Scottish universities to build planet mass hunter

The universities of Edinburgh and St Andrews have secured funding to build a spectrograph to measure the mass of distant planets by their effects on stars
Written by Tom Espiner, Contributor on

Astronomers in Scotland have secured funding to build an instrument in a hunt for Earth-like planets.

Researchers from the University of St Andrews and the University of Edinburgh will join a project to build a spectrograph called Harps-N (High Accuracy Radial velocity Planet Searcher-North) that will allow astronomers to detect and weigh planets elsewhere in the galaxy.

Harps-N location Canary Islands

Researchers from Scotland will join a project to build a spectrograph called Harps-N for the Telescopio Nazionale Galileo in the Canary Islands (above). Photo credit: University of St Andrews

"Harps-N is going to allow us to take part in measurements of low-mass planets," professor Keith Horne of the University of St Andrews told ZDNet UK on Monday. "The agreement among all of the partners has been finalised."

The project will build the spectrograph, which will be attached to the Telescopio Nazionale Galileo in the Canary Islands. The instrument will allow astronomers to analyse small movements of stars caused by the orbits of planets that are too far away to observe directly.

We're trying to understand how common or rare Earth-like planets are.
– Keith Horne, University of St Andrews

"A planet tugging on a star causes the star to move around in a very small orbit," said Horne."We measure the reflex motion of the star [to gauge planet mass]. We're trying to understand how common or rare Earth-like planets are."

The telescope will focus light on the spectrograph, which is 3.6m across, and the image will go into an optical fibre. The image is stabilised and put through a diffraction grating to separate out the constituent colours before being fed to a charge-coupled device (CCD) image sensor.

When components of the starlight are superimposed over particular colours of light, differences reflect the movement of the star. When the star is moving away from the Earth, the light shifts towards the red part of the spectrum, shifting to blue when the star moves towards the Earth.

The data from the spectrograph will be combined with information from the Kepler space telescope, which has allowed scientists to observe the transit of distant planets. The Kepler telescope escapes the distortion of light caused by Earth's atmosphere, and monitors over 150,000 stars in a single field in the Cygnus/Lyra constellation (RA=19:23h, Dec=44.5d).

When a planet passes in front of a star, it causes the star to dim for a few hours, and allows scientists to calculate the radius of the planet. Hypotheses can also be made about the planet's atmosphere by looking at how light shines through. To date, Kepler has discovered 997 stars hosting 1,235 possible planets, including 68 of Earth-like size and 54 in their star's habitable zone.

When scientists combine the data about the mass and the radius, they can calculate the density of the planet, which gives an indication of the types of material from which the planet is made. By making repeated observations, the distance of the planet from the star can also be calculated, allowing scientists to say whether the planet could have water in liquid form.

St Andrews was not initially involved in the Harps-N project, which was started by Harvard University and Geneva Observatory in 2007. Various partners have since joined the project, including the National Institute for Astrophysics (Italy) and Queen's University Belfast. Part of the funding for the project will be supplied by the Scottish Universities Physics Alliance (Supa) and the Scottish Executive, said Horne.

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