Extrasolar planets, or exoplanets, are planets that orbit stars other than our sun. Astronomers like Dr. William Welsh at San Diego State University primarily use two methods to detect these distant planets: Doppler and Transit methods. "Science Behind the News" is produced in partnership with the National Science Foundation.
Science Behind the News -- Extrasolar Planets
ANNE THOMPSON reporting:
For centuries, humans have looked to the night sky and wondered if there were any other planets out there like Earth, orbiting the distant stars. Then, in 1992, astronomers made the first discovery of a planet orbiting an alien star. Since that time, more than 700 extrasolar planets, or exoplanets, have been identified.
Dr. WILLIAM WELSH (San Diego State University): An extrasolar planet is a planet that orbits a star that's not our Sun.
THOMPSON: Dr. William Welsh is an astronomer at San Diego State University and funded by the National Science Foundation. He and other astronomers have been able to measure the size and mass of hundreds of these exoplanets, though some thousands of light years away, meaning that even if we could travel at the speed of light, it would still take us thousands of years to get there.
WELSH: So those little dots of light in the sky might have planets around them. An extrasolar planet would be a planet that orbits one of those little dots of light.
THOMPSON: To detect these distant planets, astronomers primarily use two methods: Doppler and Transit. The Doppler Method measures a star's “wobble”, something caused by the force of gravity from exoplanets themselves pulling, their stars in different directions during their orbit.
DR. WILLIAM WELSH (San Diego State University): They're moving back and forth in the sky and we're able to detect that motion through a technique called spectroscopy, which uses the Doppler technique to measure the speed of the star.
THOMPSON: The Doppler Effect is often associated with the change in the frequency of sound waves caused by a moving object, such as the sound of a race car whizzing by. Astronomers use the same principle to analyze how the movement of the star away and towards us, due to its orbiting exoplanet, changes the frequency of light waves. By using spectroscopy, it is possible to measure the frequency shifts in the star's light spectrum, implying movement of the star.
WELSH: It's making a little orbit in the sky by measuring that orbit, we can tell that there's another object there, and that other object is often a planet.
THOMPSON: Measuring the star's wobble also allows astronomers to calculate the planet's mass. The greater the wobble of the star, the greater the mass of the exoplanet. The second method used to identify exoplanets is the Transit Method. After observing a star over a period of time, astronomers sometimes notice a faint dimming of its light. This dimming is likely caused by a planet orbiting past it.
WELSH: The idea is the planet goes in front of the star and makes a little eclipse. And that little eclipse allows us to measure the change in brightness of the star. If that eclipse occurs over and over, then we know that something's orbiting the star and we can determine that there's a planet there. Now, the great thing about transits is that it gives us the size of the planet.
THOMPSON: As Welsh describes it, spotting one of these eclipses is a bit like trying to see an insect crawl across the headlight of a car miles away.
WELSH: Now, imagine a very small ant crawls across the front of that headlight, that's the amount of dimming that you would get when the earth passes in front of the Sun if we were to look at the Sun from very far away.
THOMPSON: Using Doppler and Transit Methods, astronomers are able to determine other characteristics of an exoplanet.
WELSH: The Doppler technique gives us the mass, the transit technique gives us the radius or the actual physical size of the planet. By combining those two different measurements we can get the density of the planet and find out is it a rocky planet like earth or is it a gaseous giant planet like Jupiter.
THOMPSON: Welsh's goal is not to find Jupiter-like planets, but ones more like Earth, a rocky planet with water, making it more likely that it could sustain life. One characteristic of such an Earth-like exoplanet is that it must orbit in what astronomers call the "habitable zone" of its parent star.
WELSH: The habitable zone is sometimes known as the Goldilocks Zone, where it's just the right temperature. It's not too hot and not too cold. But it's just the right temperature where we can have water. And water is what we think is required for life.
THOMPSON: To accomplish this goal, Welsh and fellow planet-hunters will continue to scour the night sky looking for exoplanets and perhaps answer an age-old question: does another Earth exist?
One of our closest celestial neighbors is a warm, rocky world, scientists say.
Writing in the journal Astronomy and Astrophysics, scientists report the discovery of an Earth-size exoplanet orbiting the star Ross 128, a dim red dwarf just 11 light-years away.
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