Johannes Kepler and the telescope that bears his name

An artist concept of the Kepler Space Telescope [1]

    For many of us who have grown up reading and watching science fiction, the idea that there might be places in the universe other than Earth that are able to support life is nearly a given. Do these places resemble Earth, or are they more exotic than we thought possible? Surely, there must be some place out there where other creatures look out from their planet and imagine what we might be like. Thanks to telescopes like Kepler, we are able to observe and collect data about a great number of planets outside of our solar system, also known as "exoplanets".

    The Kepler Space Telescope was launched in early 2009 with the goal of constantly observing over 170,000 stars simultaneously in a particularly dense patch of the constellation Cygnus. Rather than attempting to directly observe an exoplanet - a task only possible in a very limited number of cases - Kepler precisely recorded the brightness of each of those stars and watched for any points where they appeared to get dimmer. When an object passes between us and any celestial body, we call this a "transit"; these dimming events observed by Kepler correspond to transits of some object between us and a distant star, blocking some portion of the light based on the size of the object. If this happens periodically, the object is very likely orbiting that star [2]. 

A video visualizing the transit method of exoplanet detection [3]

    If we can observe at least three transfers, we can not only confirm the existence of an exoplanet, we can also learn the orbital period - how long it takes that planet to orbit the parent star. Once we know the orbital period, we can use information obtained about the mass of the star to calculate the average distance that planet is from the star, also called the semimajor axis. Now, we have an idea about the type of planet that we're looking at - is it the size of Venus or the size of Saturn? Does it orbit closely to the star, like Mercury, or far away like Neptune? Since we know the size and the distance at which it orbits, we can even begin to catalogue planets that might be able to support life based on their temperature. Specifically, we can find planets that should be able to retain liquid water on their surface, which we believe to be one of the most fundamental requirements to sustain life. 

    

    You might be inclined to wonder how we were able to determine all of that from just a slight, periodic dimming of a tiny light an extreme distance away. It turns out that all we need to know to get an idea of the distance a body is away from the object it orbits is the period of the orbit and the mass of that central object. This is often referred to as Kepler's third law (sometimes called the "harmonic law"), which is a statement of proportionality between the square of the orbital period and the cube of the semimajor axis. When generalized to any star system (rather than just our solar system), it takes the form:

    

 [4]

Where "P" is the orbital period, "a" is the semimajor axis, "G" is the gravitational constant, and M1 + M2 represent the combined masses of the star and planet (which can often be simplified to just the star). However, Kepler did not approach this finding theoretically; that would be left for Isaac Newton to accomplish later and codify as the "laws" we know today [5].

An oil painting of Johannes Kepler, located in Strasbourg, France [6].

    Johannes Kepler was born in 1571, a little over 70 years before Isaac Newton. He had originally planned to become a theologian (a religious expert), though he was quickly recognized for his mathematical abilities and was sent to the University of Tübingen where he met Michael Maestlin, a talented and prominent astronomer. Here, Maestlin would introduce Kepler to his privately-held belief in Copernican Theory - the idea that the Sun was in fact the center of the solar system. Kepler became determined to rigorously prove Copernicus's ideas [5][7].

    Before completing his theological studies, he accepted a position as a professor of mathematics at a different school, where he would produce his first major astronomical work Mysterium Cosmographicum. This brought him the fame required to work under the renowned Tycho Brahe, a Danish astronomer with an exceptionally large and detailed catalogue of observations. When Brahe died suddenly in 1601 (only a year after they had met), Kepler found himself not only his successor, but also the inheritor of the entire catalogue. Using the extreme precision of these observations, Kepler would publish the findings that would become his first two laws. The first of these is the understanding that planets move along elliptical orbits with the Sun at one focus, rather than the completely circular orbits previously argued by even Copernicus himself. The second law was the fact that the area swept out by the line drawn between a planet and the Sun was equal at any two points in an orbit over the same duration in time; this can practically be thought of as stating that planets did not always move at the same orbital velocity, but that the velocity would change based on the current distance to the Sun. Kepler's third law would not be published for another 10 years, and it would be another 10 beyond that before he would carefully detail and discuss them in his work Epitome. Though necessary realizations, Kepler's first two laws had radically diverged from Copernican thought, but the third law had provided him with the systematic defense of the theory itself that he had been seeking - that the planets and their motions were not perfect and independent of each other, but were instead integrated into a harmonic system [5][7].

References:

[1] - NASA. (n.d.-a). Kepler / K2 - Nasa Science. NASA. https://science.nasa.gov/mission/kepler/

[2] - Fentress, S. (2018, December 7). Kepler Space Telescope: The original exoplanet hunter. Space.com. https://www.space.com/24903-kepler-space-telescope.html

[3] - NASA. (n.d.-b). Transit method - NASA science. NASA. https://science.nasa.gov/mission/roman-space-telescope/transit-method/

[4] - Kepler’s third law. Imaging the Universe - Physics and Astronomy | The University of Iowa. (n.d.). https://itu.physics.uiowa.edu/glossary/keplers-third-law

[5] - Encyclopædia Britannica, inc. (2024, November 11). Johannes Kepler. Encyclopædia Britannica. https://www.britannica.com/biography/Johannes-Kepler

[6] - Wikimedia Foundation. (2024, November 30). Johannes Kepler. Wikipedia. https://de.wikipedia.org/wiki/Johannes_Kepler

[7] - Di Liscia, D. A. (2021, September 17). Johannes Kepler. Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/entries/kepler/