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Extrasolar Planets
Quick Facts
 
Oldest known:
PSR B1620-26c (13 billion years)
First Discovered:
51 Pegasi b (1996)
Largest core:
HD 149026 b
Most distant found:
OGLE-2005-BLG-390Lb (21,500 light years)
May have liquid water:
Gliese 581 c
Largest diameter:
TrES-4
Smallest found:
Gliese 436 c
Extrasolar Planets

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

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.

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

DID YOU KNOW?
Planets orbiting other stars are much too faint and far away to be seen directly.
Our solar system compared with the solar system of 55 Cancri (Courtesy of NASA)
Our solar system compared with the solar system of 55 Cancri (Courtesy of NASA)
Features

So far, over 370 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 also have been gaseous like Jupiter, but they are much hotter.

Research shows that many extrasolar planets orbit much closer to their parent stars than the Earth and other planets in our solar system do. This was quite surprising to many astronomers who thought that planets could not form close to a star. It also shows that we still have much to learn about how solar systems form.

Learn more about features of extrasolar planets from The Planetary Society.

Kepler Spacecraft (Courtesy of NASA)
Kepler Spacecraft (Courtesy of NASA)
Missions

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.

Scheduled to launch in 2009, NASA's Kepler Mission will 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.

Habitable zone according to the size of the star (Courtesy of NASA)
Habitable zone according to the size of the star (Courtesy of NASA)
Myths, Stories, and More

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. However, a star in the constellation Pegasus, named 51 Pegasi, is more similar to our Sun and planets orbiting it may be more hospitable. Swiss astronomers in 1995 found an extrasolar planet orbiting 51 Peg. Since then, more than 200 extrasolar planets have been discovered.

Discover more stories about extrasolar planets at Nine Planets.

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

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 the James Webb Telescope after it is launched in 2013, they 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.

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

IYA HOT TOPIC: Explore more Telescopes and Space Probes discoveries on NASA's International Year of Astronomy website.

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