Exoplanets: Why study them? What are the challenges? What can they teach us about finding life beyond Earth?

Universe Today has explored the importance of studying impact craters and planetary surfaces and what these scientific disciplines can teach us about finding life beyond Earth. We learned that impact craters are caused by massive rocks that can either create or destroy life, and planetary surfaces can help us better understand the geologic processes on other worlds, including the conditions necessary for life. Here, we will venture far beyond the confines of our solar system to the many stars that populate our Milky Way Galaxy and the worlds they orbit them, also known as exoplanets. We will discuss why astronomers study exoplanets, challenges of studying exoplanets, what exoplanets can teach us about finding life beyond Earth, and how upcoming students can pursue studying exoplanets, as well. So, why is it so important to study exoplanets?

“There are a variety of reasons to study planets in different planetary systems,” Dr. Jason Steffen, who is an assistant professor of physics and astronomy at the University of Nevada, Las Vegas, tells Universe Today. “One is that the planets these systems contain are different from the planets in the solar system. They are different sizes, orbit at different distances, have different histories, and are likely made of different materials. This helps us understand more about our own origins by showcasing some of the different potential outcomes of the planet formation process.”

As of this writing, NASA has confirmed the existence of 5,569 exoplanets in 4,147 planetary systems, with an additional 10,059 being classified as exoplanet candidates, meaning their confirmation is still being investigated. To confirm an exoplanet, its discovery needs to be verified by two or more detection methods. So far, NASA has used five detection methods to locate and identify exoplanets, which include transit, radial velocity, gravitational microlensing, direct imaging, and astrometry. These methods are conducted using both space- and ground-based telescopes and instruments, with the transit method having confirmed the largest number of exoplanets to date at 4,151.

Examples of space-based telescopes designed to detect exoplanets include NASA’s Kepler/K2 space telescope and NASA’s Transiting Exoplanet Survey Satellite (TESS) missions. Kepler successfully operated between 2009 and 2018 and used the transit method to confirm more than 2,600 exoplanets within our Milky Way Galaxy. The TESS mission began in 2018 and is still active, having also used the transit method to identify close to 7,000 exoplanet candidates while confirming 402 as of November 2023. Examples of ground-based telescopes used for finding exoplanets include the MEarth Project, KELT Survey, HATNet Exoplanet Survey, and SuperWASP, with the number of exoplanets each having discovered being 3, 24, 134, and close to 200, respectively. But, with all these detection methods and tools, what are some of the challenges of studying exoplanets?

“One of the largest challenges in studying exoplanets is eliminating the effects of the host star so that we can really understand what the planets are telling us,” Dr. Steffen, who was also a science team member on the Kepler mission, tells Universe Today. “For example, we need to account for the oscillations in the stellar atmosphere, or star spots, or stellar flares, in order to detect new planets. We need to be able to eliminate the majority of the light from the star in order to see the light coming from the planet itself (which is usually a billion times dimmer). We need to know the size and mass of the star in order to properly determine the sizes and masses of the orbiting planets. We are fundamentally limited in our understanding of the planets that we detect by our understanding of the stars that the planets orbit.”

The Holy Grail of exoplanet hunting is discovering an Earth-like planet complete with liquid water and a habitable environment for life to both survive and thrive. For this, astronomers search within a star’s habitable zone (HZ) since Earth is within our Sun’s HZ, allowing the flourishing of life we see on our small, blue world. While astronomers have yet to definitively confirm the existence of an Earth-like exoplanet, there are currently 69 habitable exoplanet candidates out of the more than 5,500 confirmed exoplanets that could hold this honor. Of this 69, astronomers estimate that 29 are rocky worlds that could possess liquid water with the remaining 40 potentially having liquid water or could be mini-Neptunes but unlikely to possess habitable conditions for life.

One of the most notable exoplanetary systems is TRAPPIST-1, which is located approximately 41 light-years from Earth and boasts seven Earth-sized worlds with four currently hypothesized to be orbiting within its star’s HZ: TRAPPIST-1 d, TRAPPIST-1 e, TRAPPIST-1 f, and TRAPPIST-1 g. Other nearby and potentially habitable worlds include the super-Earth, Proxima Centauri b, which is located just over 4 light-years from Earth, and the super-Earth, GJ 1061 c, which is located approximately 12 light-years from Earth. So, with all these detection methods, tools, and potentially habitable exoplanets, what can exoplanets teach us about finding life beyond Earth?

Dr. Steffen humbly conveys to Universe Today, “This is a good question, it isn’t really in my area, but I’ll take a stab at it. While we haven’t seen signs of life on another planet, we’ve seen a wider variety of planets than originally thought. This exposes us to a variety of different, potentially habitable environments. For example, the environment around different stars, or with planets of different sizes or masses, or stars with different ages and temperatures.”

Dr. Steffen proudly tells Universe Today that his favorite exoplanetary systems are Kepler-9, Kepler-11, and WASP-47, with WASP-47 being his “favorite by quite a bit.” He states this is due to the system’s unique configuration in that it was the first exoplanetary system discovered to possess a “hot Jupiter” with smaller exoplanets orbiting both inside and outside its orbit. WASP-47 is located approximately 864 light-years from Earth and contains four exoplanets: WASP-47 b, WASP-47 c, WASP-47 d, and WASP-47 e.

The hot Jupiter Dr. Steffen refers to is WASP-47 b, whose radius and mass are both slightly larger than Jupiter. Its interior counterpart is WASP-47 e, whose mass is almost seven times larger than Earth with a radius almost double, and its exterior counterpart is WASP-47 d, whose mass is slightly more than 14 times that of the Earth with a radius approximately one-third of Jupiter. The fourth planet in the system is WASP-47 c, which orbits farther out than the other three and within the star’s HZ, but whose mass and radius are just slightly larger than Jupiter.  

Dr. Steffen adds, “This indicates that WASP-47 formed from a different mechanism than most of the other hot Jupiter systems. It probably formed by migrating to its current location within a disk, instead of being violently perturbed by another planet or star—which is likely the case for most hot Jupiters.”

Much like how Universe Today previously explored planetary surfaces, the study of exoplanets also involves a myriad of scientific backgrounds and disciplines to not only help us discover exoplanets more efficiently but to analyze and interpret the data to learn more about their compositions and potential for life. Since no exoplanets have yet to be directly imaged like planets within our solar system, researching them primarily involves computer models and data analysis.

With this, Dr. Steffen tells Universe Today that upcoming students who wish to pursue studying exoplanets should, “Learn to write your own computer software, learn the basics of statistics, and learn a wide variety of physics topics—they are almost all useful in one place or another.”

Dr. Steffen concludes by stating, “We still have a lot to learn about planets and the systems where they reside.”

What new discoveries will astronomers make about exoplanets in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!