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Stardome Observatory astronomer helps discover another new planet

News of the work of Dr Grant Christie - 17/03/06

Stardome astronomer, Dr Grant Christie, is part of an international collaboration that has discovered a “super-Earth” orbiting in the cold outer regions of a distant solar system about 9,000 light-years away. The planet weighs 13 times as much as Earth, and at -193 degrees Celsius, it’s one of the coldest planets ever discovered outside our solar system.
Andrew Gould, leader of the MicroFUN collaboration and professor of astronomy at Ohio State University, pointed to new key implications of the discovery.
“First,” Gould said, “this icy super-Earth dominates the region around its star that in our solar system is populated by the gas-giant planets, Jupiter and Saturn. We've never seen a system like this before because we’ve never had the means to find them. And second,” he added, “we have found that these icy super-Earths are pretty common. Roughly 35 percent of all stars have them.”
MicroFUN searches for planets using a phenomenon predicted by Einstein called gravitational microlensing, which occurs when a massive object such as a star crosses in front of another star shining in the background. The object’s strong gravity bends the light rays from the more distant star and magnifies them like a lens. Here on Earth, astronomers see the magnified star get brighter as the lens star crosses in front of it, and then fade as the lens moves farther away.
The OGLE (Optical Gravitational Lensing Experiment) collaboration initially discovered the microlensed star in April 2005. Piecing together their observations, Gould and OGLE leader Andrzej Udalski of Warsaw University Observatory suddenly realized on May 1st that the lensed star was brightening extremely quickly, meaning that it would be exceptionally fertile ground for planet hunting.
So Gould called the MDM Observatory at Kitt Peak in Arizona, where the astronomer on duty was Ohio State graduate student Deokkeun An. Gould asked An to spare a few minutes during his night’s work on the 2.4 metre Hiltner Telescope to occasionally measure the star’s brightness. But when An and his co-observer heard how intense the signal was, they decided to put aside their own project to take more than 1,000 measurements of the event. “It's a good thing that they did -- their observations turned out to be critical to our determination that there was a planet,” Gould said.
The event was also observed over a 7 hour period by Dr Grant Christie at the Stardome Observatory, overlapping for an hour with coverage of the Arizona telescope. Christie recalled the night of 1 May, 2005 when he received the alert from Gould in Columbus, Ohio.
“Gravitational microlensing events that magnify the star’s light by 800 times are only detected once or twice a year so we immediately started observations. There was quite a bit of cloud around but we got lucky and it cleared long enough to get good observations,” he said.
The presence of the planet was first noticed by an initial computer analysis of the observational data by New Zealand astronomers Dr Philip Yock (University of Auckland), Dr Nick Rattenbury (Jodrell Bank), and Dr Ian Bond (Massey University).
But even after the astronomers had processed all the data, they faced more difficulties.
There remained a chance that the tiny warping in the signal wasn't caused by a planet. Ohio State graduate student Subo Dong had to write special software to speed his computer models to weed out the other possibilities.
The models confirmed the presence of a Neptune-mass planet, 13 times heavier than Earth, orbiting a star about half as big as our sun.
“This solar system is quite different from our own and many others in that it appears to be devoid of giant Jupiter-like planets,” said Dr Ian Bond of Massey University.
Gould suspects that the new planet is a bare, icy terrestrial one -- a cold super-Earth. “Judging from the absence of Jupiter-like planets in its vicinity, that solar system may lack the gas necessary to make gas planets,” he said.
Until a decade ago, scientists had no evidence of what other solar systems were like. Since then, some 170 planets have been discovered, and most of them have been gas giants similar to Jupiter. Only a handful of Neptune-mass planets have ever been detected, and this is just the second in the cold outer regions of their solar systems.
This discovery brings astronomers one step closer to their goal of finally detecting a planet similar to Earth. Gravitational microlensing is presently the only technique sensitive enough to detect such small planets.
The name of the new planet is OGLE-2005-BLG-169Lb. It refers to the 169th microlensing event discovered by the OGLE Collaboration toward the Galactic bulge in 2005, and “Lb” refers to a planetary mass companion to the lens star.
The astronomers have submitted a paper on the new planet to a leading international journal, Astrophysical Journal Letters.

Dr Grant Christie of Stardome Observatory was awarded the New Zealand Order of Merit in the recent New Years Honors list for his contributions to astronomy.

Grant has been involved with Stardome Observatory since its opening, and his passion has led to him being part of an international collaboration of astronomers and his second significant planet discovery.

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