Combing the skies for Earth-like planets

CAMBRIDGE, MASSACHUSETTS – Mankind is always going to be most interested in those extra-solar planets which most closely resemble Earth. Now, researchers at the Harvard-Smithsonian Center for Astrophysics have created what they call an “astro-comb” to help astronomers detect lighter planets, more like Earth, around distant stars.

The standard approach for detecting extra-solar planets is to observe their influence through spectroscopy, which analyzes the energy spectrum of the light coming from the star. This reveals the identity of the atoms in the star (each element emits light at a certain characteristic frequency) and also tells researchers how fast the star is moving away or toward Earth, thanks to the Doppler effect.

The Harvard researchers have taken this one step further. Although an Earth-like planet might weigh millions of times less than its star, the star will be jerked around a tiny amount by the gravity interaction between star and planet. The only problem is that until now, spectroscopy has not been sophisticated enough to take full advantage of this phenomenon.

Until now, standard spectroscopy techniques have been able to determine star movements to within a few meters per second. But, in tests, the Harvard researchers have been able to calculate star velocity shifts of less than one meter per second. Smithsonian researcher David Phillips says that he and his colleagues expect to reach a velocity resolution of 60cm/sec – maybe even 1 cm/sec.

The astronomers achieved their improvement using a frequency comb as the basis for the astro-comb. A special laser system is used to emit light at a series of frequencies, evenly spaced across a wide range of values. This can be used to calibrate the energy of light coming in from the distant star. In effect, the frequency comb approach sharpens the spectroscopy process.

The astro-comb method has been tried out on a medium-sized telescope in Arizona and will soon be installed on the much larger William Herschel Telescope in the Canary Islands.

The group will present their findings at the 2009 Conference on Lasers and Electro Optics/International Quantum Electronics Conference, which takes place May 31 to June 5 at the Baltimore Convention Center.