Subscribe to receive SETI Institute news weekly in your inbox.



embed video
What is an Exoplanet? Video featuring Dan Peluso on Youtube:

Simply put, exoplanets are planets that lie beyond our solar system. So first, we have to understand the definition of a planet.

Planets are worlds that orbit our Sun, like Mars, Jupiter, and of course, our own Earth. In 2006, the International Astronomical Union adopted the infamous new definition of planet, which left out Pluto (now a dwarf planet):

  • Planets must orbit our Sun
  • Planets must be large enough that their gravity forces their shape to be spherical
  • Planets must clear their orbits of other material, so they are the largest body near their orbit

The prefix exo is short for extrasolar, meaning beyond the Sun, so exoplanets are just planets that orbit stars other than our Sun.


Planets in the Solar System
The planets in our solar system, to scale in terms of size, but not distance. Credit: WP via Wikipedia

As of October 1, 2020, we’ve discovered 4,354 exoplanets in 3,218 planetary systems. About 22% of those systems have more than one planet.

The main types of exoplanets we’ve discovered so far:

  • Gas Giants: large planets with thick gas atmospheres, like Saturn and Jupiter
  • Neptune-Like: ice giants with cold gas atmospheres, similar in size to Uranus and Neptune
  • Super-Earths: a mix of rocky and gas-shrouded exoplanets, bigger than Earth, but smaller than Neptune
  • Terrestrial: rocky, similar in size to our own Earth

You may have heard of Hot Jupiters, a commonly found type of exoplanet. They have the properties of gas giants, but orbit very close to their stars, giving them high surface/atmosphere temperatures. They also orbit around their star very quickly – in some cases, less than 1 day!

Another interesting example of exoplanets are known as lava worlds, Earth-sized exoplanets with extremely high temperatures. One example is known as Kepler-78b, which orbits its star at only 1% of the distance from Earth to our Sun! Kepler-78b’s surface temperature most likely reach higher than 3,680 °F / 2,000 °C!


Exoplanet Types
Credit: NASA

In our solar system, Earth is the only world that has been proven to have life. There may be life beneath the icy surfaces of moons orbiting Saturn and Jupiter, but we haven’t discovered it yet.

Do habitable exoplanets exist? We don’t know for sure, but there is a growing list of candidates!


Habitable Planets
The current list of potentially habitable exoplanets (as of October 5, 2020), ranked by distance from Earth (in light years). Credit: Planetary Habitability Laboratory, University of Puerto Rico at Arecibo

The closest discovered exoplanet which could possess life is just 4 light years away! Known as Proxima Centauri b, this Super-Earth is slightly larger than Earth and inhabits the closest planetary system to us! It orbits Proxima Centauri, a red dwarf star that is part of a triple star system called Alpha Centauri.


Earth, Proxima b
Artist’s impression of Proxima Centauri b as compared with Earth. This image shows the similar sizes of these worlds. Credit: PHL @ UPR Arecibo, NASA EPIC Team

If there are living beings on Proxima Centauri b that have come to consciousness like us, they just might be looking back at us with their own telescopes wondering, just like we do, if they are alone in the universe.


Recent Articles