courtesy of NASA)

Are people on Earth alone in the universe? For millennia, human beings have pondered this question. Ancient Greek philosophers like the Epicureans speculated that other worlds could exist. Medieval scholars thought that other planets might harbor forms of life.

In 1991 astronomers detected the first three extrasolar planets orbiting a dying pulsar star in the constellation Virgo. It is extremely unlikely that any life could exist on these planets. In 1995, Didier Queloz and Michel Mayor discovered the first planet orbiting another star similar to our Sun. Since the launch of Kepler in 2009, many extrasolar planets have been identified that may be similar to our own planet Earth.

Discover more stories about extrasolar planets at Nine Planets.

Earth Matters

: Artist's concept of an Earthlike planet (Courtesy of NASA)

Could there be another planet like Earth? Searches targeting thousands of stars nearest our Sun may reveal evidence of planets that could sustain life. Some of these planets might be very much like Earth. "Earth-type" planets must be solid bodies (unlike the gas giants Jupiter, Saturn, Uranus, and Neptune in our outer Solar System) with masses roughly between 0.1 - 10 Earth masses. For life as we know it to exist on the surface of a planet without needing special technology to survive, that planet's temperature and atmospheric pressure must support the existence of liquid water and provide energy for complex life-forming chemical reactions. The planet can't be too close or too far from its star, or the temperature and atmospheric pressure won't be just right. That "just right" distance is known as the habitable zone.

With the help of high-powered telescopes, scientists are hoping to find a planet outside of our Solar System where life could exist. In 2008, using data from the Hubble Space Telescope, scientists observed carbon dioxide on a distant extrasolar planet called HD 189733b, which is orbiting its own star. This discovery is exciting because it shows that we can probe the ingredients of an extrasolar planet's atmosphere. While HD 189733b is about the size of Jupiter and is too hot for life to exist, scientists continue to explore. Using the Hubble Space Telescope and the Spitzer Space Telescope as well as planned future space observatories, they will continue to search for combinations of gases such as ozone, methane, water vapor, and carbon dioxide that, when found together, would be good biosignatures (signs of life) on far away planets.

Discovering New Worlds

The discovery of new worlds is a great example of how tools, observations, and theories help us explore the Universe. Mercury, Venus, Mars, Jupiter, and Saturn have been observed since the beginning of civilization and can be seen with the unaided eye. The word planet comes from the Greek word for wanderer, which is very fitting since the movements of planets in the sky are different from the stars.

Galileo's observations with telescopes, including his discovery of Jupiter's moons, craters on our Moon, and sunspots laid a foundation for our modern understanding of these "heavenly bodies" as other worlds. His observations were critical to the acceptance of the idea that the Sun rather than the Earth is at the center of our Solar System.

The planet Uranus was the first planet discovered through telescopic observations by William Herschel and his sister, Caroline, in 1781. John Couch Adams and Urbain Le Verrier predicted the existence of Neptune before it was actually observed. They used Isaac Newton's studies of gravity to predict where Neptune should be based on irregularities in the orbit of Uranus. Sure enough, Johann Gottfried Galle and Heinrich d'Arrest discovered Neptune at its predicted position in 1846. Over the past two centuries, numerous small, icy or rocky worlds have been discovered orbiting the Sun, including Pluto, which was discovered by Clyde Tombaugh in 1930 and is called a dwarf planet.

Our Sun is one of several hundred billion stars in the Milky Way Galaxy. The most distant objects that orbit our Sun are billions of miles away, but other stars in our Galaxy are trillions, even quadrillions of miles away! Since then, over 700 planets have been discovered orbiting other stars, and this number often increases on the scale of weeks or even days. These planets are known as exoplanets or extrasolar planets, and amateur astronomers can detect a number of them. The Extrasolar Planets Encyclopedia is a terrific place to learn about new discoveries.

Discover more about Earth and extrasolar planets from NASA's Planet Quest.

Getting closer to Earth

Finding these new worlds makes the search for new planets one of the most exciting areas of astronomical research, but all the scientists involved would trade their discoveries for one very special planet - a copy of our own Earth. The closest we have seems to be Gilese 581g, discovered in September 2010.

This planet weighs in at just over three times the mass of Earth, and therefore probably has a rocky surface. Critically, its discoverers believe that it lies in the habitable zone of its parent star. In this region - also known as the "Goldilocks Zone" because it is neither too hot not too cold - liquid water can exist. Gilese 581g might be a watery world like our own. In a few more years, we should have the technology and the telescopes to scan its atmosphere for signs of life - just one more step in our search for an Earth like ours.

Learn more about this recent discovery on NASA's website.