A New Generation of Astronomers Is on the Hunt for the Next Earth

A New Generation of Astronomers Is on the Hunt for the Next Earth
October 04 12:15 2016 Print This Article

(WIRED) – ABOUT A MONTH ago, astronomers announced they had found a new exoplanet—this one, orbiting in the habitable zone of the nearest star to Earth. Proxima b is exciting because it’s nearby, and someday someone might send a spaceprobe to it. Plus, it has a mass close to Earth’s—making it more likely to be livable.

But Proxima b is also notable because scientists know its mass at all. Many of the 44 potentially habitable exoplanets have been found by Kepler-style transit searches, which watch and wait for planets to pass in front of stars and eclipse some of their light. But that only gives you a measure of a planet’s radius. Proxima b popped out of a different technique, one that, for the most part, hasn’t been sensitive enough to see planets like Earth. And advances in that technique—including a new instrument called EXPRES—could improve detection enough for scientists to find and weigh lots of other Earth-mass planets.

That exoplanet-finding method, called a radial velocity search, works by detecting the seesaw of a star pulled around by its planet’s gravity. The back-and-forth movement in each orbit takes the star ever so slightly toward us, and then ever so slightly away from us. Rinse and repeat, at regular intervals. When the star moves in our direction, its light waves appear a little squished—bluer. When it moves away, the waves appear stretched—redder.

Transits tell us a planet’s radius. Radial velocities tell us a planet’s mass. But a planet’s overall density, and so its composition, only comes from their powers combined. If scientists only knew how wide you were and not how much you weighed, they wouldn’t know if you were gassy or rocky. The same is true of planets. So being able to detect Earth-sized planets with the radial velocity method will help scientists to figure out if they actually are Earth-like. That’s an express goal of EXPRES, and the Yale lab led by astronomer Debra Fischer that is developing it.

The Hardware

EXPRES is a spectrograph, a device that can measure the regular, repeating shifts from radial velocity changes. They split incoming light up by wavelength—like a prism, if prisms provided lots of data. The previous state-of-the-astronomical-art, an instrument called HARPS, was installed on a 3.6-meter telescope in Chile by the University of Geneva in Switzerland in 2003. But HARPS doesn’t comb the star’s spectrum finely enough to see the littlest, lice-like planets. And it doesn’t take stars’ natural noise into account enough.

“We’re used to the idea that the exoplanet research is this booming industry,” Fischer says. But she begs to differ. For the past five years, she says, “our ability to detect planets has absolutely flattened out.” Proxima b, a planet 1.3 times the mass of Earth, is the smallest-mass planet yet found with the radial velocity technique. But scientists only found it because Proxima is so close (cosmically speaking) to us, as well as very close to its low-mass star. Because of those proximities, the back-and-forth showed up stronger than it would have in a different system.

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