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Akld Astronomers Help Discover a New Solar System

15th February 2008

Media Release - Stardome Observatory

Embargoed until Friday, February 15, 2008 at 8:00am NZ time to coincide with publication in the journal Science.

Auckland Astronomers Help Discover a New Solar System That Looks Very Familiar

The closest match yet found to our solar system

Stardome Observatory astronomer, Dr Grant Christie, is member of an international team that has discovered two planets orbiting a dim star nearly 5,000 light years away.

Relative to the parent star, these two planets bear a striking resemblance to Jupiter and Saturn. The similarity extends to their relative sizes, the spacing of their orbits and their temperatures.

This makes the new solar system look like a half-sized model of our one which is also dominated by two gas giant planets. It is the closest match yet found to our solar system and supports current theories of how it formed.

The results are reported in the February 15 issue of the prestigious journal Science.

Theorists have wondered whether gas giants in other solar systems would form in the same way as ours did. This discovery seems to suggest they do, said Dr Scott Gaudi, assistant professor of astronomy at Ohio State University, who led the analysis team and was the lead author.

The find also suggests that our galaxy hosts many planetary systems arranged like our own.

The two planets were revealed when the star they orbit crossed almost exactly in front of a more distant star as seen from Earth. For a two-week period from late March through early April of 2006, the nearer star magnified the light shining from the farther star.

This natural magnifying effect is called gravitational microlensing, and this was a particularly dramatic example: the light from the more distant star was magnified 500 times making the event especially sensitive to planet detection.

The Optical Gravitational Lensing Experiment (OGLE), led by Professor Andrzej Udalski first detected the event, dubbed OGLE-2006-BLG-109, on March 28, 2006 from the Warsaw University Observatory in Chile. The Microlensing Follow Up Network (MicroFUN), led by Professor Andrew Gould at Ohio State University, then joined with OGLE to organize astronomers worldwide to gather observations of it.

Stardome Observatory in Auckland, which has been part of the MicroFUN collaboration since 2004, responded the next night. "The skies were mostly clear and this allowed us to get 30 hours of data over seven nights, two of those coinciding with crucial phases of the event", explained Dr Christie.

"Stardome has now observed over 100 microlensing events so sooner or later you'll hit a winner", he added. Dr Christie is a lead author on the paper.

Jennie McCormick, also part of MicroFUN since 2004 and a lead author, obtained 10 hours of observations over three nights at crucial times from her private observatory in Farm Cove, Pakuranga. "For an amateur with a small 0.25 metre telescope, being able to work alongside the professionals on a really exciting and important event like this is just mind-blowing," she said.

The early phases of the event were observed by Paul Tristram using the NZ/Japan MOA 1.8 metre telescope at Mt John Observatory in Canterbury but cloud prevented observations while it was at maximum brightness.

The current discovery relied on 11 different ground-based telescopes in countries around the world, including New Zealand, Tasmania, Israel, Chile, the Canary Islands, and the United States. In total 69 scientists from 11 countries contributed.

Dr Gaudi took the lead in analysing the data as they came in. As he studied the light signal, he saw a distortion that he thought was caused by a Saturn-mass planet. Then, less than a day later, came an additional distortion he wasn't expecting: a "blip" in the signal that appeared to be caused by a second, larger planet orbiting the same star.

"It was exciting to see the picture evolving even while the event was still taking place", Christie recalled. "The theorists guessed early on we were detecting a new planetary system".

Over the next few months, Gaudi demonstrated that this two-planet interpretation was correct. Then David Bennett at the University of Notre Dame in Chicago refined Gaudi's preliminary model using sophisticated software, and revealed additional details about the system.

This is the third time a Jupiter-mass planet was found by microlensing, Gaudi explained. In the previous two cases, additional planets would have been very difficult to detect, had they been there.

The fact that astronomers found the planets during the first event that allowed such a detection suggests that these scaled-down versions of our solar system are very common, he added.

The newly-discovered planets appear to be gas planets like Jupiter and Saturn -- only about 80 percent as big -- and they orbit a red dwarf star about half the mass of the sun. The star is dim and cooler than ours, issuing only five percent as much light.

Although the star is much dimmer than our sun, temperatures at both planets are likely to be similar to that of Jupiter and Saturn, because they are orbiting closer to their star.

"The temperatures are important because these dictate the amount of material that is available for planet formation," Gaudi said. "Most theorists think that the biggest planet in our solar system formed at Jupiter's location because that is the closest to the sun that ice can form. Saturn is the next biggest because it is in the next location further away, where there is less primordial material available to form planets."

Gaudi described this microlensing event as the most complicated one ever studied. The astronomers carefully modelled their data on computers, and explored all possible explanations for the light signal. It took a year and a half of intensive analysis before they were confident that they had found the two planets.

While it is possible there may be small rocky planets in the new planetary system, they were not detected. Stardome Observatory has now contributed to the discovery of four extra-solar planets as part of the MicroFUN collaboration.


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