New planet found: Icy “Super-Earth” dominates distant solar system

Author: William G. Gilroy

An international collaboration of astronomers that includes David Bennett, an associate professor of physics at the University of Notre Dame, has discovered asuper-Earthorbiting in the cold outer regions of a distant solar system roughly 9,000 light-years away from our planet.

The planet has a mass of about 13 times that of the Earth, a temperature of about -330 degrees Fahrenheit, and orbits its star at about two-thirds of the distance between Jupiter and the Sun.

This is the second cool,super-Earthto be found recently by the gravitational microlensing method, but in this case, there is a crucial difference.

The data indicate that there are no gas giant planets in Jupiter, Saturn or even Earth-like orbits,said Bennett, one of the lead authors of the paper describing the planet discovery.So, it is expected that this ‘super-Earthis the dominant planet in its solar system even though it is only about 4 percent the mass of Jupiter. In fact, it is likely that there are more solar systems dominated by ‘super-Earthsin our Galaxy than there are solar systems like our own.

Andrew Gould, lead author of the paper, said:Weve never seen a system like this before, because weve never had the means to find them. The gravitational microlensing method gives us this capability.

Gravitational microlensing is based on an effect predicted by Albert Einstein in 1915. During a gravitational microlensing event, the almost perfect alignment between a background source star, a lens star, and an observatory allows researchers to discover a planet that orbits the lens star. The technique enables the detection of a planets gravitational field on the light seen from the more distant background star.

When two stars are perfectly aligned as seen from Earth, the gravitational field of the foreground star acts as a lens to magnify the background star. Astronomers dont see the planet or the star that it is orbiting, but the effect of their gravity reveals the existence of the planet around the lens star.

This latest discovery brings the number of planets discovered by the microlensing method to four, with two of the four being of the icy, super-Earth varietya class of planets that cannot yet be detected by any other method.These two recent discoveries can be compared to the number of discoveries expected from all the microlensing events that have already been observed.

A comparison of these two ‘super-Earthdiscoveries to the number expected from all the microlensing events indicates that about 40 percent of the stars have an icy, ‘super-Earthplanet,Bennett said.

The discovery was made by 36 astronomers from the MicroFUN (Micro-Lensing Follow-Up Network), OGLE (Optical Gravitational Lensing Experiment), and Robonet (a global network of 2-meter robotic telescopes) collaborations. The name of the newly discovered planet isOGLE-2005-169Lb,which refers to the 169 th microlensing event discovered by the OGLE Collaboration toward the Galactic bulge in 2005. TheLbrefers to a planetary mass companion to the lens star.

Crucial roles in the discovery of OGLE-2005-169Lb were made by OGLE team leader Andrezej Udalski of Warsaw University Observatory and graduate students Deokkeun An of Ohio State and Ai-ying Zhou of Missouri State University.

Udalski noticed that this microlensing event was reaching a very high magnificationMay 1, 2005, and he quickly alerted the MicroFUN group to this fact, since high magnification events are known to be very favorable for planet detection. MicroFuns regular telescopes were unable to get many images, so MicroFUN leader Gould called the MDM Observatory in Arizona where An and Zhou were observing and asked them to obtain a few measurements of the stars brightness over the course of the night.

Instead, An and Zhou made more than 100 measurements. This large number of MDM measurements was crucial for the determination that the observed signal must, in fact, be due to a planet. The discovery would have been missed if the graduate students had simply done as Gould had requested.

Bennetts research is supported by the National Science Foundation and NASA.

* Contact: * _David Bennett, associate professor of physics, 574-631-8298,

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