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New View of Exoplanets Will Aid Search for E.T.

Astronomers have developed a new method to probe the atmospheres of extrasolar planets, which should greatly expand the search for planets that have the right temperature and composition for life.

The technique allowed researchers to precisely calculate the mass of a planet named Tau Bootis b for the first time since its discovery 15 years ago.

“The coolest thing about this technique is we basically can now see the planet itself and its orbital movement,” said astronomer Simon Albrecht of MIT, who co-authored a paper describing the method and the Tau Bootis b findings, which appeared June 28 in Nature.

Researchers have several ways of learning about exoplanets. One of the most common and useful methods is used by the Kepler space telescope, which watches to see if the brightness of a star periodically dips, indicating that a planet is passing in front and eclipsing its light. When the exoplanet is just at the star’s edge, starlight can seep through the planet’s atmosphere, carrying a fingerprint of the atmospheric composition when it arrives at telescopes on Earth. Researchers can also sometimes block out a star’s light and directly image an exoplanet, but only when it is farther from its star than Pluto is from our sun.

Alternatively, astronomers closely observe a star to see if it wobbles slightly, signifying that a planet is gravitationally tugging on its host star. With this technique, no light from the planet is typically observed. Albrecht, working with a team led by astronomer Matteo Brogi of Leiden University in the Netherlands, tweaked this later method to get new information about the planet orbiting the star Tau Bootis, a yellow-white star slightly larger and hotter than our sun located 51 light-years away in the constellation Bootes. Since 1996, astronomers have known that Tau Bootis hosts a Jupiter-mass planet that flies around the host star every three days.

By looking carefully at the light coming from Tau Bootis, the researchers were able to tease out certain wavelengths of light that were changing in a characteristic way.

For a day and a half, the wavelengths would get longer, or redshifted, as the planet moved away from us. Then the wavelengths would grow shorter, or blueshifted, for the same amount of time, precisely matching up with the known orbit of the exoplanet. This is known as the Doppler effect, occurring because the frequency of a light or sound wave changes when it’s moving, such as when an ambulance’s pitch increases as it gets closer.

These wobbling wavelengths allowed the team to accurately trace the planet’s orbit, thereby measuring its mass, which is now known to be about six times that of Jupiter. The method also provided information about the planet’s atmosphere, indicating that it contained carbon monoxide.

Soon, the team hopes to look for other molecules, such as methane and hydrogen, and is already applying their technique to probe planets around other stars, said Albrecht. With better telescopes, they may be able to pick up biosignatures such as carbon and oxygen in the atmospheres of Earth-like planets.

“In the future, it will be one of the ways that we can search for these molecules of life,” said Albrecht.

Since only about 1 in 100 exoplanets transit their host star, “we can increase our list of potential targets by a factor of 10 or more,” said planetary scientist Heather Knutson of Caltech, who was not involved in the work. “It really opens up the door for a whole range of exciting measurements.”

But the information gleaned about Tau Bootis’s exoplanet came from one of the most favorable and nearby exoplanets and represents the limit of current telescopes, said astronomer David Charbonneau of Harvard, who was also not involved in the new work.

Truly remarkable results will likely have to wait for the next generation of enormous telescopes, like the Giant Magellan Telescope or the European Extremely Large Telescope, which will be able to pick up far more starlight and probe with higher resolution.

“The main interest for me is not this results in particular, but the results down the road,” said Charbonneau.

Image: An artist’s impression of Tau Bootis b in orbit around its parent star. ESO/L. Calçada