Astronomical Society of Coonabarabran

THE SEARCH FOR EXTRA-SOLAR PLANETS INTENSIFIES

Siding Spring takes part in global telescope project

By Liz Cutts

The race is on to answer one of astronomy's hottest questions - how does a planet form and evolve?

A massive global telescope network project to find transiting planets will provide information on how planets are made.

Most of what we are now learning about planets orbiting other stars is derived from the study of those that transit, or partially eclipse, their home star.

Our home solar system may be down by a planet with the recent reclassification of Pluto, but the number of giant planets discovered in orbit around other stars continues to grow.

Preparations are now underway for a transiting extra-solar planet search in the Southern Hemisphere.

The HAT-South project will involve a global network of six identical, small-sized, wide-field telescopes spread over three sites - Chile, Namibia and Australia.

Two of the telescopes will be installed at the Siding Spring Observatory at Coonabarabran in a setup that allows for close to 24 hour time-coverage, with very efficient transit detection capabilities.

The Australian National University (ANU) is working in partnership with Harvard/Smithsonian Center for Astrophysics and the Max Planck Institute for Astronomy on the massive 'HAT-South' project to locate transiting planets in the Southern Hemisphere.

'HAT-South' is based on the HAT (Hungarian-Made Automated Telescope) prototypes that have already discovered seven transiting planets in the Northern Hemisphere.

"HAT-South will be much more capable, with more sky coverage, smaller pixels, larger aperture and more observing sites," explained Professor Penny Sackett, astronomer with the ANU Research School of Astronomy and Astrophysics.

"The telescopes have been funded by the US National Science Foundation and will be installed in early 2009."

When Giant Magellan Telescope (GMT) goes into operation in 2017 it will be possible for astronomers to learn more about the evolution of literally hundreds of planets.

However, this will only be successful if scientists can first discover the many transiting planets in the Southern Hemisphere where both GMT and Australians can best study them.

"Astronomers are now able to measure the mass, radius (and thus density), orbital size, eccentricity, and relative angular momentum vector for most transiting planets," adds Professor Sackett.

"This information is beginning to shed light on how planetary systems form.

"Even more excitingly for understanding the complex evolution of the planet itself, the best ground and space based instruments are measuring the temperature and atmospheric content of a few prime systems."

Professor Sackett explains that a planet passing in front of its parent star as seen from Earth will cause that star to momentarily drop in brightness for a few hours. This will happen periodically, once a "planet year."

"The size of the dip depends on the size of the planet relative to its star," says Professor Sackett. "A Jupiter-like planet transiting a Sun-like star will cause about a 1% dip in the star's brightness.

"An Earthlike planet would cause only a 0.01% dip and so is difficult to detect with telescopes on the ground.

"By detecting these dips we now know of the existence a class of planets -- "Hot Jupiters" that orbit searingly close to their parents with periods as short two days.

"How these planets form and evolve is one of the hottest questions in astronomy."

HAT-South will monitor hundreds of thousands of moderately bright stars simultaneously and nearly round-the-clock for about two months before switching to another portion of the sky to repeat.

Huge numbers of stars must be monitored to find the few that have planets with orbits sufficiently aligned to cause a transit.

All data will be combined into a 24-hour data stream and sent back to each collaborating institute for analysis.

"When a planet-like drop in a stellar light curve is identified, a large verification program will be launched on that star," states Professor Sackett.

"If the planet candidate survives these tests that can reveal most types of "planet imposters," high resolution spectrographs will be used to measure the planet's mass and confirm its identity."

Based on the success of the precursor HATnet project and detailed computer simulations, astronomers expect 25 Southern Hemisphere transiting planets every year from HATSouth, making it the most efficient of any such survey in the world.

hat_network

Figure 1: (HAT network) At any hour of any day a HAT-South telescope will be observing for transiting planets.

hat_diagram

Figure 2: (HAT diagram) The engineering diagram for the HAT-South telescope to be installed at Siding Spring.

artists_concept

Figure 3: (artists image) Artists concept of a transiting planet TrES-4 in the constellation of Hercules. (Image NASA)