What Are Exoplanets?

Planets in the habitable zone or "sweet spot" are in orbit at a very specific distance from their stars. The habitable zone is the range of distances between a planet and star that allow life to exist. Exoplanets in a habitable zone have suitable climates for water to exist as a liquid and form oceans. Calculations to determine the habitable zone for a specific exoplanet are based on the exoplanet's distance from its star. Other factors, such as the exoplanet's atmosphere and the greenhouse effect, are also taken into account.

Exoplanets are difficult to detect with a telescope. Glare from the star obscures the view of orbiting planets. Astronomers look for exoplanets indirectly by observing effects on their stars. One common indirect method of detection is Doppler spectroscopy. This method is also known as the radial velocity or wobble method. A star with orbiting planets does not have a perfect orbit because the planets pull on the star. The star's orbit is off-center and makes the star look like it is wobbling.

One of the first exoplanets discovered with the wobbly method was found in 1995. It is a large, hot planet approximately half the size of Jupiter with a very fast 4-day orbit. The combination of the exoplanet's rapid orbit and immense size exerted enough force on the star to make the star's wobbling appearance very obvious. The wobble method measures changes in a star's radial velocity to estimate the size of an orbiting planet.

The exoplanet discovered in 1995 is called 51 Pegasi b but is now known as Dimidium. It is 50 light-years away from Earth in the Pegasus constellation. The discovery of Dimidium was a breakthrough for astronomers because it was the first exoplanet found orbiting a star, 51 Pegasi, that is similar to our Sun. Dimidium is the prototype for the class of planets labeled "hot Jupiters."

NASA launched the Kepler Space Telescope in 2009 as a space observatory to find exoplanets outside of our solar system. The main focus was finding exoplanets similar to Earth. The Kepler Space Telescope was in operation for nine years and found 2,682 confirmed exoplanets. Scientists are still working on confirming another 2,900 possible planets found by Kepler.

Kepler detected exoplanets with the transit method. Stars appear to "dim" when an orbiting planet passes between the star and Earth. Each passage of the planet between the star and Earth is called a transit. The transit method detects exoplanets by measuring the dimming effect. The presence of an orbiting planet is suspected when dimming occurs at regular intervals.

NASA's Spitzer Telescope is an infrared space telescope launched in 2003. Observations from the Spitzer Telescope initiated a huge step forward in planetary science. Spitzer can detect light on planets outside of our Solar System. It is the first instrument capable of direct observation of exoplanets instead of the indirect wobble or transit methods. Direct observation lets scientists study and compare exoplanets. The infrared observatory also helps scientists determine temperature, winds, and the composition of the atmosphere on distant exoplanets.

Most exoplanets have been discovered through indirect imaging, but relatively recent direct imaging methods are superior in many ways. False positives are rare using direct imaging methods, while the transit method has a false positive rate of approximately 40%. Exoplanets detected with the radial-velocity, or wobble, method require extensive follow-up by astronomers to confirm the presence of a planet. Direct imaging also provides information that scientists use to estimate a wide range of planetary conditions.

The exoplanet WASP-12b was found by the SuperWASP planetary transit survey in 2008. It is an important discovery because WASP-12b is being consumed by its host star. Astronomers watch the process to learn more about the formation and dissolution of planets. The destruction of a planet by its host star is actually a very slow process. Astronomers estimate that it will take approximately 10-million more years for WASP-12b to completely disintegrate.

Gliese 436 b is an enormous exoplanet in the Leo constellation. It is also providing astronomers and other scientists with new knowledge. Gliese 43 b is almost as large as Neptune, and it is covered in burning ice. The extreme pressure and temperatures above 570°F on Gliese 43 b create a unique environment that keeps water in a solid form when it should be vaporized.

There are currently 16 known exoplanets with a high probability of sustaining life. Another 33 exoplanets may have the conditions necessary for life to exist, but scientists are still evaluating them. The exoplanets HD 85512 b, Kepler-69c, and Tau Ceti f were considered habitable at one time, but updated habitable zone models and new observations have shown they cannot sustain life. HD 85512 b and Tau Ceti f are actually outside of their respective habitable zones, and Kepler-69c has an atmosphere and landscape similar to Venus.