Keck and Kepler to

join forces

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 10 March, 2009

Two of astronomy’s most powerful planet hunters will join forces this summer to scrutinize distant Earths.

The twin ten-metre W. M. Keck telescopes located in Hawaii have already played an important role in the search for giant planets orbiting distant stars. Now that the Kepler mission is on its way to picking out Earth-sized planets, Keck will be used to make follow up measurements to determine the composition of these far away worlds.

lThe twin 10-metre Keck telescopes are on a mission to discover extrasolar Earths hand in hand with the space-based Kepler spacecraft. Image: Laurie Hatch.

“Keck and NASA have a long-standing partnership to push astronomy research to its fullest potential,” says Keck Observatory Director Taft Armandroff. “This Keck-Kepler collaboration gives that partnership a compelling new scientific focus.”

Kepler launched successfully from NASA’s Kennedy Space Center last Friday and will study 100,000 stars in a single region of the sky for three and a half years. To identify exoplanets it will watch for the periodic dimming in a star’s brightness that betrays the presence of a planet transiting in front of its host star. But, says regular Keck user and Kepler mission co-invesitgator Geoff Marcy, “Kepler does not tell astronomers with certainty if the object taking a bite out of the starlight is a planet or another star.” And this is where the Keck telescope plays a supporting role in the Kepler mission.

Once Kepler team members locate an Earth candidate and determine as best they can that they’re not looking at two stars orbiting each other, they will pass the object to Marcy and his colleagues, who will use the Keck I telescope and its instrument HIRES (High Resolution Spectrometer) to monitor how the light coming from the parent star changes as the planet candidate journeys around the star.

A transit occurs when a planet crosses in front of its star from the perspective of the observer. When Earth-like planets transit their parent star, they block out about 1/10,000 the starlight. Image: NASA.

HIRES splits a star’s light into its component wavelengths, or colours. That is, its spectrum. As the planet orbits behind its parent star, as seen from Kepler’s point of view, gravity will slightly pull on the star, shifting its spectrum to redder wavelengths. As the planet crosses the face of the star, it will pull the star in the other direction, and the star’s spectrum will shift toward bluer wavelengths. HIRES will detect these shifts which will enable astronomers to calculate the star’s radial velocity, the speed at which the star moves toward or away from Earth. Based on this speed, Marcy and his team will be able to calculate the mass of the proposed planet.

“Keck’s HIRES is the only game in town that can measure spectral shifts caused by an Earth-sized planet. No other telescope is big enough,” says Marcy. “That is why NASA is really heavily dependent on the Keck telescopes right now.”

Calculating the planet candidate’s mass is crucial in determining the object’s composition, and can allow astronomers to distinguish between a planet and a star. Furthermore, Marcy and his team can use the Keck-calculated mass and Kepler-calculated diameter to determine the planet’s density. “In a sense it’s as if we are taking the planets and dunking them in a bathtub to see if they float,” says Marcy. “A rocky planet like Earth [with an average density of roughly five grams per cubic centimetre] would sink.” On the other hand, gas giants, which have a density close to that of water, would “float”.

The Keck-Kepler duo are set to become a formidable planet-hunting team, and Marcy and colleagues plan to start studying Kepler’s candidate Earths with Keck I and HIRES at the end of July 2009.