What Are Exoplanets?
An exoplanet is any planet that is not in our solar system. The majority of exoplanets orbit stars in their own solar system. But there are a few free-floating exoplanets, called rogue planets, that are not gravitationally tied to a star and instead orbit the galactic center.
While the first evidence of exoplanets dates back to 1917, the first confirmed detection was not until the 1990s and we have since discovered thousands. So far, most the exoplanets that have been discovered are in a relatively small region in the Milkey Way.
How Do Astronomers Find Exoplanets?
There are five main methods astronomers use to find exoplanets. The two most prominent are the transit and the radial velocity method.
- Transit (Searching for Shadows) – If there is a planet orbiting another star, it will eventually pass between its Sun and us, the observer. When this happens the stars light ever so slightly dims as the planet blocks a portion of the light from reaching us. This is how astronomers can tell there is a planet orbiting another star.
- Radial Velocity (Watching for Wobble) – Though planets are relatively small compared to most stars, their gravity can still impact their star. As the planet orbits the star it can cause the star to slightly wobble. This changes the light we see coming from the star. If the star wobbles towards us its light will appear more blue. If it wobbles away from us, it appears more red.
- Direct Imaging (Taking Pictures) – This method is self-explanatory and is how we view the planets in our own solar system.
- Gravitational Microlensing (Light in a Gravity Lens) – We know that objects that have mass have gravity. This gravity allows it to warp the space around it. Light flows and moves with these warps. If there is a planet it warps the space around it as it orbits its star. The light from this star then bends with the warp as the planet passes by. We can measure this distortion and know there is a planet.
- Astrometry (Minuscule Movements) – Similar to radial velocity, with this method astronomers are watching the wobble of a star. But with astrometry they watch how the star’s wobble causes it to change position relative to other stars.
These methods are quite complex so please feel free to watch these great animations to help you understand how the planets and stars interact with each other.
NASA’s Exoplanet Space Telescopes
NASA has multiple telescopes that contribute to the search for exoplanets. The Kepler Space Telescope was specifically designed to search our region of the Milkey Way for Earth-sized and smaller planets near what is known as the habitable zone. The Habitable zone, also known as the “Goldilocks Zone”, is the area around a star where a rocky planet could sustain liquid water on the surface.
If there is sustained liquid water, there is the possibility for life. Earth is in the habitable zone around our Sun. The Kepler mission specifically looks for these zones and planets and determines the fraction of stars that might have such planets. While Kepler was retired in 2018, all the data being collected is still being analyze and used to find new exoplanets. So far more than 2,700 exoplanets have been confirmed from Kepler data.
NASA’s Spitzer Space Telescope is an infrared telescope and was not designed to hunt for exoplanets. However, its infrared instruments turned out to be a great tool in the search for exoplanets. It was a key player in the discovery of the TRAPPIST-1 system. The Transiting Exoplanet Survey Satellite (TESS) was launched in 2018 to succeed Kepler.
Its target is to look for planets around bright dwarf stars, the most common type of star in the Milky Way. The next generation of exoplanet observatories are the James Webb Space Telescope (JWST), launched 2021, and the future Nancy Grace Roman Space Telescope (NGST). While JWST’s main focus is not to hunt for exoplanets it is a key part of why it was built.
What Types of Exoplanets Are There?
By measuring exoplanets’ size, mass, temperature, and many other properties, we can tell lots of things about them. We can see a range in compositions from rocky Earth and Venus like planets to gas-rich exoplanets like Jupiter and Saturn.
Exoplanets are made up of many of the same elements we find in our solar system and that make up our planets, but the mixes of elements can differ greatly. So far, we have found a wide variety of planets from worlds covered in seas of lava to puffy planets like Styrofoam.
Astronomers have currently grouped exoplanets into four types: terrestrial, super-Earth, Neptune-like, and Gas Giants.
- Gas Giants – These planets are very large and comprised of mainly helium and/or hydrogen. These planets are similar to our Jupiter and Saturn where they do not have hard surfaces but have large clouds and storms above their small solid core. Some gas giant exoplanets can be much larger than Jupiter and are found closer to their stars than we would find in our solar system. There are 1,605 confirmed gas giant exoplanets.
- Neptune-Like – These exoplanets are similar in size to Uranus and Neptune. Neptune-like exoplanets typically have rock or a heavy metal core with an atmosphere dominated by helium and hydrogen. There are 1,801 confirmed Neptune-like exoplanets.
- Super-Earths – Unlike any planet we see in our solar system, super-Earths are more massive than Earth but lighter than our ice giants Neptune and Uranus. Super-Earths can be made of rock, gas, or a combination of the two. They are between 2-10 times the size of Earth. There are 1,586 confirmed super-Earth exoplanets.
- Terrestrial – In our solar system, Mercury, Venus, Earth, and Mars are considered terrestrial, or rocky, planets. Exoplanets get classified as terrestrial if they are rocky and are between one half to two times the size of Earth. There are 190 confirmed terrestrial exoplanets.
While we are not the first generation of humanity to believe in the possibility of life outside our solar system, we are the first to have the technology to one day maybe discover if there is life on another planet.