Astronomers comb the stars for planets

Thanks to the ability of astronomers to detect the presence of extrasolar planets orbiting distant stars, scientists today are able to examine hundreds of solar systems - but this also raises a problem - how do you spot the smaller earth-like planets in a wealth of data?

An artist's conception of extrasolar planet HD 189733 b – the “astro-comb” could help locate many more

Now US researchers have created an "astro-comb" to help astronomers detect lighter planets, more like Earth, around distant stars.

Researcher David Phillips of the Harvard-Smithsonian Center for Astrophysics said: “I expect to reach a resolution which when applied to the activities of large telescopes presently under construction, would open new possibilities in astronomy and astrophysics, including simpler detection of more Earth-like planets.”

Currently spectroscopy can reveal the identity of the atoms in the star (each element emits light at a certain characteristic frequency), it can also tell researchers how fast the star is moving away or toward Earth, courtesy of the Doppler effect, which occurs whenever a source of waves is itself in motion. By recording the change in the frequency of the waves coming from or bouncing off of an object, scientists can deduce the velocity of the object.

It can also be used to deduce the presence of a planet? Though the planet might weigh millions of times less than the star, the star will be jerked around a tiny amount owing to the gravity interaction between star and planet. This jerking motion causes the star to move toward or away from Earth slightly in a way that depends on the planet's mass and its nearness to the star.

Right now standard spectroscopy techniques can determine star movements to within a few meters per second (m/sec). In tests at the Harvard-Smithsonian Center for Astrophysics, researchers are now able to calculate star velocity shifts of less than 1m/sec, allowing them to more accurately pinpoint the planet's location.

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, which resides on a mountaintop in the Canary Islands.