How Do Astronomers Find Exoplanets?

Artist's conception of exoplanet Kepler-16b. Image courtesy NASA/JPL-Caltech/R. Hurt.
Artist’s conception of exoplanet Kepler-16b. Image courtesy NASA/JPL-Caltech/R. Hurt.

By: Sara Knight
BU News Service

Last month astronomer Erik Petigura announced it was likely that our galaxy may have up to 40 billion Earth-like, habitable exoplanets swirling around their Sun-like stars. Petigura’s conclusion, which resulted from his analysis of Kepler satellite data, marks a huge milestone in the search for exoplanets – a field that has experienced a rapid expansion in the past decade. The ultimate goal of this extra-solar quest – to find planets with conditions able to support life as we know it – is only attainable if researchers can not only locate these other worlds, but discern their composition. Given that the nearest exoplanet is 4.37 light-years, or 26.22 trillion miles, away from Earth, it is no simple task. So how do researchers find and analyze alien worlds?

It comes down to tenacious observation and a lot of math. First, astronomers must set a satellite or telescope to record a given patch of space – for example, the Kepler satellite focuses on an astral window of about 100,000 stars. Then they wait for minute fluctuations in the amount of light a star gives off, which indicates a body, maybe a planet, passing in front of the star. Researchers confirm the light-blocking object as an exoplanet only after noting that the light fluctuates in a regular pattern, which can take years depending on the length of the planet’s orbit.

Once the light-blocker is verified as a planet the researchers ascertain its volume and mass. Luckily for them, finding the planet’s volume is relatively straight-forward – the amount the star’s light dims as the planet passes by denotes its size. Finding the planet’s mass is trickier: researchers must determine the strength of the gravitational attraction between the planet and star. The larger the attraction, the more massive the planet. The star itself is also “in orbit” around the center of mass between itself and the planet – it is just so massive that its orbit is more of a wobble than a proper ellipse. As the star wobbles, the frequency of its light changes in the visible spectrum – a phenomenon known as the Doppler effect. By observing these fluctuations, astronomers can figure out how much a star wobbles, and therefore the mass of its orbiting planet.

Artist's conception of exoplanets in the Milky Way. Image courtesy Wikimedia Commons.
Artist’s conception of exoplanets in the Milky Way. Image courtesy Wikimedia Commons.
After obtaining the figures for the planet’s volume and mass, finding the planet’s density is as simple as dividing the mass by the volume. Once astronomers figure out the density, they can extrapolate the sorts of materials that may make up the world. For example, an exoplanet with an extremely high density is probably composed of heavier materials, like rocks and metals.

In 2010, astronomers began using light frequency analysis not only to find the mass of the exoplanet, but also to infer its atmospheric makeup. While the planet passes between the star and their observation point, the chemicals in its atmosphere will give faint light signatures, which astronomers analyze using a chemical spectroscope. By noting which elements shift in the star’s chemical lineup when the planet passes by, they can infer the chemicals present in the exoplanet’s atmosphere.

So far, astronomers have found a huge variety of alien planets – some made mostly of metal and with 20 Earth-hour years, others made mostly of super-hot gas and that have silicate glass particles as rainfall. While the exoplanets with what we on Earth would consider extreme conditions are the coolest to read about, the ones that are able to sustain liquid water most interest astronomers. Those relatively ho-hum planets reside in the habitable zones of their stars – zones that astronomers believe life as we know it in our solar system may exist. And despite the report that there are probably 40 billion of them out there, we have only spotted twelve of them so far. But by remembering the scope of our search this humble figure seems a little less discouraging. After all, we can only focus on a small fraction of space at a time and stars are oriented randomly throughout the Universe – who knows what we’re missing?