For a long time it was thought that the eight or nine planets that orbit our sun [* See note] 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. We call any planet that orbits a star outside our solar system an exoplanet.
The first distant star with a planet was discovered in 1995. As of this date (August 2015), astronomers know of 1,172 stars that have exoplanets, and they have confirmed the existence of 1,887 exoplanets. There are nearly 4,000 additional exoplanets that have been spotted but not yet confirmed. But these numbers change so often that by the time you read this, there could be more that have been discovered. To find out the latest count, visit NASA's New Worlds Atlas site.
The first exoplanet orbiting a star similar to our own sun was found in 1995. However, its orbit was not similar to Earth's, and the planet resembled Jupiter in many ways. It didn't take long before many more exoplanets had been discovered orbiting distant stars. Most of them are gas giants like Jupiter that orbit too close to their suns for life to exist.
A few exoplanets have been found that have orbits more like those of the planets in our solar system. The exoplanets are still too far away for astronomers to see what they really look like or to determine if any life exists on them. Most of the images we have on this site are not actual photographs but artist drawings because the images we do have don't show much detail. For example, view this image of the first ever, actual photo of an exoplanet, taken in 2004. It was taken with an infrared camera. Perhaps someday we will be able to visit these discoveries, see them “up close,” and learn more about these exoplanets.
Finding an exoplanet or extrasolar planet by simply “seeing” it is nearly impossible due to the distances from earth and comparison in size between an exoplanet and its sun. To extend the visual ability of astronomers, science is constantly developing new technologies to aid the human eye.
There are currently 10 methods of detecting exoplanets. These include astrometry, eclipse timing variations, imaging, microlensing, orbital brightness modulation, pulsar timing, pulsar timing variations, radial velocity, transit, and transit timing variations. Let's take a look at just a few of these means of finding an exoplanet.
How 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 exoplanet's atmosphere to determine what kind of gases the exoplanet's atmosphere is made of. By timing this transit, they can also determine the size and the length of its orbit.
Several countries have projects to discover and study exoplanets in transit. The Canadian Space Agency launched its first space telescope, the Microvariability and Oscillations of Stars (MOST) project, in 2003 to study exoplanets in transit. In 2006, 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.
Scientists are learning new things about planets in our own solar system as they transit the Sun. See these images of Venus from 2012.
Another method used to determine a planet's existence is known as doppler shift and is also called radial velocity. It works similar to the way the sound of a car changes pitch as it gets closer to you, passes, and then moves away. That change in pitch is because the sound waves get compressed when the object is close by and spread out when it moves 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 compressed and decompressed way.
All planets exert 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 a planet pulling on that star. This is known as astrometric measurement.
This technique relies on the fact that gravity from planets, stars and other bodies bends light. Light coming from a very distant star can be bent by the gravity of an intermediary star. Astronomers watch for changes in the brightness of the distant star as an intermediary star passes in front of it. The nearer star acts as a lens to brighten the light from the distant star. If the star's image brightens even more for a short period, that's a sign there's an exoplanet in the neighborhood.
On March 6, 2009, NASA launched a special telescope designed to scan regions of space for minute changes in brightness. It measures those changes in brightness to identify planets crossing their suns in transit. Learn more about Kepler at NASA's site.
In addition to Kepler there are a number of other exoplanet detection projects. The Spitzer Space Telescope launched in 2003, the Keck Interferometer telescope stationed in Hawaii, the Hubble Space Telescope, the Deep Impact mission launched in 2005 (later extended and renamed EPOXI), the CoRot mission, and the MOST project are just a few of the missions underway with the goal of locating bodies similar to earth. The hope is that we might discover life on at least some of those other exoplanets.
A planet that could host life as we know it would have to meet certain conditions. It would need to have water and specific gases such as carbon dioxide and ozone in its atmosphere. Also, temperatures on its surface would need to stay within a certain range in order to maintain water in a liquid state. In addition to these conditions, astronomers 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.
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.
An important discovery took place in April 2014 when scientists discovered a planet similar to Earth in size and which orbits at what is called a habitable distance from its sun since water would be liquid at that range. Known as Kepler 186f, the planet is likely rocky. Learn more about this discovery at JPL.
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 just the ones discovered so far. There could be many, many more in our galaxy. 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!!!
Yes, with the demotion of Pluto to dwarf status, nine is inaccurate. But there continues to be a huge debate about how many planets exist in our own solar system. For a period of time, early in human 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 to dwarf planets in 2006. Since we continue to discover new bodies in our solar system, including new moons for some of the gas giants as well as additional dwarf planets, the number of planets in our solar system could be in the hundreds if some of those bodies had not been relabeled. Many fought this change, and 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 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 “numbers and letters” system will continue rather than trying to give each planet a separate name like we do in our own solar system — Mercury, Venus, Earth, etc.