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Chart of relative sizes of planets
Gliese 581 51 Pegasi PSR B1257+12
Gliese 581. Star type: M, Distance: 20.3 light years, number of planets: 4
51 Pegasi. Star type: M, Distance: 50.9 light years, Number of planets: 1
PSR B1257+12. Star type: pulsar, Distance: 2000 light years, number of planets: 3

Are there planets beyond our solar system? Extrasolar planets are planets orbiting stars other than our Sun.
Our solar system is neat and orderly. Orbiting our Sun are four small rocky planets (Mercury, Venus, Earth and Mars), the asteroid belt, four big gas giant planets (Jupiter, Saturn, Uranus and Neptune) and the belt of icy bodies.
What would you think of a solar system that was completely different? How about a giant planet, much bigger than Jupiter, orbiting a star so closely that they're practically touching? What about planets orbiting not one, but two stars in a binary star system? For a long time, imagining other solar systems was the job of science fiction writers. Beginning in the mid 1990s, astronomers began finding evidence of planets orbiting stars other than our Sun. At first, the only planets that could be detected were really big. However, as telescopes and computers advanced, astronomers began finding smaller and smaller planets. Since Jupiter is the biggest planet in our solar system, scientists get really excited when they find planets the size of Jupiter or smaller.

Astronomers have found more than two hundred Extrasolar planets. The first one was detected in 1995 by the Swiss astronomers Michel Mayor and Didier Queloz around the star 51 Pegasi. Since then Extrasolar planet hunting has continued with great excitement. Most planets discovered early on were large like Jupiter and only take a few days to orbit their star. Astronomers call them "Hot Jupiters". Later smaller or distant planets and even some planetary systems with two or more planets were detected. Using the precise spectrometers and large telescopes scientists have discovered about 13 planets with masses below 20 Earth's masses. The smallest one, so far, has only 1.5 times larger in diameter than the Earth. However, none of these planets has been directly seen. They all were discovered using the indirect methods.
Astronomers use a number of different ways to detect planets orbiting around other stars. One way is called the radial velocity technique, where observers look for small wobbles of the stars which might be caused by planets orbiting them. Most of the known Extrasolar planets were discovered in this way. However, this works only for Jupiter sized planets and as we know such large planets are likely inhospitable for human life, unless they have a habitable moon. There is also a photometric technique, where scientists monitor the brightness of stars to look for slight changes that would be caused if a planet were to pass in front of the star. Using this technique, we may register not only the Earth size planets, but also indicate some gases like ozone or water vapor, which may indicate life. The real challenge in this technique is to distinguish the planet from the star which is approximately the same size.
While detection of Extrasolar planets similar to the Earth seems only matter of time, searching for traces of life is challenging. Traces of water vapor, oxygen, ozone and methane are likely indicators of extraterrestrial life. According to scientists the relatively large amounts of oxygen and ozone in Earth's atmosphere are a result living organisms.
How does ozone indicate evidence of life? Without constant reproduction of oxygen by living organisms, the ozone layer in Earth's atmosphere would quickly vanish because the molecules have a tendency to combine with other molecules and solar UV light easily destroys them. So to find life on other planets scientists are looking for evidence of oxygen or ozone.
The smallest Extrasolar planet found so far orbits the red dwarf Gliese 581, located in constellation Libra about 20.5 ly way of from our Sun. Astronomers, detected the planet using the radial velocity method, which also allows them to estimate the mass. It is about 5 times the mass of the Earth and orbits around the Gliese for 13 days. Astronomers are very excited about this planet, because it lies in the habitable zone and may have liquid water. Since the host star is about 1/3 of the Sun's mass and 50 times fainter, its habitable zone is located closer to the star. With average temperatures between 0° C and 40° C and radius only 1.5 times Earth's radius the planet should be either rocky or covered with water.
There are two other planets orbiting around the same star. One is Neptune sized with an orbital period of 5 days, and other is about 8 Earth masses and orbits in 84 days.
SOFIA icon

The Stratospheric Observatory for Infrared Astronomy - SOFIA helps us study the Universe in infrared light using a 3.5 m telescope located on the board of an aircraft flying about 42,000 feet above Earth's surface. SOFIA will help us learn more about the birth of the stars, formation of the planetary systems and the origin of complex molecules in the interstellar molecular clouds.

The Kepler Space Observatory

The Kepler mission is dedicated to finding Earth-sized planets using the photometric technique of transit detection. To accomplish this task Kepler is observing from space to eliminate the effects of Earth's atmosphere. Kepler will also monitor about 100,000 main sequence stars in Cygnus-Lyra region.

jupiter vs earth icon

While missions like Kepler are hunting for the more small Exoplanets, space agencies are planning the next generation of missions. These will try to detect not only very tiny planets but also the traces of life indicating gases like ozone.

asteroid belt

The area between the orbits of Mars and Jupiter which contains a large number of asteroids, including Ceres, the largest asteroid in the Solar System.

Michel Mayor

Swiss astronomer and emeritus professor at the University of Geneva. In 1995, he and Didier Queloz discovered the first exoplanet, 51 Pegasi b.

Didier Queloz

A Swiss astronomer, who along with Michel Mayor, discovered 51 Pegasi in 1995.

51 Pegasi

A yellow dwarf star found in the constellation Pegasus.
In 1995 it became the first star other than our Sun, to be found to have a planet orbiting around it.


A straight line passing through the center of a figure, usually a circle, and whose endpoints lie on the periphery of the figure.


The third most abundant element in the universe and a key component of the air we breathe.


A triatomic molecule, made up of three oxygen atoms.
Ozone is a large part of Earth's ozone layer, which absorbs most of the Sun's damaging ultraviolet light.



A colorless, odorless gas and the main component of natural gas.


A type of raditation that's invisible to the human eye, since its wavelenghts are shorter than those of visbile light.
Ultraviolet light can be found in sunlight, and is responsible for sunburns when exposed to it for too long.

Red dwarf

A small and relatively cool star, about half the mass the size of our Sun and with temperatures under 4,000 K.
Red dwarfs are the most common types of known stars.

Gliese 581

A red dwarf star located in the constellation Libra.


One of the zodiac constellations and home to the stars Gliese 581, Zubenelgenubi, Zubeneschamali, Zubenelakrab, Brachium.

Light Year

The distance that light travels (in a vacuum) in one year. Equivalent to about 9.5 trillion kilometers, or about 6 trillion miles.


The amound of matter in an object, measured in kilograms. Mass is different from weight, which is measured in pounds and can only be measured under acceleration such as a gravity field. A 1-kilogram mass weighs about 2.2 pounds on the surface of Earth, but weighs only 0.8 pounds on the surface of Mars.

Orbital period

The time taken by an object to do a complete orbit around another object.