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Auckland Astronomers help discover a new solar system that looks very familiar

News from Stardome Observatory - 15/02/08

Stardome Observatory astronomer, Dr Grant Christie, is a 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.


The findings are being reported in the 15 Feb issue of the renowned journal "Science" as the cooperative effort of 4 international teams (MicroFUN, OGLE, MOA, and PLANET/RoboNet) and further researchers, comprising in total 69 scientists from 11 countries, with Dr Scott Gaudi from Ohio State University (USA) being the lead author.

Both professional and amateur astronomers joined in. People from three other microlensing collaborations -- the Microlensing Observations in Astrophysics (MOA) Collaboration, the Probing Lensing Anomalies NETwork (PLANET), and the RoboNet Collaboration -- all contributed observations and are co-authors of the study with MicroFUN and OGLE.

The authors hailed from the Ohio State University, University of Notre Dame, Warsaw University Observatory, Stardome Observatory (Auckland), Tel-Aviv University, Farm Cove Observatory (Auckland), Mt. John Observatory (Tekapo), Lawrence Livermore National Laboratory, Princeton University Observatory, Universidad de Concepción, University of Cambridge, Chungbuk National University, Korea Astronomy and Space Science Institute, Campo Catino Astronomical Observatory, Nagoya University, Massey University, University of Auckland, University of Canterbury, Victoria University, Konan University, Nagano National College of Technology, University of Manchester, Tokyo Metropolitan College of Aeronautics, University of Exeter, Université Pierre et Marie Curie, Liverpool John Moores University, University of St. Andrews, University of Tasmania, Université Paul Sabatier-Toulouse, Dartmouth College, and the University of Oxford.

This work was sponsored by the National Science Foundation; NASA; the Polish Ministry of Scientific Research and Information Technology; the SRC Korea Science & Engineering Foundation; the Korea Astronomy & Space Science Institute; Deutsche Forschungsgemeinschaft; the Particle Physics and Astronomy Research Council; The European Union’s Framework Programme for Research and Technological Development; The Israel Science Foundation; the Marsden Fund of New Zealand; the Japan Ministry of Education, Culture, Sports, Science and Technology; and the Japan Society for the Promotion of Science.

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