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 universe that have planets orbiting around them. The first star with a planet was discovered in 1995. And now at this point in time, (as of June 2016) astronomers know of 2429 stars out there that have planets and 3268 planets have been discovered. Almost 2500 ‘possible’ candidates still needing to be analyzed to determine if they are indeed planets. These numbers are changing daily. For a current update to these numbers check out NASA’s New Worlds Atlas. If you click on the option to explore data while you are there, you will find out additional details about these exoplanets.
Exoplanets or extrasolar planets are the planets that orbit stars other than our own sun or that wander without the benefit of a star to shine on them. Special telescopes and methods are needed to find them. So let’s learn about just a few of the many methods scientists use to study and seek out exoplanets.
Seeing an Exoplanet
Finding a planet by simply “seeing” it, even with a great telescope, is extremely hard due to the distances between Earth and any exoplanet circling its star. Early images of Pluto were just a blurry blob and Pluto is relatively close by comparison. (See early Pluto picture.) It wasn’t until the New Horizons flyby in 2015 that scientists could see Pluto’s surface and get a real idea about its geography. (See Pluto’s surface up close.) Exoplanets are so much further away than Pluto, that astronomers really need additional help to see them. The comparison in size between the planet and its star makes finding exoplanets very difficult too. A very few have been found visually – the first one in 2008. They are generally very large; similar to Jupiter. A few of them are termed rogue planets as they travel around space not orbiting a star of their own. Learn more about Rogue Planets at Space.com.
The first exoplanet, 51 Pegasi b, was found in 1992 orbiting a star similar to our own sun. Its orbit was not similar to Earth’s and the planet resembled Jupiter in many ways. Astronomers used the Hubble telescope to identify this first discovery. 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 stars 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 here on Science Trek’s site are not actual photographs, but artist drawings, because the images scientists have taken don’t show much detail. For example, here is one of the first actual photographs taken of an exoplanet. It was taken in 2004 with an infrared camera. It doesn’t look like much of a planet. Even its star doesn’t look like much of a star. They are just blurry objects – sort of like those early images of Pluto. Perhaps someday we will be able to visit these discoveries, see them “up close,” and learn more about these exoplanets.
Borrowing a name from a children’s story in which things had to be “just right” to be useful, astronomers have been searching for a planet that is “just right” for life – The Goldilocks Planet. If a planet capable of having life were to be found, it would need to be just the right distance from its star - not too hot and not too cold so that water would stay liquid. This distance from a star is known as the habitable zone. It would need to have just the right gases such as carbon dioxide and oxygen – in just the right amounts that would be similar to the gases found on earth. It would have to be just the right size so as not to have too much gravity and not too little. It has been thought that this “just right” planet would be “just right” for life. Learn more at NASA.
Recently in 2016, three earth sized planets were discovered orbiting a small red dwarf star that is known as the TRAPPEST-1. This star is a cooler star than our own sun and only about the size of Jupiter. The location of the star is relatively close to earth – just 40 light years, but because of its temperature is impossible to see with the naked eye or even with home telescopes. The three planets found are much closer to their star than Earth is to its sun, but because of the lower temperatures, it is still considered a possible “just right” environment for life to exist. To learn more about the TRAPPIST-1 star and the new planets discovered visit NASA’s site dedicated to this discovery. Here you will find a map of the star’s location in the Aquarius constellation.
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.
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 exoplanets 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!!!
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 star for a portion of time. By timing this transit, they can also determine the size and the length of its orbit. The first exoplanet found using this transit method was observed in 1999. See an image of Venus on June 8, 2004 in transit across our own sun. And Mercury as it transits the sun on May 11, 2016. This video shows multiple views of the transit.
Astronomers can also look at the small amount of light that shines through the planet’s atmosphere to determine what kind of gases make up the atmosphere or even the structure of planet itself – solid rock or gases. This is known as spectroscopy. In 2001 astronomers were able to take measurements of an exoplanet’s atmosphere for the first time. Learn more about how spectroscopy works at NASA.
The United States is just one of many countries working to locate exoplanets. 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 European Space Agency launched a satellite, in 2006, designed to observe exoplanets based on their transits. The goal is to find Earth-like rocky planets. This system, known as the CoRoT mission, is very sensitive to light changes. CoRoT found its first exoplanet in 2007.
Gravity from planets, stars and other bodies bend space. Light coming from a star can be bent by the gravity of a planet or other body passing between it and our telescopes. 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. Imagine someone stretching a blanket tightly while rolling a ball (a planet) on the blanket’s surface. The blanket will dip and bend where the ball lies. This is similar to the bending of space that helps astronomers identify an exoplanet.
Another 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.
All planets have a gravitational pull on their own star. Astronomers watch for a star to move slightly back and forth or wobble as a means of detecting planets pulling on that star. This wobble has been compared to a child and an adult riding a teeter-totter. To see some helpful illustrations about this, go to the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.
On March 6, 2009, NASA launched a special telescope to scan a specific area of space for minute changes in brightness. It measured to identify planets crossing their stars in transit. Kepler discovered its first rocky planet in 2011. This was important because Earth is also considered a rocky plant. Previous discoveries were all gaseous like Jupiter. In 2013 the Kepler had a malfunction and nearly ended its mission. But some very skilled scientists were able to repair the problems from earth and get Kepler back to work. This newly repaired and repurposed Kepler is now called K2. A possible Goldilocks planet was then found in 2014 – just the right size and just the right distance from its star. In 2015 Kepler discovered its 1000th planet. To read more, check out Kepler’s mission at NASA.
Science continues to grow and to utilize technology to enhance the search for more and better details about exoplanets. The Spitzer Space Telescope, launched in 2003, allowed astronomers to create an actual map of the surface of an exoplanet using the temperatures of the cloud cover over the planet in 2007. NN Explore is set to begin in 2018 – a joint venture between NASA and the National Science Foundation to study the skies using radial velocity. In 2020 (or there-a-bouts) NASA will launch the WFIRST – Wide Field Infrared Survey Telescope. It will be more powerful than the Hubble. It will be able to analyze exoplanet atmospheres and study other space phenomenon. Scientists and astronomers continue the search in hopes of finding that “just right” planet and hopefully alien life.
Our Solar System’s Nine Planets?
Yes, 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. Scientists have discovered many smaller planets in our solar system and so the number of planets 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 star, but rather the order they are discovered.
The star 51 Pegasi is 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 star 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
For a chance to learn about specific planets that have been located and find out what scientists know about each planet, visit JPL’s Alien Worlds page. Here you can find out details about some of the planets that have been discovered so far.