Are there planets out there like ours? Could they be capable of sustaining life? It’s questions like these that have been motivating Dr. Peter Plavchan, assistant professor of astronomy at Missouri State University, as he has been studying planets that orbit other stars besides our sun, known as exoplanets.
In 2012, astronomers announced the discovery of an Earth-mass planet around the nearest star system to our sun, the Alpha Centauri system.
“Unfortunately, the planet is not habitable,” said Plavchan. The planet completes one orbit around its sun, the star Alpha Centauri B, in about three and a half days. “This makes it incredibly hot.”
This discovery is still fairly controversial and many different groups want confirmation that the planet exists. Proving that it exists will have to wait though — for the next two years the stars Alpha Centauri A and B are too close together in the sky, so it will be extremely difficult to distinguish the light from each of the stars independently.
“In my paper we focus on constraining the actual mass of the planet from the gravitational interactions of the exoplanet with the stars in the system,” said Plavchan, who collaborated on the study with Dr. Xi Chen, a scientist at the NASA Exoplanet Science Institute, and Missouri State student Garrett Pohl. “We use something called the ‘radial velocity’ or ‘doppler technique,’ which relies on the fact that the planet tugs on the star. As the planet goes around the star, the star actually wobbles back and forth.”
The Doppler Technique
If you’ve ever heard an ambulance go down the street next to you, you hear the sound become higher in pitch as it approaches and lower in pitch as it drives away from you. That is the shift in the frequency of sound due to the velocity of the ambulance with respect to you. Astronomers use this same concept when determining the mass of a planet.
“Instead of changes in pitch, we are looking for very subtle changes in color of the light coming from the star,” said Plavchan. “These subtle changes represent the fact that the star is wobbling due to the planet orbiting it.”
For the past three years, Plavchan has been an executive committee member for the NASA Exoplanet Exploration Program Analysis Group (Exo-PAG), which studies various exoplanet-related topics. This group exists to address questions like: How many exoplanets have the right temperature to have liquid water on their surface? How many of them will we be able to image in the future with a very big telescope?
“We currently know of 1,800 different exoplanets, and statistically we know that there are over 100 billion just in our galaxy alone,” said Plavchan, noting that more than 700 of those 1,800 exoplanets have either been discovered or confirmed using this wobble method. “It’s really exciting to know there are planets out there.”
Plavchan also recently published a paper that focuses on the Doppler technique and its applications, as well as its prospects for the future.
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