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How Do We Find Exoplanets?

Astronomers have found more than 3700 worlds outside of our solar system in the past 25 years. How do they find them?

By
Sabrina Stierwalt, PhD,
March 5, 2018
Episode #276

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image of an exoplanetAstronomers have discovered more than 3700 exoplanets, or planets outside of our solar system, including over 600 multi-planet systems, with another nearly 5,000 candidate dedications awaiting confirmation. These exoplanet discoveries include massive planets similar to our Jupiter, smaller Earth-sized planets, and even a solar system home to 7 rocky planets similar in density to Earth, Venus, and Mars.

Many of these discoveries owe thanks to NASA’s Kepler mission and its successor K2, a project surveying our galactic neighborhood for small, Earth-sized planets using a dedicated space-based telescope. NASA’s next major exoplanet hunting effort, called TESS, short for Transiting Exoplanet Survey Satellite, will survey over 200,000 nearby stars and is expected to find thousands of new worlds. TESS is set to launch sometime after April 2018 on the SpaceX Falcon 9 rocket.

So how do astronomers find exoplanets, and how much do we actually know about these worlds outside of our solar system?

Transiting Exoplanets

The method used by both the Kepler and TESS space-based missions, and thus the method that has led to the most exoplanet discoveries, is the transit method of exoplanet detection. When a planet passes in between us and a star, the planet is said to be transiting. In our own solar system, we can view transits of Mercury and Venus when their orbits bring them in between us here on Earth and our Sun.

Even though planets are small relative to stars, when the planet’s path moves across our view of its host star, the planet blocks a small amount of light coming from that star. Astronomers can measure the dip in the brightness of the light coming from the star when the planet is in front of it as well as the length of time over which the dip in brightness is observed. Knowing the mass of the star, and with a little help from Kepler’s Third Law of planetary motion, these two measurements can tell you how far away the planet is from its host star. The size of the planet’s orbit is key for getting a first estimate of whether the temperature there might be suitable for liquid water and thus life. Astronomers can also deduce the size of the planet itself if they have an estimate of the host star’s size.

Since we do not know beforehand which stars have planets around them and also which stars have the perfect alignment so those planets will pass in between us and their host star as they orbit, nor can we predict when these transits might happen, using the transit method to find exoplanets requires continuous monitoring of hundreds of thousands of stars. Thus, NASA has invested in dedicated space telescopes like TESS and Kepler that devote their time to this kind of monitoring.

Radial Velocity Method

Even though the transit method for finding exoplanets is more commonplace now thanks to these planet-hunting space missions, the first surge in exoplanet detections were actually made using a different technique known as radial velocity, or the Doppler method. To understand this method, we first have to understand a little about planetary orbits.

We think of planets as orbiting their host stars, but actually the star and the planet together orbit their shared center of mass. When you have two orbiting objects of equal mass, their center of mass will be at the midpoint between them so they will appear to chase each other in their orbits around a central point. Stars, however, are much more massive than planets, so the center of mass in such a system is much, much closer to the host star. This imbalance results in a large, extended orbit for the planet and only a tiny wobble in the host star’s position. Although tiny, this wobble is still measureable.

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