Exoplanets, also called “extrasolar planets” are worlds orbiting other stars. Thousands of possible exoplanets have been found through ground-based and space-based observatories. Nearly 2,000 have been confirmed; the rest are awaiting further observations so that astronomers can be sure that they are planets. Astronomers estimate that there could be trillions of planets around other stars.

Facts about Exoplanets

  • The known exoplanets fall along a range of sizes, masses, and orbital positions. Sizes and masses range from smaller and less massive than Earth to super-Jupiter types of worlds. Orbital positions range from very close to the parent star to very distant.
  • Astronomers are starting to find and measure atmospheres around distant exoplanets. This allows them to understand what gases exist in those gaseous envelopes.
  • Among other characteristics, astronomers can measure the surface temperatures, orbits, magnetic fields, and colors of exoplanets. As detection methods improve, they will be able to find out more about distant worlds.
  • At least one exoplanet has been found to have an exomoon, while another one is leaving behind a trail of material as it vaporizes while orbiting too close to its star.
  • The region around a star where liquid water could exist on the surface of a solid planet is called the habitable zone. Worlds orbiting in that zone are considered to be prime candidates where life could be supported.
  • More than 22 percent of Sun-like stars have Earth-sized planets in their habitable zones. These are important places to concentrate a search for possible life-bearing worlds.
  • The Kepler Mission was launched to search out distant worlds. It continues its search today. Other missions that have found distant worlds include the Hubble Space Telescope, the CoROT mission from the European Space Agency, the WISE mission, and the Herschel spacecraft. Ground-based observatories continue to be an important part of the search for distant worlds.

Types of Exoplanets

Astronomers group types of exoplanets as follows: Earth-size, Earth-like, Super-Jupiters, gas giants, rocky worlds the size of Earth, rocky giants, Super-Earths, mini-Neptunes, and gas dwarfs.

Earth-size and Earth-like exoplanets are those that are similar in size to our planet, while Earth-like planets have several characteristics in common with our planet, including similar atmosphere and possible liquid water on the surface. These include rocky Earth-size worlds and rocky giants.

Super-Earths are Earth-type planets that are larger than our home world, and contain more mass than Earth does. They are less massive than any gas or ice giants that happen to be in the same system. The first super-Earth was found orbiting the pulsar 1275+12 in 1992.

In the realm of gas giant exoplanets, super-Jupiters are those giants that are much larger than Jupiter. Mini-Neptunes are often referred to as gas dwarfs, and are usually smaller than Uranus or Neptune. They can have up to 10 times the mass of Earth and have very thick atmospheres.

Examples of Exoplanets

Gamma Cephei Ab: The first exoplanet detected, found in 1998 around the star Gamma Cephei. It was not confirmed until 2003, when better detection techniques were developed.

PSR 1267+12 B and C: The first pulsar planets. These were found in 1992, orbiting the rapidly spinning remains of a massive star that exploded as a supernova. Astronomers found a third planet in this system, and are still trying to figure out when those planets formed and how they survived the supernova explosion. One of the planets is a Super-Earth.

51 Pegasi b: The first planet around a star like the Sun. Astronomers found it using the Observatoire de Haute-Provence in France, a ground-based facility. This planet is also known as a “hot Jupiter” because it appears to be a very warm gas-giant-type world.

Kepler 186f: the first Earth-size planet circling in the habitable zone of its star. Found by the Kepler Mission in 2014.

Kepler 11-f: orbits a Sun-like star and has at least 2.3 times the mass of Earth. It may be a gas dwarf, due to its low density and possible hydrogen-helium atmosphere.

Mu Arae c: a hot Neptune type planet orbiting very close to its parent star, Mu Arae. This is the first hot Neptune discovered.