A Fourth Planet for Mu Arae

by Paul Gilster on August 25, 2006

Hunting for exoplanets isn’t a matter of peering into telescopes and seeing faint specks of light. It’s all about combing through data — reams and reams of data thankfully digitized — for the telltale signatures of planets. And it’s fascinating to reflect that in many cases the signatures we seek are in our possession in the form of already gathered radial velocity data. We must continue to re-examine our growing stellar libraries, which in the case of radial velocities only get richer over time as planetary influences become more pronounced.

And so we come to Mu Arae, a G-type dwarf star much like our Sun and catalogued as HD 160691. A new study by Krzysztof Gozdziewski, Andrzej Maciejewski, and Cezary Migaszewski re-examines this already intriguing planetary system to discover yet another planet, the fourth to be found there. These radial velocity measurements were made by the Anglo-Australian Planet Search project and build on the earlier detections of three worlds, two being Jupiter-style companions with orbits projected at 630 days and 2500 days respectively. A third planet has been characterized as a ‘hot Neptune’ in a 9-day orbit.

The new analysis finds an interesting fourth planet of about 0.5 Jupiter masses in a 307-day orbit. That fourth world makes Mu Arae the second known four-planet system, the other being 55 Cancri, and in both cases the planetary orbits are nearly circular, an architecture not so different from our own Solar System. Given longer data runs we thus begin to see solar systems in greater detail and can apply our ever more precise computing tools toward resolving their structure. Key to all this, of course, is making the software used to highlight these planetary nuances better and better.

Teasing planets out of the datasets takes time, as becomes clear in a second study, this one from the Geneva exoplanet search team, that delves into the Mu Arae system. From the paper:

“More than 25% of the known extrasolar planets populate systems with at least two planets. In most of these systems, the various planets were discovered sequentially, starting with the planet inducing the highest radial-velocity amplitude at short or intermediate period. The following planets unveiled themselves when, after a while, the residuals became high compared to the expected measurement precision and showed some structure as a function of time.”

Mu Arae turns out to be a classic case in point. Using data from the HARPS (High Accuracy Radial Velocity Planet Searcher) instrument at La Silla to update older observations, the Geneva team is now able to characterize the entire Mu Arae system in greater detail. The team confirms the presence of the Neptune-class planet orbiting every 9.64 days, and also the previously known gas giant now pegged as orbiting every 643 days. The other Jovian class world is given an orbital period value of 4206 days (this one is still in need of work), while the newly discovered planet orbits every 310.6 days.

The complete dataset involved in this study covers 8 years of observations and 171 nightly-averaged measurements whose precision varied depending on the instrument used. As the paper comments: “With increasing number of planets, and thus free fit parameters, the amount of possible solutions increases drastically, and the only way to constrain the orbits is to acquire many new and precise data points.”

Which is what the exoplanet hunt is all about, throwing dramatic light on our continuing analysis of radial velocity data and projects like systemic, an undertaking that has gathered all available radial velocity data for known planet-bearing stars. The current Sky & Telescope has an article on systemic under an apt title: “Virtual Planet Sleuths,” while the collaboration’s creator, Greg Laughlin (UC-Santa Cruz) discusses planet formation in the current issue of American Scientist.


Adam August 28, 2006 at 4:18

Hi Paul

The quote…

“With increasing number of planets, and thus free fit parameters, the amount of possible solutions increases drastically[...]”

…is kind of something to remember when analysing the data. The blurrier the data, the bigger the range and number of planets. Clean data is awfully hard to get for planets at the limits of detectability – how many ‘hot neptunes’ will evaporate with better data?

I guess the ‘scopes will just have to squint longer at the stars…


Basel eshtewy July 30, 2007 at 17:12

This Red star with four planets one of them is very close to its star having a tail and other two planets having circular orbits while the fourth planet is with eliptical orbit are very will known to us in Egypt and arab countries. Its name is Al Najm Al Thakeb while the closest planet is called Al Tarek. The four planets are from our solar syetm and this star is in orbit going around our solar system and it is supposed to be in its way back to us. It shall affect our earth once it become close to our planet…


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