By Dr. Gary Deel, Ph.D., J.D.
Faculty Director, School of Business, American Military University
The third of three articles on advances in the study of the Solar System
Unfortunately, Kepler was not capable of finding all of the potential exoplanets orbiting the stars it surveyed. This was primarily due to Kepler’s reliance on transit photometry. That meant that it could only detect exoplanets if they happened to be orbiting in an ecliptic plane around their host stars such that they would transit between Kepler and the host star.
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But if an exoplanet were to orbit in any other plane outside the range that would create a transit visible to Kepler, the telescope would not be able to find it. As such, Kepler’s efforts have been bolstered by other missions that use different types of exoplanet-hunting technology, such Doppler Spectroscopy.
When a Planet Orbits a Star, its Orbit Is Dictated by the Gravitational Pull from the Star
When a planet orbits a star, its orbit is dictated by the gravitational pull from the star. But because the planet also has mass, its gravity affects the host star’s movement in a reciprocal way. Accordingly, a planet doesn’t simply orbit a star. Rather, the planet and the star mutually orbit a common center of gravity between them. Think of two children of similar size interlocking their hands and spinning in circles, both moving around the center of the mass between them – their clasped hands.
Because stars are usually so much larger than the planets they host, the common center of gravity is generally much closer to the star than the planet. This is why planets appear to do all of the moving and stars don’t appear to move at all. Now, if you picture a mother and her child interlocking hands and spinning in circles, you will note now how the child moves much more than the adult due to his smaller mass relative to that of his mother.
Depending on the actual size difference, it might even look like the mother isn’t moving at all; instead, she appears to be just rotating in place while spinning her child around her. But she is moving. She’s moving around the common center of mass between them, however slightly. It’s just that her movement is less noticeable because the center of mass is so much closer to her than it is to her child.
Likewise, Stars also Move in Subtle Wobble Patterns around a Common Center of Gravity
Likewise, stars also move in subtle wobble patterns around the common center of gravity between them and their orbiting planets. And Doppler Spectroscopy uses highly sensitive detectors to measure the change in light wavelength emanating from a host star as it wobbles in its orbit around a common center of gravity with a planet.
When an object moves toward us or away from us, the light it emits or reflects in our direction changes in wavelength. Objects moving toward us become slightly “blue-shifted.” That is, the wavelength gets shorter and the light appears slightly more blue. Alternatively, objects moving away from us become slightly “red-shifted” as their wavelengths get longer and they appear a little more red.
The amount of blue-shifting or red-shifting varies with the speed of the object moving toward us or away from us. But the degree of change caused by the wobble as the star orbits a common center of gravity with a planet is imperceptible to the naked eye. Only precise scientific instruments, such as the High Accuracy Radial velocity Planet Searcher (HARPS) in the European Southern Observatory, can detect this subtle change. When a fluctuation repeats on a regular basis — as would be expected from a planet orbiting its star — it is an indication that an exoplanet is present.
Notwithstanding Kepler’s limitation in being able to find only exoplanets that transit across the face of distant stars, its discoveries have nonetheless changed the way we view our universe, the prevalence of exoplanets, and the search for life in other parts of our galaxy.
However, Kepler was so successful that, six months before its decommission, NASA launched the Transiting Exoplanet Survey Satellite (TESS) as a sequel mission to Kepler. TESS uses the same transit approach taken by Kepler to look for exoplanets, but its equipment is more sophisticated and it is able to search an area of the night sky 400 times larger than Kepler could.
Now less than two years into the TESS mission, it has already found more than 1,200 exoplanet candidates, and it is expected to find more than 20,000 in total during the course of its mission life. Collectively, Kepler, TESS, and other exoplanet missions to date have discovered more than 4,000 exoplanets orbiting more than 3,000 stars.
With over 100 billion stars in the Milky Way galaxy, these searches won’t be ending anytime soon. However, we shouldn’t forget the critical contributions that Kepler made in this effort, and how it revolutionized our understanding of our place in the cosmos.
Thanks to Kepler, we now have a keen understanding of just how replete with exoplanets our universe actually is. We also know a great deal about their numbers, sizes, colors, compositions, temperatures, and suitability for life.
Kepler moved the bar forward a great deal on the frontier of exoplanet science. It laid a strong foundation on which future missions such as TESS will continue to build. In spite of its limitations, Kepler will forever carry a strong legacy as a major milestone in astronomical research.
About the Author
Dr. Gary Deel is a Faculty Director with the School of Business at American Military University. He holds a J.D. in Law and a Ph.D. in Hospitality/Business Management. Gary teaches human resources and employment law classes for American Military University, the University of Central Florida, Colorado State University and others.