Exoplanets, worlds orbiting other stars


  • Exoplanets are planets that orbit other stars.
  • Scientists think that most stars have at least one exoplanet. These worlds are a prime target for the search for life beyond Earth.
  • You can help The Planetary Society advocate for the Roman Space Telescope, NASA’s next exoplanet mission. You can also support our efforts to help scientists find 100 Earth-sized exoplanets around nearby stars.

Why study exoplanets?

For centuries, fictional depictions of planets orbiting other stars have fired our imagination. From the desert world of Arrakis in Dune to the lush jungles of Yoda's planet Dagobah in Star Wars, we humans have been fascinated with the idea of exotic, far-off worlds. 

We now know that worlds beyond our solar system — known as exoplanets — do exist. In fact, there are a whole lot of them: Scientists have found over 4,000 exoplanets, and think that most stars have their own solar systems. Some exoplanets are surprisingly similar to fictional worlds we've imagined, while others have turned out to be more exotic than anything we could have dreamed.

Our universe is estimated to have over 100 billion galaxies, each with hundreds of billions of stars. If most stars have one or more planet around them, there may be billions of trillions of planets in the universe.

Every exoplanet discovery teaches us something new about how the universe works. When we just had our own solar system’s eight planets to study, we had a limited view of what kind of planetary systems are possible in the cosmos. Now, with over 4,000 exoplanets cataloged, the horizons of planetary science are broader than ever. We also sometimes get to see other solar systems forming, which teaches us about our own origins. It's like watching our very own "How It's Made" show through the world's telescopes.

There are lots of reasons to learn about exoplanets, but perhaps the most compelling is that we could find another world that hosts living organisms. If we discover life beyond Earth, it could change the course of human history. And with continual advancements in exoplanet research, this discovery could happen in your lifetime.

Exoplanet Proxima b
Exoplanet Proxima b This artist’s impression shows the surface of exoplanet Proxima b orbiting the red dwarf star Proxima Centauri—the closest star to Earth. The planet is similar in size to Earth and lies in its star's habitable zone, the not-too-hot, not-too-cold region where temperatures are suitable for liquid water to exist on the surface.Image: ESO/M. Kornmesser

How do we study exoplanets?

Even through a powerful ground-or-space-based telescope, stars look like tiny points of light. Planets are even tinier and are very difficult to spot next to their bright host stars. Therefore, scientists rely on indirect methods, like looking at the stars themselves for signs that planets might be orbiting them.

As these techniques have evolved, we’ve gone from merely being able to tell if a star has a planet to actually being able to detect characteristics of exoplanets. Through increasingly sophisticated technologies, we can not only figure out basic exoplanet traits like mass and diameter, but whether a world is solid or gaseous or even has water vapor in the atmosphere.

A new NASA mission called the Roman Space Telescope (RST), is an astrophysics and exoplanets mission that could launch as early as 2025 and will be able to detect Earth-sized, rocky worlds. The Trump administration tried and failed to cancel RST in three recent NASA budget proposals. 

One of the main goals of exoplanet research is to planets that resemble Earth. You’ll often hear the term “Earth-like” used to describe worlds that resemble our own. Generally speaking, this means the planet might have liquid water and an atmosphere that could support life as we know it. Life on other worlds may be unrecognizably different from what we know here on Earth, and even life on our planet is extraordinarily diverse. But it makes sense to start by looking for worlds that resemble our own.

Earth-sized or Earth-like?

It’s important to distinguish “Earth-like” from “Earth-sized” when reading news stories about exoplanets; just because a planet is about the same size as Earth doesn’t mean it could support life (Hello, Venus!).

How do we look for Earth-like planets?

The most basic condition for being able to sustain liquid water is the planet’s position in relation to its star. Scientists call the region around a star where liquid water can exist on the surface of a planet the habitable zone: not so close to the star that water all evaporates, and not so far from the star that it all freezes. This area is also known as the “Goldilocks zone” (not too hot, not too cold).

We can tell if a planet is in the habitable zone based on the distance of the planet from its host star and the temperature of that star. A bigger, hotter star’s habitable zone is farther out than that of a smaller, cooler star.

There could be as many as 40 billion planets in the habitable zone of stars right here in our Milky Way galaxy. So far we’ve found about 20. That may not seem like many, but consider this: it's possible we’ve already found a planet that hosts life, and we just don’t know it yet.

How do we figure out if a planet in the habitable zone is, in fact, habitable? Or inhabited?

Imagine what our Sun and Earth would look like from a few hundred light-years away. Using our present-day technologies, you might be able to determine that Earth is a small, rocky planet that sits in the Sun's habitable zone. 

Next, you could look for signs of water in the Earth's atmosphere, to rule out it wasn't a dry, desert planet like Mars. Our current technologies are just barely capable of doing this, while upcoming ground and space-based observatories will be able to do it even better

By then, you might be able to conclude Earth was habitable. But to tell if there was actually life on the surface, you'd need to find gases in the planet's atmosphere like ozone that are possible byproducts of life. Our current and near-future telescopes won't be able to do this, but several proposed space telescopes might. 

Even then, you probably wouldn't be able to say for certain that you'd found life. Ultimately, it could take something like a telescope that uses our Sun's gravity as a giant lens to capture a sharp picture of an exoplanet and see signs of life on the surface. This may sound impossibly futuristic, but the technology is not beyond our reach. 

What can you do to advance exoplanet research?

Since 2009, Planetary Society members have supported work by Debra Fischer, one of the world's top exoplanet researchers. These projects have greatly improved our ability to search for Earth-like exoplanets.

We've been working with Fischer on exoplanet projects since 2009. People like you helped us fund FINDS, a new instrument to detect tiny star wobbles caused by orbiting Earth-sized exoplanets, and Exoplanets Laser, a cutting-edge telescope calibration system. We also supported a search for exoplanets around Alpha Centauri, the closest star system to Earth.

Right now, we're working with Fischer on a project to find 100 Earth-like planets. Our members and supporters are crowdfunding the replacement of a photonic crystal fiber (PCF).

It may sound like science fiction, but a PCF isn’t what powers lightsabers in Star Wars. A PCF is an ultra-sensitive cable used to transport light. Fischer’s teams are using it to send light from the Lowell Discovery Telescope and a solar telescope into the exoplanets laser calibration device Planetary Society that grew out of research members helped fund in 2014. Unfortunately, the cable degrades with every use, and if it is not replaced soon, Fischer's work will come to a halt.

You can also help us support the Roman Space Telescope, NASA’s next exoplanet mission. Despite being the astrophysics community's highest-priority mission this decade, the Trump administration tried three times to cancel the mission, citing a lack of funding. Fortunately, Congress rejected those proposals the past two years, though it must do so again for 2021. The Planetary Society has submitted a statement urging Congress in January 2020 to continue funding the mission.