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Exoplanet Facts

What Is An Exoplanet?

Artist's rendering - WASP-12b the hottest exoplanet yet discovered Courtesy NASA/JPL-Caltech

For a long time it was thought that the eight or nine* planets that orbited our sun might be the only planets in the universe. But as it turns out, there are lots of stars in the heavens that have planets orbiting around them. The first star with a planet was discovered in 1995. And now at this point in time, astronomers know of 385 stars out there that have planets and 453 planets have been discovered.

Exoplanets or extrasolar planets are the many planets that orbit stars other than our own sun. They are great distances away and can not be detected with the naked eye. Special telescopes and methods are needed to find them.


SunriseHow does a scientist go about finding a planet outside of our solar system? One method is to observe it transiting or passing in front of its star. This causes it to block out a tiny bit of light from its sun for a portion of time. Astronomers can also look at the small amount of light that shines through the planet's atmosphere to determine what kind of gases the atmosphere is made of. By timing this transit, they can also determine the size and the length of its orbit. The Canadian Space Agency launched the Microvariability and Oscillations of Stars (MOST ) project in 2003 to study planets in transit with their first space telescope. The French launched a satellite designed to observe exoplanets based on their transits. It is very sensitive to light changes and is known as the CoRoT mission.

See an image of Venus on June 8, 2004 in transit across our own sun.

Doppler Shift

Star fieldAnother method used to determine a planet's existence is a method known as doppler shift, also called radial velocity. It works similar to the way you are aware when a car is getting closer to you, it passes you and then moves away.

That change in pitch is because the sound waves get compressed when the object is close and are spread out when it is further away. The same is true with light waves that come from a star. If the star is shifting in relation to a planet the light waves will behave in this compression and decompression method.

Astrometric Measurement

All planets have a gravitational pull on their own sun. Astronomers watch for a star to move slightly back and forth or wobble as a means of detecting planets pulling on that star.

Terrestrial Planet Finder
Courtesy NASA/JPL-Caltech - Artist Rendering - Terrestrial Planet Finder

Gravitational Microlensing

Gravity from planets, stars and other bodies bend space. Light coming from a star can be bent by this gravity. Astronomers watch for this bending of light by a planet to act as a lens to focus on the star which will also appear to change position.

Direct Detection

Finding a planet by simply "seeing" it is extremely difficult due to the distances and comparison in size between the planet and its sun. To extend the visual ability of astronomers, science is constantly developing new ways to aid the human eye.

Kepler Project

On March 6, 2009, NASA launched a special telescope to scan an area of space for minute changes in brightness. It will measure to identify planets crossing their suns in transit. To read more click here.

In addition to Kepler there are other projects underway to try and locate a body similar to earth with the hope that we might discover life on another planet.

Kepler Space Telescope
Artist Rendering - Kepler Space Telescope Courtesy NASA/JPL-Caltech

Additional Projects

The Spitzer Space Telescope launched in 2003, the Keck Interferometer telescope stationed in Hawaii, the Hubble Space Telescope, the EPOCh Deep Impact mission launched in 2005, the CoRot mission, and the MOST project are just a few of the missions underway in hopes of detecting exoplanets.


The first exoplanet was found in 1995 orbiting a star similar to our own sun. Its orbit was not similar to Earth’s and the planet resembled Jupiter in many ways. It didn’t take long before many planets had been discovered orbiting distant stars, but most of them are Jupiter like, gas giants, that orbit too close to their suns for life to exist.

Gas giant around a distant star
Artist Rendering Courtesy NASA/JPL-Caltech

But a few planets have been found that have orbits more like those of the planets in our solar system. The planets are still too far away for astronomers to see what they really look like or to determine if any life exists. The images we have here are not actual photographs, but artist drawings, because we can’t take great images of them yet. Here is a link to one of the few actual photographs taken. Perhaps someday we will be able to visit these discoveries and learn more about these exoplanets.

Another Earth?

A planet that could house life would have to have certain conditions. It would need to have certain gases in its atmosphere such as carbon dioxide, water, and ozone, and a certain range of temperatures on its surface that could maintain water in a liquid state. In addition to these conditions astronomers will watch for slight changes in seasonal changes in gases that would indicate growing seasons for either plant or animal life. But all of this is, as of yet, far into the future.

Rocky planet orbiting a distant star
Courtesy NASA/JPL-Caltech

And what would that life be like? Would it be little green people or perhaps just a bacteria growing in an ocean? No one knows for sure, but the search goes on in hopes of finding out.


Our galactic neighborhood
Courtesy NASA/JPL-Caltech

How Far Away?

Most of the planets that have been detected lie within a 300 light-year distance from our sun. But what is a light-year? A light-year is based on the distance light can travel in a year. Or more precisely, 9,500,000,000,000 (9 trillion, 500 billion km) kilometers per year. So take that thirteen digit number above and multiply it by 300 to find out how far away a known exoplanet is from our sun. That’s of the ones discovered so far. There could be many, many more in our galaxy.

Andromeda galaxy
Andromeda Galaxy Courtesy NASA/JPL-Caltech

And what of the other galaxies in the universe? The next nearest galaxy is Andromeda which is 21 quintillion km away. That’s a 21 with 18 zeroes!!!

*Nine Planets

Our solar systemYes, nine is inaccurate. But as planets go there continues to be a huge debate about how many planets exist in our own solar system. For a period of time, early in man’s history, even our sun and our moon were considered to be planets. Then as time went by it was discovered that the moon was a satellite that orbited the earth and that the earth orbited the sun, not the other way around. The outer planets were the last to be discovered and even Pluto wasn’t found until 1930.

As many people know, Pluto and a number of other solar bodies were demoted in 2006 to dwarf planets. There are many who fought this change and so the number of planets in our solar system could be hundreds if some of the bodies had not been relabeled as dwarf planets. New bodies are being discovered regularly, including new moons for some of the gas giants and additional dwarf planets. For those of us who attended school between 1930 and 2006, change is hard and many of us still think of the solar system as having 9 planets.

Naming An Exoplanet

Exoplanets do not have really catchy names like the planets in our own solar system. They are instead, named after the star they orbit followed by a letter of the alphabet. The letter "a" is never used, so the first planet found orbiting a star is given the letter "b." With each new discovery, another letter is added. The letters are not assigned by the order the planet is from its sun, but rather the order they are discovered.

51 Pegasi
51 Pegasi NASA courtesy of nasaimages.org

The sun 51 Pegasi is a star about 50 light-years away from us. In 1995 a planet was discovered orbiting 51 Pegasi. It was named 51 Pegasi b and orbits its sun every 4 days.

Some of the stars don’t even have interesting names, but have names that are largely numbers and letters. Since there are bound to be lots more planets discovered, it seems likely that this system will continue rather than to give each planet a separate name like we do in our own solar system - Mercury, Venus, Earth, etc.

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