"Studying Worlds Beyond our Solar System: How and Why We Find Exoplanets" By William Cerny
On October 30th, 2018, after almost a decade of scouring the skies for exoplanets, NASA’s Kepler Space Telescope ceased its operations after running out of the crucial hydrazine fuel it was using to keep itself pointed on target. Tasked with scanning a particular region of the Milky Way, this long-running scientific mission discovered a staggering 2,662 extrasolar planets, provided data for nearly 3,000 scientific papers, generated new planetary taxa, challenged our planetary formation models, and gave us visions of worlds so exotic and bizarre they almost defy possibility. While Kepler’s mission may be over, its immense trove of data will be a source of discoveries for years to come, and its successor orbital and ground-based telescopes will undoubtedly conduct pioneering research in the future.
What exactly made Kepler’s discoveries so groundbreaking? Detecting exoplanets on their own, a technique known as ‘direct imaging,’ is exceedingly tough, even equipped with the world’s most sensitive instruments. This is mainly because exoplanets are many orders of magnitude smaller and dimmer than their host stars, and at interstellar distances their orbital distances appear lost in the star’s glare. The task is so difficult, in fact, that less than a quarter of a percent of all exoplanets discovered to date have been directly imaged.
Because of this immense challenge, scientists have turned to more oblique methods, including the technique employed by Kepler. By regularly monitoring and analyzing the apparent brightness of a distant star over time, researchers can watch for periodic dips in brightness that represent a passage or ‘transit’ of an exoplanet in front of it, as seen from a telescope. This method is limited in that it can only see planets that orbit edge-on to an Earth-based observer, but it has nevertheless proven extremely successful and can yield reliable estimates of an exoplanet’s size – though disentangling potential transits from starspot activity can also pose a challenge.
Another method relies on looking for the wobble or change in ‘radial velocity’ caused by the gravitational influence of a planet on its host star. Thanks to relativity, this wobble creates a Doppler shift in the star’s emitted light. Spectroscopy, the study of splitting of light into its component wavelengths, can then be used to detect the changes in light. This method has also proven successful and has led to the detection of almost 700 exoplanets to date. An additional benefit of this method is that it can put constraints on the mass of an exoplanet. This information, along with size estimates from transit data, can be used to approximate a planet’s density and composition.
Another detection technique in use today is called gravitational microlensing. Just like a magnifying glass, gravity can bend and concentrate the path of light as it passes near massive objects. If a star serendipitously passes in front of another from an observer’s viewpoint, the foreground star can considerably magnify the background star’s light. The same magnification occurs if there are planets orbiting the foreground star. Although this method bears comparable flaws to the direct detection methods, it has been demonstrated to be sufficient, and has led to the detection of about 70 exoplanets.[1}
Naturally, this all begs the question: why go to all this effort to learn about planets trillions of kilometers away? And what have we learned from studying them?
The main focus of the massive hunt for exoplanets is perhaps the most inspiring: to find planets beyond our Solar System that might potentially host life. Crucial to determining whether an exoplanet could host life is the planet’s position in the so-called “Goldilocks Zone,” the range of distances a planet can be from its star to have a chance at possessing liquid water, which is vital for all carbon-based life. To date, the Kepler Space Telescope has tentatively identified 361 exoplanets as candidates due to their position in their respective star’s habitable zones. Naturally, there are many other factors critical to supporting a hospitable planetary environment, including a strong magnetic field to protect from dangerous quantities of stellar radiation, the presence of an atmosphere, and the relative stability of the planet’s host star.
Some scientists have proposed an “Earth Similarity Index” to assess the viability of these candidates to support life. This index sorts planets based on the closeness of their orbital distances to that of Earth’s and the energy flux of their host star compared to the Sun. Kepler-438b, a planet at the top of the list, has a radius equivalent to 1.1 Earth radii, a mass expected to be between .6 and 4 times the mass of the Earth, and an equilibrium temperature within 30 Kelvin of Earth’s. These values are all within ranges consistent with our understanding of the conditions that which allow for development of life. Further study of these distant worlds will expand and refine the pool of candidates that might potentially bear life.
Studying these thousands of exoplanets isn’t limited to looking for life. Research into these diverse worlds has fundamentally reshaped our understanding of planetary classification and distribution and continues to challenge our existing models of Solar System formation. For example, the discovery of a significant population of large gas-giants that orbit far closer and faster around their host stars than Mercury, our own innermost planet, demanded the creation of the new planetary classification ‘Hot Jupiters.’ These planets have called into question theories about stellar system formation, many of which suppose that extreme temperatures in star formation evaporate the atmospheres of inner planets, leaving primarily heavier elements such as Iron and Silicon to dominate their compositions. Discovery of another population of planets, now dubbed ‘Mini-Neptunes,’ has also indirectly raised new questions, as scientists uncovered a statistically significant ‘gap’ in the frequency of planets with radii more than 1.6 times that of Earth’s but smaller than those of these Neptune-like gas worlds. This presents a conundrum in explaining how both rocky and gaseous planets form. While these new taxa are just two examples of the striking scientific insights to be gained from studying exoplanets, many ongoing and future projects will undoubtedly produce novel discoveries that will further challenge our assumptions about the universe and provide a window into these distant worlds.
 “Exoplanet and Candidate Statistics.” NASA Exoplanet Archive, NASA/Caltech, Nov. 2018, exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html.
 Chen, Rick. “Kepler By the Numbers – Mission Statistics.” NASA, 26 Oct. 2018, www.nasa.gov/kepler/missionstatistics.
 “Earth Similarity Index (ESI).” Mapping the Habitable Universe, Planetary Habitability Laboratory @ UPR Arecibo, Jan. 2018, phl.upr.edu/projects/earth-similarity-index-esi.
 Cartier, Kimberley. “The Kepler Revolution.” EOS, 1 Aug. 2018, eos.org/features/the-kepler-revolution.