Exoplanets

Planets that orbit a star outside our solar system.

For a long time, it was thought that the eight or nine planets that orbit 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 planet outside of our solar system was discovered in 1992. And now, at this point in time (as of August 2022) astronomers have identified 5,069 planets and 3,797 planetary star systems. Currently 8,833 “possible candidates” still need 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. While you are there, you can explore more about these newly discovered worlds.

Exoplanets or extrasolar planets are the planets that orbit stars other than our own sun. Our sun is just one of at least 100 billion stars in our galaxy, and most stars are thought to have one or more planets. Scientists estimate there may be more than a trillion planets in our galaxy alone. In addition, there may be up to 2 trillion galaxies beyond our own, so we will never run out of places to hunt for exoplanets. Today we know that exoplanets are very common. However, because they are so far away, exoplanets are not easy to find. Let's learn about a few of the ways scientists study and seek out exoplanets.

Finding a planet by simply “seeing” it, even with a powerful telescope, is very difficult due to the distances between Earth and any exoplanets out in space. Even our neighboring stars are trillions of miles away – much further away than Pluto, the distant dwarf planet in our solar system. Early images of Pluto showed just a blurry blob, and it wasn't until the New Horizons flyby in 2015 that scientists could get a good look at Pluto. With exoplanets being so much further away than Pluto, it's clear that at such distances scientists cannot just look through a telescope and take a picture of an exoplanet. In addition, stars are very big and bright compared to planets circling them, so a small, faint planet is easily hidden in the glare of a huge, overwhelmingly bright star. NASA astronomers describe looking for an exoplanet orbiting a star as being similar to “looking for a firefly next to a lighthouse from a mile away.” Only a few exoplanets have been found through direct observation. Most of the time, astronomers use indirect ways to detect and study these distant planets.

The first exoplanets were found in 1992 orbiting a pulsar star, which is a collapsed star nothing like our sun. But in 1995, an exoplanet called 51 Pegasi b, a Jupiter-like planet about 50 light-years away from Earth, was discovered orbiting a star similar to our own sun. This discovery launched a new field of astronomical research. Since then, thousands of exoplanets of great variety have been found orbiting distant stars. Some exoplanets are rocky, while others are gas planets. Some are believed to have water or ice, while others have seas of lava. Some orbit two or more stars at once. Some travel so close to their stars that they complete their orbit in just a few days. Some have very strange weather – one recently discovered exoplanet seems to have metallic clouds and rain made of liquid gemstones! In addition to exoplanets that orbit stars, there are also free-floating exoplanets known as rogue planets that travel around space unattached to any stars at all.

A few exoplanets have been directly photographed using infrared telescopes. You can see the photographs and learn more about direct imaging of exoplanets. However, most of the images we have of exoplanets are not actual photographs, but artist's drawings. Scientists are often able to determine the size, mass, temperature, and composition of exoplanets, and with those details, they are able to estimate their appearance.

These newly discovered planetary systems are very diverse and quite different from our own solar system. Every exoplanet discovery teaches us something new. The ongoing search for unknown worlds is exciting and full of promise for new discoveries. In the future, as more and more exoplanets are discovered and methods for detecting them advance, we will continue to learn more about how the universe works.

Borrowing a name from The Three Bears 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 have the right temperature to allow for liquid water to exist – just the right distance from its star, not too hot and not too cold. 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 similar to the kind of atmosphere found on Earth. It would have to be just the right size so as not to have too much gravity or too little. It is thought that this “just right” planet would be “just right” to support life — and for most people, the possibility of life on other worlds is the most intriguing part of the search. Learn more about the Goldilocks Zone at NASA.

Recently, seven Earth-sized rocky planets were discovered orbiting a small red dwarf star known as TRAPPIST-1. This star is a cooler and smaller star than our own sun, located about 40 light-years away from Earth. Up to four of these planets are believed to be orbiting in the star's habitable zone and to have temperatures suitable to the presence of water – possibly a “just right” environment for life to exist. We don't yet know whether these planets have Earth-like atmospheres, but the new James Webb Space Telescope will probe for gases such as carbon dioxide, methane, oxygen, and water vapor that could offer hints as to whether life could be possible.

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

Most of the exoplanets detected so far are located within a small region of our Milky Way galaxy, within a few thousand light-years of our sun. The closest exoplanet we know of is Proxima Centauri b, located 4.2 light-years away. But what is a light-year? A light-year is based on the distance light can travel in a year. Or more precisely: One light year is 9,500,000,000,000 (9 trillion, 500 billion) kilometers, or nearly 6 trillion miles. So our closest known exoplanet is 4 times that distance! Of course, there are much more distant exoplanets in our galaxy, and many galaxies beyond our own. The nearest galaxy is Andromeda, which is 2.5 million light-years away – in terms of kilometers, that is 23 with 18 zeroes!

With such huge distances, how do scientists go about finding exoplanets? Often, they study the effect these planets have on the stars they orbit. Let's take a look at some of these methods for discovering exoplanets.

One method is to observe a planet transiting or passing in front of its star. (See a view of Mercury as it transits the sun.) When a planet passes, it blocks out a bit of light from its star, so the star looks a little dimmer. Scientists observe how the brightness of the star changes during transit. By timing this transit, scientists can figure out the size of the planet, its distance from its star, and the length of its orbit. The greatest number of known exoplanets have been discovered using the transit method. Learn more about the transit method of detecting and studying exoplanets.

During a transit, scientists can also learn about an exoplanet's atmosphere. As it transits, a small amount of light shines through the atmosphere. That light can be analyzed to determine what kinds of gases make up the atmosphere, which helps in estimating the planet's temperature. This is known as spectroscopy. It is another tool to help figure out a planet's habitability, or suitability for life.

Gravity from large planets, stars, and other bodies can “bend” space. Light coming from a star can distort and change direction when affected by the gravity of a planet or other body passing between it and our telescopes. Gravitational microlensing occurs when a planet or star's gravity focuses the light of another star in a way that makes it appear brighter. Astronomers watch for this bending of light by a planet that acts as a lens to focus the star's light. Just as a magnifying glass can focus sunlight on a small spot, the gravity of a planet can focus the light of a distant star onto the observer. This allows astronomers to detect the presence of an exoplanet. 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 way to search for exoplanets is to look for “wobbly” stars. This method is known as radial velocity or Doppler shift. All planets have a gravitational pull on their own stars. Astronomers watch for a star that wobbles or moves slightly back and forth, as a means of detecting planets pulling on that star. But how do they “see” these wobbly stars?

It is similar to the way the noise of an ambulance siren sounds higher in pitch when it is closer to you, and then lower in pitch when it moves away from you. The change in pitch happens because 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 from a star. If the star is shifting in response to a planet, the light waves squeeze together and then stretch out depending on the nearness of the planet. This change in wavelength affects the color of the light astronomers observe coming from the star. NASA has an animation that illustrates this. This method is most effective for detecting larger exoplanets that have a greater gravitational effect on their stars.

Taking direct pictures of exoplanets is difficult. Not only are exoplanets very far away, but they are much dimmer than the stars they orbit. Reflected light from a planet is lost in the brightness of its massive host stars. So scientists are experimenting with techniques to block the glare of the stars, providing a better view of possible exoplanets around them. One light-blocking method is a coronograph, a device built into a ground-based telescope that blocks a star's light before it reaches the telescope's detector. Another method intended for use with a space-based telescope is a starshade, a spacecraft that blocks starlight from a star being observed by astronomers. See a NASA animation of how this allows an exoplanet to be seen. Although very few exoplanets have yet been directly imaged, it is hoped that in the future new instruments will be able to take direct pictures of an exoplanet's surface.

Learn more about the ways NASA scientists find exoplanets.

After the first exoplanets were detected in the 1990s, scientific exploration continued to advance, utilizing improved technology in the quest to learn more about exoplanets. The Spitzer Space Telescope, launched in 2003, was the first to directly observe infrared light from an exoplanet. In 2007, the Spitzer Telescope allowed astronomers to create an actual map of the surface of an exoplanet using the temperatures of the cloud cover over the planet.

On March 6, 2009, NASA launched a special telescope called Kepler, designed to scan a specific area of space containing 150,000 stars and to watch for small changes in brightness. Its goal was to detect exoplanets crossing their stars in transit. Kepler discovered its first rocky planet in 2011. This was important because rocky planets (such as Earth) may have the potential to support life. Previously discovered exoplanets were gaseous like Jupiter. In 2013 the Kepler had a malfunction and nearly ended its mission. However, some very skilled scientists were able to repair the problems from Earth and get Kepler back to work. This newly repaired and repurposed Kepler was called K2. A possible Goldilocks planet, an Earth-sized planet located in its star's habitable zone, was found in 2014 – just the right size and just the right distance from its star. Many other such discoveries followed, and by the time Kepler's mission was completed in 2018, it had discovered more than 2,600 exoplanets. Kepler revealed that there are more planets than stars in our galaxy.

The Transiting Exoplanet Survey Satellite (TESS) was launched in 2018 to scan a much larger area and to image hundreds of thousands of the sky's brightest stars. The Hubble Telescope has provided images of debris disks around stars, where planets form. NASA's new James Webb Space Telescope will teach us much more about exoplanet atmospheres and conditions on the planets. For example, in July 2022 it captured clear images of the atmosphere surrounding one exoplanet that indicate the likely presence of water.

Other countries' space exploration agencies, including the Canadian Space Agency and European Space Agency, are also working to locate exoplanets with space telescopes and satellites. All over the world, scientists and astronomers continue the search in hopes of finding that “just right” planet that supports life.

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.

The night sky is full of exoplanets, and it's only been within the last 30 years that we have started to discover their mysteries. To learn about specific planets that have been located, visit NASA's Strange New Worlds page. Here you can find details about some of the amazing planets that have been discovered so far.

Our knowledge of exoplanets, stars, and galaxies is expanding every day. It's an exciting time for space science! One day you may be part of making future discoveries. Learn more about space at these Science Trek pages:

• Astronomy
• Galaxies
• Planets
• Asteroids and Comets
• Satellites
• Astronauts
• Mars