By Emily Levesque, University of Washington
The transit method received a giant boost when the Kepler space telescope was launched by NASA in March of 2009. The telescope was designed to continuously monitor the brightness of stars in our local Milky Way neighborhood. From that data, astronomers could search for any telltale periodic drops in brightness from these stars and possibly reveal the presence of an exoplanet. How? Read on more to find out.
The Kepler Telescope
The method of detecting exoplanets based on transits proved to be immensely successful purely thanks to its efficiency. Instead of studying the motion of a single star, or the ticking of a single pulsar, the Kepler space telescope could stare at 150,000 stars simultaneously, quickly and efficiently building up an enormous library of stellar light curves for astronomers to analyze.
Named for Johannes Kepler, the famous astronomer who untangled the mathematics of planetary orbits, Kepler ultimately detected more than 2,500 exoplanets, and most exoplanets we know of today have been discovered using the transit method.
The Optical Gravitational Lensing Experiment
While it seems to be a more difficult approach, gravitational lensing has also been successfully used to detect planets. The Optical Gravitational Lensing Experiment, or OGLE, began in 1992 and used small ground-based telescopes to monitor hundreds of millions of stars in the center of the Milky Way and the nearby Magellanic Clouds. It specifically focused on distant crowded star fields to maximize the odds of spotting a microlensing event from a foreground star-and-planet system.
OGLE succeeded in 2004, detecting a planet one-and-a-half times as massive as Jupiter. It was seen orbiting a small orange star, a bit cooler than our own Sun.
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Star HR 8799
Thus, finally, astronomers successfully captured actual images of exoplanets orbiting other stars! The most spectacular example might be the star HR 8799, orbited by four planets. Using one of the Keck 10-meter telescopes, in Hawaii, astronomers were able to observe all four of these planets repeatedly over seven years, combining the observations into an incredible animation. In it one can see the shifting twinkling of the star’s light as it passes through our atmosphere, mostly blocked by the black central circle of the coronagraph. The four bright spot circling the star are the system’s planets! The speedy innermost planet, on the right, is a little closer to this star than Uranus is to our Sun; the faint slow-moving planet on the upper left, is tracing an orbit far bigger than that of Pluto’s.
An Accomplishment of Modern Telescopes
As we watch this, we’re seeing an entire new planetary system, orbiting a star not unlike our own Sun, with stunning clarity. It’s a sight that once used to solely belong to the imaginations of science fiction writers and movie makers, and an incredible accomplishment of modern telescopes.
The HR 8799 system is just one example of the many weird new planetary worlds we have to explore. Kepler-16 is a binary star system with a Saturn-sized planet orbiting both stars. A view from that planet might give us a real-life example of the famous binary sunset, seen from Tatooine, in the movie Star Wars.
Even our closest stellar neighbor besides the Sun, Proxima Centauri, is orbited by an exoplanet, one just 20% more massive than our own Earth. Proxima Centauri is only four light-years away; the odds of a star that nearby, with its own Earth-sized planet, seem almost too good to be true, yet there it is.
And yet, we’ve still only searched a tiny fraction of potential exoplanet host stars. This makes one wonder: Just how common are exoplanets?
Exoplanets Hospitable to Life
To conclude, detecting planets around other stars has gone from a science fiction pipe-dream to an everyday occurrence in astronomy. There are several compelling reasons to continue studying exoplanets in the hopes of understanding exactly how common they are, and exactly where they are. Some of these planets occupy what we call the ‘habitable zone’, a region where planets could, based on their distance from their host stars, be hospitable to life.
So, what does that mean for how we search for exoplanets and how we study them? It is important in the context that it would allow us to study the environments of distant planets, enabling us to determine whether these distant worlds are harboring life, and what—or who—might be living on them.
Today, astronomers can count more than 4,000 worlds orbiting stars light-years from our own sun, and that number is increasing every day. From this research, immense and detailed new subfields of astronomy have been born, focusing on a large-scaled census of exoplanets and detailed studies of this rich variety of newly-discovered distant worlds.
Common Questions about Modern Space Telescopes and Exoplanet Exploration
Named for Johannes Kepler, the famous astronomer who untangled the mathematics of planetary orbits, the Kepler space telescope ultimately detected more than 2,500 exoplanets, and most exoplanets we know of today have been discovered using the transit method.
Astronomers successfully captured actual images of exoplanets orbiting other stars. The most spectacular example might be the star HR 8799, orbited by four planets. Using one of the Keck 10-meter telescopes, in Hawaii, astronomers were able to observe all four of these planets repeatedly over seven years.
Proxima Centauri is orbited by an exoplanet, one just 20% more massive than our own Earth. Proxima Centauri is only four light-years away; the odds of a star that nearby, with its own Earth-sized planet, seem almost too good to be true, yet there it is.