Our understanding of the formation of planetary systems has historically been based on the observations about our own Solar System. A planet is a roughly spherical object orbiting a star that has sufficiently strong gravity to clear its orbital path of other debris. The four terrestrial planets (Earth, Venus, Mercury, and Mars) of our Solar System are located in the inner region relatively closer to the Sun, whereas the four giant, or Jovian, planets (Jupiter, Saturn, Uranus, and Neptune) are located in the outer region relatively farther away from the Sun. If our solar system is typical of other solar systems, then we can conclude that giant planets are formed and located in the outer regions of a planetary system, while terrestrial planets are formed and located in the inner regions.

An exoplanet is a planet outside our Solar System. Over 3500 exoplanets have been confirmed to date, and we believe that exoplanets are ubiquitous in the universe and that there are probably more exoplanets in the universe than stars. It is 100% certain that an average Solar-type star harbors at least one planet. Also, our current detection technology does not allow us to find smaller planets, such as planets the size of Mars and Mercury, but that may change with future observations. Many exoplanets have physical and orbital characteristics very unlike that of our own Solar System’s planets. They are so different in fact, that scientists have come up with distinct categories for these exoplanets.

Figure 1  (below) describes the following types of exoplanets alien to our Solar System:

• super-Earths: planets that are more massive than Earth, but considerably less massive than Neptune.
• mini-Neptunes: planets more massive than super-Earths, but still less massive than Neptune.
• super-Jupiters: planets more massive than Jupiter.
• hot-Jupiters and hot-Neptunes: planets as massive as Jupiter and Neptune with orbits smaller than that of Mercury — so they are much closer to the Sun, and therefore hotter.

In our Solar System, the terrestrial planets are much smaller than the Jovian planets. Earth is the largest terrestrial planet, but about 1000 Earths can fit inside Jupiter. Our classic understanding of planetary systems did not originally include the existence of “hot Jupiters” or giant planets found close to their host stars. These new types of exoplanets force us to revisit our understanding of the formation and evolution of planetary systems.

In our Solar System, Earth has comparatively less water than many other objects in our Solar System (see Figure 2 below). Imagine an exoplanet that is terrestrial in nature having significantly larger oceans than Earth. Density estimates allow us to predict the composition of some of the exoplanets, and we have discovered many water-worlds. How do you think that life would evolve on such a planet? Would it even be possible? If possible, would it be similar to anything we have experienced on Earth, or would it be truly ‘alien’?

These existential questions are just a few of the many exciting problems to ask when considering the full diversity of exoplanet sizes, temperatures, and water content. GJ 1214B is one example of a “water-world” exoplanet (illustrated in Figure 3 below). This exoplanet is about 7 times more massive than Earth and also has approximately at least 6000 times higher net water content than Earth. Imagine the possible diversity of ocean-life on this exoplanet. The only ocean-life we know is on Earth, but scientists are certain that some of the moons of Jovian planets have subsurface oceans. Such oceans are thought to exist underneath the surface of the moons and may harbor life. Space missions in the next two-three decades are likely to explore such oceans, for example, the subsurface ocean on Europa, the moon of Jupiter.

In this article, only a few types of exoplanets are described, but probably every single exoplanet has a unique set of characteristics unlike what we have witnessed in our Solar System. The existence of different types of exoplanets (see Figures 4 and 5 below) inspires us to explore the possible mechanisms that could have produced such an incredible diversity of exoplanets. In a follow-up post, I will describe a mechanism that can be responsible for producing the diversity of exoplanets.

Additional References:

1. Winn, J. N., & Fabrycky, D. C. 2015, ARA&A, 53, 409
2. Charbonneau, D., Berta, Z. K., Irwin, J., et al. 2009, Nature, 462, 891

Edited by Clara Boothby and Briana K. Whitaker