By Dr. Gary Deel, Ph.D., J.D.
Faculty Director, School of Business, American Military University
The second of three articles on advances in the study of the Solar System
NASA launched the first-of-its-kind Kepler Space Telescope in 2009 to hunt for exoplanets in the Milky Way galaxy.
Kepler showed enormous success right out of the gate, discovering its first exoplanet within the first few months of operation. And by 2014, just five years into the mission, Kepler had already amassed a catalog of more than 700 exoplanets orbiting more than 300 different stars.
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In addition to detecting the existence of exoplanets, Kepler was also able to use the data collected on changes in light emissions to deduce the sizes of those exoplanets.
If a small planet transits in front of its star, it blocks only a little bit of light. But if a larger planet transits across the star, it blocks more light. Instead of an ant on the flashlight lens we discussed in the analogy in Part I, imagine something larger, like a cricket. By measuring with precision the difference in light emission during an exoplanet’s transits, Kepler was able to determine the size of the exoplanet. In doing so, Kepler found exoplanets ranging in size from those smaller than the Earth to some larger than Jupiter.
Kepler Enabled Astronomers to Speculate about Which Exoplanets Were Gaseous and Which Were Rocky in Composition
With this information and our knowledge of the planets in our own solar system, Kepler also enabled astronomers to speculate about which exoplanets were gaseous and which were rocky in composition. Curiously, Kepler also discovered what is today called the “Fulton Gap,” an apparent rarity among exoplanets with sizes between one-and-a-half and two times that of Earth. We still do not have an explanation for this disproportionality in exoplanet size.
Studying the photometry data from Kepler, experts could also work out rough approximations of the colors of different exoplanets. Different colors and albedos reflect different levels of light. The details of the light observed from exoplanets in their orbits — particularly when the sides that face Kepler are illuminated by their host stars — can be used to speculate about exoplanet color. In 2014, exoplanets HD 189733b and GJ 504 b were the first to have their colors extrapolated. The former is a deep blue color and the latter is a magenta color.
Kepler was also able to deduce the distance from which a planet orbits its host star by measuring the time intervals between transits. We know that the orbital speed of an object depends on how far the object’s orbit is from its center of rotation. For example, in our own solar system Mercury orbits the Sun the fastest because it is closer to the Sun than the other planets. By contrast, the gas giants in the outer solar system orbit more slowly the further out they are.
Scientists Can Use the Known Speeds of Orbiting Exoplanets to Determine Their Orbital Distances
With this understanding, scientists can work backwards and use the known speeds of orbiting exoplanets to determine their orbital distances. So, for example, suppose an exoplanet orbits its host star at a rate of one revolution every 365 Earth days. If the host star is similar in mass to our own Sun, we can conclude that such a planet orbits its star at a distance of roughly one astronomical unit (AU), the same distance from which Earth orbits the Sun.
Using the exoplanet transits, Kepler also allowed for some inferences to be drawn about orbital eccentricities. These were based on changes in speed within a single orbit. Orbital eccentricity is a measure of how circular or elongated an orbit is. Of the exoplanets that have so far been catalogued, it appears that most exoplanets with circular orbits reside very close to their host stars.
The Majority of Exoplanets Surveyed Have a Greater Number of Orbital Eccentricities
In comparison to our own solar system, where most planetary orbits are fairly circular, the majority of exoplanets surveyed thus far have higher orbital eccentricities. Many astrobiologists believe that a circular orbit is an important component to life on a planet because a consistent distance from the host star stabilizes temperatures. So exoplanets with high orbital eccentricities are not particularly good candidates for potential life.
Finally, Kepler produced one more bit of information about the exoplanets it found. Knowing the likely composition of an exoplanet, the distance from which the exoplanet orbits its host star, and the brightness of the star itself, experts can make educated guesses as to the surface temperatures on the exoplanets.
For example, we know that our own Sun creates different temperatures on the planets in our solar system based on their distance from the Sun. So if we know the orbiting distance of an exoplanet, we can estimate its surface temperature.
Of course, the mass and brightness of other stars — which translate into heat emitted — can be very different from that of our own Sun. So temperatures of exoplanets orbiting their host stars might be very different from those of planets orbiting the same distance from the Sun. But again, experts can reconcile these differences using mass and brightness observations, and careful calculations.
Some Exoplanets Are Frigid and Icy While Others Are Boiling Hot or Even Molten
In doing so, experts have determined that some Kepler exoplanets are frigid and icy while others are boiling hot or even molten. Kepler also found that more than one in every five stars surveyed hosts an Earth-size planet within their habitable zones — the ranges of orbital distances where exoplanet surface temperatures would allow for the presence of liquid water. This is obviously promising news for the prospect of finding life outside our solar system.
It is also worth noting that some exoplanets might have surface characteristics that could result in unexpected surface temperatures, notwithstanding our measurements of orbital distance and host star mass and brightness. For example, in our own solar system the hottest planet is Venus, the second planet from the Sun.
This is counterintuitive as one would think that Mercury would be hottest, being closest to the Sun. However, Venus suffers from a kind of runaway greenhouse gas effect. Its atmosphere is engorged with carbon dioxide and other gases that trap the Sun’s heat within, causing the planet’s surface to bake at over 900 degrees Fahrenheit.
There is no way to know from the Kepler data if any of the discovered exoplanets have atmospheric characteristics similar to Venus, but future missions might be able to follow up on Kepler’s findings and study the atmospheres of exoplanets using light spectroscopy.
Unfortunately, despite its best efforts, Kepler was not capable of finding all of the potential exoplanets that might be orbiting the stars it surveyed.
In the final part of this series, we’ll examine Kepler’s primary limitation, and the space telescope’s overall contribution to space exploration.
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.