A mission that nearly ended prematurely continues to reveal novel insight into distant star systems as well as our own solar system.
When the Kepler mission rocketed into space in 2009, it promised to seek out Earth-like worlds orbiting sun-like stars beyond our solar system. The mission was highly successful until reaction wheel failures in 2012 and 2013 called its future into question by limiting the space telescope’s ability to collect relevant data. Fortunately, since then the Kepler spacecraft has continued to uncover new exoplanets: worlds that orbits other stars in our galaxy.
Kepler identifies exoplanet candidates using the transit method: it searches for periodic dips in the brightness of a star that could indicate a planet is traveling in front of it. Scientists then analyze Kepler data to determine whether the object is actually an exoplanet rather than an astrophysical imposter mimicking the appearance and behavior of a planet.
A key goal of the Kepler mission is to identify roughly Earth-sized exoplanets in the habitable zone: those that orbit at a distance from their star that might allow liquid water to exist on the surface. Kepler has also identified a number of other exoplanet types, including ice giants as well as hundreds of gas giants that are similar to Jupiter. In fact, 1% of all exoplanets are believed to be gas giants, with some being warmer than Jupiter, some colder, some larger, and some smaller. One subset is known as “hot Jupiters”: gas giants that orbit their stars extremely closely—even closer than Mercury revolves around our sun. The orbits of these gas giants are so short that it takes less than 10 days for them to complete a full circuit. As a result of this close proximity to their stars, surface temperatures on hot Jupiters reach upwards of 2,000 degrees Fahrenheit. The planets are tidally locked to their parent stars, so that one side always faces the star while the other side is always dark, resulting in drastically different temperatures on each side. A recent study found that the scorching side of these hot Jupiters tends to be cloud free, while the darker, colder side is often heavily clouded.
Hot Jupiters present several mysteries to scientists, including the question of how a planet with an atmosphere similar to Jupiter’s could form so close to a star. Observations from space-based telescopes, including Kepler and NASA’s Spitzer Space Telescope, have led scientists to theorize that these gas giants may form far from their parent stars. Over time, the orbit of the Jupiter-like exoplanet is impacted by gravitational forces from nearby planets or stars, pushing the gas giant into an eccentric orbit that ultimately brings it extremely close to its own star. As time passes, gravitational and tidal effects from the parent star smooth out the orbit of the hot Jupiter. Once the orbit stabilizes, the exoplanet may revolve tightly around its star for billions of years.
Hot Jupiters are not the only Jupiter-like exoplanets that Kepler has spotted. “Warm Jupiters”—neither as cold as Jupiter itself nor as scorching as hot Jupiters—are another form of gas giant spied by the space telescope. These exoplanets orbit their stars at roughly the same distance as the terrestrial planets revolve around our sun. A recent study by astronomers from the University of Toronto, published in the Astrophysics Journal and based on an analysis of four years of Kepler data, found that there are two types of warm Jupiters. The first probably migrated into its current position, much like the hot Jupiters, while the second likely formed in its current position. Although it is not known how a gas giant could form so close to a star, the theory is grounded in the fact that this type of gas giant is accompanied by one or more smaller companion planets. This is in stark contrast to migrating warm Jupiters and hot Jupiters, which are isolated. For companion planets to remain so close to a gas giant, the warm Jupiter would have had to form near them from the start; otherwise, the gravitational effects of a migrating warm Jupiter would likely propel any nearby exoplanets completely out of the star system. Scientists continue to examine data from Kepler and its extended mission, K2, to learn more about the behavior of these exoplanets.
Given its unique vantage point, Kepler does more than hunt exoplanets: it frequently acts as witness to marvels occurring in our own solar system. In 2014, the Kepler spacecraft captured images of Comet Siding Spring shortly after its close flyby of Mars. Nearly two years later, in September 2016, Kepler provided a novel view of Comet 67P/Churyumov/Gerasimenko as it soared past, accompanied by the European Space Agency’s (ESA) Rosetta spacecraft. At that point, due to its proximity to the sun’s position in the sky, Comet 67P was not visible from Earth. From space, Rosetta captured local images of the comet, while Kepler offered a broader perspective on the gas and dust in the comet’s tail. Scientists will use data from the space telescope to assess how much mass the comet loses each day during its race through the solar system. This is just one example of the ways in which Kepler continues to contribute to our understanding of star systems and more, both near and far.
The Kepler and K2 missions are run by Ames Research Center for the Science Mission Directorate (SMD). Kepler mission development was managed by the Jet Propulsion Laboratory (JPL). The spacecraft’s flight system is operated by Ball Aerospace & Technologies Corporation in conjunction with the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.
Read an APPEL News article about a new statistical method that enabled scientists to more than double the number of verified exoplanets identified by Kepler.
Read an APPEL News article about software developed to delve deeper into the data produced by Kepler.
Learn more about clouds in the atmospheres of hot Jupiters.
View a video of a transit graphic depicting a planet moving across the face of its parent star.